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Nitrogen Donors in Organometallic Chemistry and Homogeneous Catalysis.

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REVIEWS
Nitrogen Donors in Organometallic Chemistry and Homogeneous Catalysis
Antonio Togni* and Luigi M. Venanzi
7
/-
Homogeneous catalysis has been responsible for many major recent developments in synthetic organic chemistry.
The combined use of organometallic
and coordination chemistry has produced a number of new and powerful
synthetic methods for important classes
of compounds in general and for optically active substances in particular. For
this purpose. complexes with optically
active ligands have been used, most of
them coordinating through phosphorus.
More recent developments have highlighted the use of “nitrogen-donors”,
particularly as they are easily obtained
from the “chiral pool”. However, the remarkable achievements in this area have
been based on an empirical approach.
This article attempts to bridge the gap
between the synthetic and the coordination chemist. The first section discusses
the rates of formation and dissociation
of complexes with nitrogen donors, their
relationship to the rates of product formation, and presents the factors which
induce homolytic cleavage of M- C
bonds. It also provides a summary of the
main types of organometallic complexes
formed by metal centers coordinated to
nitrogen donors and their reactivity pat-
terns. The second section highlights the
most significant, homogeneously catalyzed reactions involving complexes
with nitrogen ligands. Foremost among
them are the asymmetric aspects of hydrogenation (particularly those involving boranes as reducing agents). hydrosilylation, cyclopropanations, Diels Alder reactions. aldol condensations,
alkylation of aldehydes, conjugate addition reactions, Grignard cross-coupling
reactions, allylic alkylations, oxidation
reactions, olefin epoxidations. and dihydroxylation of olefins.
1
\
1. Introductory Remarks
The development of organometallic chemistry and homogeneous catalysis during the last forty years, and particularly that
involving transition metals, has been closely associated with the
realization that their formation, stability, and reactivity are governed by the fundamental rules of coordination chemistry.[’]
Thus, an awareness of these rules is essential for any rational
discussion of organometallic reactions.
When applying these rules to carbon donors one must take
into account the fact that the great majority of compounds
containing M -C bonds also contain coligands (often referred to
as “spectator ligands”, although their real role is far from that
of being mere spectators) which have donor atoms other than
carbon. Foremost among these are the P-donors. Two other
essential coligands in organometallic chemistry are the hydride
ion and CO. itself a C-donor. Indeed these two ligands played
a central role in the early developments of the synthetic applications of organometallic chemistry, particularly because of their
dual role as “reagents” and coligands.
Textbooks of organometallic chemistry and homogeneous
catalysis”’ make sporadic reference to the use of N-donors, but
[*I
Prof. DI-.,A. Togni. Prof. Dr. L. M. Venanzi
Laboratonurn fur Aiivrganische Chemie der Eidgenossischen
Technischen Hochschule
ETH-Zent run1
Universitiitsrrosse 6 . CH-8092 Zurich (Switzerland)
Tclefax: Int. code + (1)252-8935
hitherto no attempt has been made to systematize their role, as
has been done for P-donors. Furthermore, although phosphorus donors and their higher homologues have received much
attention, and numerous books and reviews describing their
coordination chemistry have been published,[3] nothing of the
kind has occurred for the organic nitrogen donors, possibly
because their complexes have not been extensively used for synthetic organic applications. Therefore, this article will discuss
the role of the N-donors in the above areas, often by comparing
and contrasting them mainly with P-donors.
2. Fundamental Concepts of
Organometallic Chemistry
The organometallic chemistry discussed here is concerned
with the formation, transformation, and rupture of M -C
bonds. Nevertheless, as this review emphasizes the role of
complexes with nitrogen donors in organic synthesis, reactions that are not organometallic in the sense defined above
will also be included in the section dealing with catalytic processes.
The metal centers which will feature prominently are those of
the transition and post-transition series, although the organometallic chemistry of the lanthanides and actinides is being
actively in~estigated.[~]
Organometallic compounds of some of
the alkali and alkaline earth elements, as well as those of boron
A. Togni and L. M. Venanzi
REVIEWS
and aluminum. are frequently used as reagents or catalysts in
the reactions described later. Furthermore. there is no fundamental diffci-encc bctlbcen their chemistry and that of the transition and post-transition elements: They share an essential a n d
common feature in their "coordination beha\ ior". even though
boron and aluminum frequently form compounds having threecenter. two-electron bonds.'s1
As its title implies, this review will be mainly concerned with
compounds containing nitrogen donors as coligands and their
organomctallic chemistry. For this purpose it will be useful to
sunim;irize briefly the main features of coordination bchavior.
o f the carbon donors and their properties, and of the nitrogen
donors .
2.1. Coordination Behavior
Any atom. generally referred to as the "central atom". which
possesses low energy orbitals containing fewer electrons than
required for thc formation of the maxim~iiiipossible number of
conventional. two-clectron cov;ilcnt bonds, can easily form
morc bonds than its "classical" covalency implies. In other
words. a molecule containing such a central atom is a Lewis
acid. Conversely. ii Lewis acid is a molecule containing such a
central atom and is said t o be "coordinatively unsaturated".
This is a mkijor feature in the context of this article, 21s many
Lewis acidic complexes containing nitrogen donors play a
role in catnlytic processes (see, for example. Sections 5.6 and
5.7).
As will become apparent later. the most prominent application ofcomplexes containing nitrogen donors is the synthesis of
optically active organic molecules. For most organometallic reactions of interest here, one can envisage it pathway of the type
shown in Scheme 1.
A
The ligand o r ligands denoted by L in this Scheme m a y fulfill
variety of functions:
1 . modulation of the electron density at the central atom M
and. thcrefore, its reactivity:
2. blocking of a number of coordination sites at M and, consequently, control of the multiplicity and symmetry of the valence orbitals available for the organometallic reaction:
3. provision of the environment within which the organometallic reaction will take place.
Furthermore, the dynamics of the M - L interactions play :I
crucial role in successful organometallic reactions. This is obviously connected with the dissociation rate of the M--L fragmcnt : if it is much faster than the rate-determining step for the
formation of A- B. the ligand L will not exert a significant
influence on the organometallic reaction. Important exceptions
to this simplified behavior are those catalytic systems in which
ligand-induced acceleration is observed (see Section 5.12.2). I t
follows that, when considering the role of a coligand L ( a nitrogen donor in our case). not only the thermodynamics'h1but also
the kinetics of complex formation between M and L must be
considered.
Before discussing these factors it is appropriate to say a few
words about the organometallic steps of a reaction sequence. An
essential feature o f a catalytic reaction is that it includes one step
that is either irrccersible or virtually irreversible. Most frequent;I
REVIEWS
Nitrogen Donors
iy, this is a reductive elimination reaction,121as shown in the last
step of Scheme 1, where either a C-H or a C-C bond is formed.
Furthermore, in principle, one can envision the same organometallic reaction with a variety of coligands coordinating
through different donor atoms; their electronic nature and
geometry will favor, or disfavor, one o r more steps of the same
reaction. Thus for the same metal, a given catalytic cycle may be
more or less disfavored by the use of a particular ligand, or, for
the same ligand, by a suitable choice of metal.
At this point it is necessary to discuss undesirable organometallic reactions. Foremost among them are those leading to
the homolytic cleavage of M-C bonds and, consequently, to
loss of regiochemical or stereochemical selectivity (Scheme 2).
A
L,M-R
B
L,M-X-CH2R
L,MIR
R'
C
D
L,M-R
+
E'
-
-
L,M'
+
L,MH
+
X=CHR
L,M
+
R-R'
L,M'
+
R-E
'R
Scheme 1.Proces5ea leading to the cleavage of an M - C bond. A) homolytic cleavage 171; B) /i-hydrogen abstraction [XI: C ) reductive elimination [9]; D) electrophilic
attack iit ;I coordinated carbon atom [lo].
Nucleophilic displacement of a coordinated C-a-donor is generally associated with its transformation into a x-donor and will
not be considered here.
While reactions B-D in Scheme 2 can constitute integral
parts of desirable catalytic cycles, reaction A is generally to be
suppressed. and therefore metal complexes in which homolytic
cleavage can easily take place should be avoided.
The ease of cleavage A is related to the stability of the resulting fragments L,M' and 'R. However. the role of the metal-containing fragment is the more complex given the variety of metal
centers. their spin states, and the number and nature of the
accompanying ligands. The foremost consideration is that the
unpaired electron, associated with the metal fragment produced
after the homolytic cleavage, will be localized on the metal center. thereby lowering its oxidation state by one unit.
A good example of this effect is shown in Scheme 3.["] The
"normal" mode of Fe-C cleavage of the d6 Fe" compound, a
p-H abstraction reaction, changes to a radical decomposition
f7\r
&*''Et
N(I
L N
-Et
_-.
H3C-CH3
+
H,C=CH,
50 %
I l+
+
l / x [Fe(bpy)J,
50 %
-e
C
N
"*
:.$'8Et
N(I
*Et
L N
-
H3C-CH2-CHz-CH3
45 Yo
+
H3C-CH3
48 Yo
+
H2C=CH2
8YO
Scheme 3. Diffei-ent decomposition pathways of dialkyl Fe" and Fe"' complexes
(/(-hydrogen ellminiition vs. homolytic cleavage) [ I l l .
after the corresponding ds Fe"' complex is formed. In general,
whenever the metal complex can give another stable species with
the metal center in an oxidation state one unit lower, homolytic
cleavage will be favored. Complexes of the metal centers d 7 CO"
and d' Cu" provide good illustration of this behavior. On the
other hand, homolytic cleavage in d 8 Ni" complexes is uncommon.["]
Thus the preferred metal centers for organometallic reactions
should have even-numbered electron configurations, particularly do, d6 (low spin), d8 (low spin), and d". However, numerous
exceptions are known (e.g., d 7 Rh" in Section 5.5.3, or d 5 Fe"'
in Section 5.6).
These redox reactions do not prevent the use of catalyst precursors containing metal centers with electron configurations
such as d7 and d', as, upon addition of an organometallic
reagent, reduction will take place, and the metal center of the
resulting complex will be in the oxidation state best suited for
catalysis (see, for example, the cobalt catalyst in Section 5.2.2).
However, the use of a catalyst precursor containing the metal
center in the appropriate oxidation state is usually preferable, as
fewer side-reactions are then likely.
Any discussion of ligand exchange processes must be approached from their reaction mechanisms. These can be conveniently described in terms of the two extreme models illustrated
in Scheme 4.f131
In the absence of specific information about a
LnM
A
B
__
__ __
L+1)M
Ly_l)M
+
C
LM
,
+
C
+ L
Ly-ilMC
L,MC
L{,_,)ML'
+
L
Scheme 4.The limiting models for the mechanism of a ligand exchange process. A
dissociative. B: associative.
given ligand-exchange reaction in organometallic chemistry,
one can judiciously use the following generalizations :
1 . Coordinatively saturated complexes, particularly those in
which the central atom has either an effective atomic number
(EAN) of 18, or a half-filled d shell, are likely to undergo
ligand exchange by a dissociative process.
2. Coordinatively unsaturated complexes, that is, those with
EAN < 18, muy undergo ligand exchange by an associative
mechanism.
It is apparent from these generalizations that one cannot always expect a direct correlation between velocity of a ligand
exchange and thermodynamic stability of a given ML, complex.
However, for dissociative ligand-exchange processes their rates
are presumably closely related to the strengths of the M--L
interactions in the ML, complexes.
Before discussing the factors influencing ligand exchange
rates, it is useful to have an idea of their range. The most complete set of data relates to water exchange in most metal cations,
which extends from lO-'to lo' s-1.1141 However, perhaps more
relevant to the chemistry discussed
\
\
here are the dissociation rates in
complexes with polydentate ligands.
Some data for murexide [valid for reaction (a)], which, under the experiNH
A. Togni and L. M. Venanzi
REVIEWS
mental conditions used, acts as a uninegative, terdentate ligand
coordinating through the donor atoms indicated by the arrows
in the structure sketched below, are listed in Table 1 . [ 1 5 ]
Tdble 1. Some rate constants for reaction (a) in 0.1 M KNO, a t pH 4.0 and 10 C.
Ml+
k,
[W's-']
C'l'
2 6 x 10'
> 6 X 10'
ca. 9 x 106
Srz
Mn"
Co'
+
15x10'
1 . 0 1~0 3
1.2x10*
2.0 x 10'
1.1 x 108
N12+
Cu2
+
Znz
Cd' '
+
A-
M"
I
[s-)]
tt.6~10'
in3+
>6
S c 3 + [a]
Cd.
x lo5
7.5 X 10'
5.1~10'
0.41
5 . 2 10'
~
2.0~10'
7.x x 10'
Y 3 + [b]
La3+[b]
G d 3 + [b]
T h 3 + [b]
L u 3 + [b]
k,
k ,
[M-lb-l)
[SC']
2.0x 10"
4.8 x 10'
1 . 7 10'
~
8.6~10'
5 . 2 10'
~
3.0 x 10'
1.3~10'
50
1.5 x 10'
5.5 X 10"
3.2~10'
4.3 x 10'
3.3 x 10'
4.5~10'
[a] At pH 3.0 and 12 C [h] At 12 C
As can be seen, even these rates span a very wide range, which
is comparable to that of water exchange if the same metal ion is
used. Several factors influence these rates. Foremost among
them are the electron configuration, ionic charge, size. spinstate, coordination number, and geometry of the metal center,
the ionic charge of the complex. the nature of the coligands, and
the nature of the chemical reaction.
One further general point is that the kinetics of substitution
reactions for transition metal complexes with the same outer
electron configuration are faster for metal centers of the first
transition series than for the corresponding centers of the second and third transition series. as can be seen for the examples
shown in Scheme 5.['']
M-CI
Me
+
PEt,
Me
PEt,
[s-'l
M
T I'CI
k,,,
Pt
Pd
25
-40
1.7~10-~
5.8~
Ni
-65
1 . 6 ~ 1 0 ~ ~
Scheme 5. Ligand exchange reactions for Nil', Pd", and Pt" complexes with pyridine
([pyl = 6.2 x l o 3 M) [16].
Another consideration should be kept in mind: ligand-disproportionation reactions [Eq. (b)] can easily occur. Obviously,
boundary is fixed by the need to have a reasonably Fast catalytic
reaction. while the higher boundary is coupled to the speed of
the organometallic transformation leading to the product, as
this must clearly be faster than the ligand-exchange rate. This is
merely a guideline. and significant deviations can be found in
practice as the intrinsic lability of a given metal center can be
decreased or increased in a variety of ways. Therefore, the range
of "useful" metal centers can be considerably extended.
When nitrogen donors are used as coligands one has to solve
a problem which does not arise with Lewis bases coordinating
through phosphorus atoms. As has been pointed out when discussing Scheme 1, the influence of the ligand L will be felt if the
M-L exchange rate is significantly slower than the rate of the
organometallic reaction. as is the case of a phosphorus donor.
which has the great advantage of labilizing the ligand t r a m to it.
The labilizing effect of a given ligand upon the rates of substitution of other hgdnds present in the same complex has been
extensively studied. In the present context one only needs to
consider the trnns effect, that is. the labilizing effect of one
ligand upon another situated in trans position.["I Some data on
this effect are summarized in Table 2.
Table 2. The more common donors arranLged in order of decreasing rruns effect. and
approximate values oftheexchange rates ofthe i r r m lipandsin a Ptl'complex [13a].
fr(iii.s effect
Donor ligand
k,,,
very large
large
moderate
weak
CO, C H - . C 2 H 4 .PR, z H
SC(NH,)?. NO;. 1.. S C W . CH;. C,H,
Br-. C1py. NH,. O H -
c11
500
ca. 10-1
As can be seen, in a given complex one can have a substitutionally inert, coordinated phosphane ligand and trans to this
P-donor, a kinetically very labile ligand. This, unfortunately, is
not the case for nitrogen donors, because they are among the
ligands that exert the weakest trans effect. Nevertheless, coordinated amines d o impart a smaller labilizing effect on the residual
ligands, as can be seen from the data shown in Table 3.['41However, generally. in order to have reasonable reaction rates in the
case of complexes containing nitrogen coligands one must
choose labile metal centers and find a way of reducing the lability of the metal-nitrogen bond(s).
Table 3. The labihring effect on M-OH, bonds of coordinated aliphatic amines. as
expressed by the water exchange reaction.
k,,io
+
[Ni(H,O), mLJz++ H,O tl [Ni(H,O),, nLn]z+ H,O
Complex [a]
processes of this type are to be avoided as, in such cases, the
organometallic reaction at one center is not influenced by the
nitrogen ligand, while the second center, being strongly coordinated to two L moieties, may not be available for the
organometallic reactions. This disproportionation can be prevented by the use of nitrogen ligands with bulky substituents.
Useful reactivity is likely to be found for metal ions that show
intermediate exchange rates (lo-' to l o 2 s - I ) . The lower
10'
cil.10'
ca. I
ki,,o
bl
3 x 10'
2.5 x 10'
6.1 x 10'
44x10'
1.2 x 106
2.Y x 106
3.x x 10"
[a] en = ethylenediamine, dien = diethylenetriamine. trien = triethylenetetrdmine.
tach = 1,3.5-triaminocyclohexane.[h] At 25 C : k,,,,, i s defined as k,(6):kO,,(6-n),
where n is the number of bound nitrogen ligands: k , = rate constant for the formation reaction.
Angew. Clirni. l i i r . Ed. EnR/. 1994, 33. 497-526
REVIEWS
Nitrogen Donors
This can be achieved by the use of polydentate or anionic
ligands of rigid structure optimally designed to fit the size of the
metal center (see. for example, the polydentate ligands in Section 5.12.1). The lower lability of chelating systems has been
amply demonstrated in classical coordination compounds. As
can be seen from Table 4,[''] monodentate ligands are much
Me
-
Me
PY
PY
Table 4. Rate constants [s-'1 for the dissociation of mono- and bidentate nitrogen
heterocycles [s- '1 from Nil' and Co" complexes: T = 25 " C , p H z 6.0.
Complex [a]
M = Ni
M
IM(H,0)4pv)12 '
[MW , O L ( b ~ y ) l '+
[M(H,OHphen)l' '
40
5x
1 x 10-5
8x
[a] py
=
pyridine. bpy
=
bipyridine, phen
=
CO
6x
1 x 10-2
= phenanthroline.
more labile than the corresponding bidentate ligands. Indeed,
with ter- and tetradentate ligands measurable dissociation rates
can only be measured in acidic solution.['" However, as acidic
media are only used for workup of organometallic reactions, it
is useful to have an idea of the inertness of a chelating nitrogen
ligand in neutral solution. Thus, the bpy dissociation rate constant for the complex [Ni(H,0),(bpy)12+ is 5 x
s - ' , in
contrast to 40 s- for the corresponding monopyridine compIex.['']
Particularly relevant for the transfer of chiral information
from a metal complex to a product is the rigidity of the coligand(s). Thus, as can be seen in the data of Table 4. the rigid
phenanthroline exchanges at a slower rate than the more flexible
2,2'-bipyridine ligand.
Finally, ligand-exchange rates in polydentate ligands d o not
necessarily give a complete picture as far as transfer of chirality
is concerned, as a facile process involving the opening and closing of one arm of a polydentate ligand could lead to a decrease
in transfer of chirality without complete detachment of the
ligand from the metal. Thus, macrocyclic and related ligands
prove to be particularly valuable as "spectator" ligands.
2.2. Characteristics of Coordination Compounds
with M - C Bonds
Because there are many types of C-donors it should be noted
that even those classified as "one-carbon centers" may bind to
more than one metal center and, if available, may use more than
one electron pair for bonding; furthermore, while most C-donor
ligands also act as reagents in organometallic reactions, some of
them, particularly six-electron donors such as the cyclopentadienyl anion and benzene, mainly function as coligands ("protecting groups"), that is, they block a number of coordination sites
at a metal center. leaving available for further reaction a given
number of free coordination sites in well-defined spatial relationships. Furthermore, they can direct incoming reagents and
leaving groups along specific pathways and also strongly influence the electronic properties of the reacting fragment, L,M.
The most stable organometallic compounds of a given metal
center are those in which all low-energy orbitals are involved
in bonding (preferably two-electron bonding). F o r instance,
PtIMe, is tetrameric with the structure shown in Scheme 6, but
Ang"tc Clioi~.rnl. Ed. Engl. 1994. 33. 497- 526
Scheme 6. The reaction of [PtIMe,], with pyridine (the mcthyl group\ at one Pt
atom are omitted for clarity) 1191.
mononuclear complexes are readily obtained by reaction with
additional ligands, for example pyridine, which gives the complex [PtIMe,(py),].[191The platinum centers in both complexes
are 18-electron systems with octahedral coordination geometry.
Catalyst precursors are often di- or oligomeric and can become
monomeric under the reaction conditions used (see. for example, the aluminum Lewis acid in Scheme 36, Section 5.6).
The absence of unpaired electrons on a metal centers of coordination compounds implies first that the metal center should
have an even number of electrons and second that the complex
should be of the low-spin type, that is, one where there is maximum pairing of all the available metal d electrons. This second
requirement dictates the choice of the metal center and of its
ligands. Thus, all metal centers of the second and third transition series give virtually only low-spin complexes. Furthermore.
their metal centers in oxidation states having an odd number of
electrons (generally only one) are relatively unstable and easily
reduced. As C-donors are good reducing agents. paramagnetic
organometallic compounds of these elements are rare.
Mention has been made earlier that the complexes of metal
centers of the second and third transition series, in their stable
oxidation states, are intrinsically kinetically inert (see
Scheme 5 ) . Thus, their use as catalysts generally requires coligands other than nitrogen (trans effect). An important exception
is the [Os"(NH,),] system discussed in Section 5.1 3.
A metal center from the first transition series can be chosen by
taking into account that the tendency of a given metal ion to
form low-spin complexes is generally expressed i n terms of two
parameters. The first, the spin-pairing energy P,, expresses the
metal-based, unfavorable energy change upon spin-pairing,
while the second, the ligand field stabilization energy A , expresses the ligand-based, favorable energy change upon spin-pairing.
Values for these parameters for some metal centers and ligands
are listed in Table 5.[201
The data confirm that the metal ions of the first transition
series show a lower tendency to give low-spin complexes and
thus require coligands such as phosphanes to be formed.
However, all the d6 low-spin complexes, particularly if in an
octahedral coordination environment, give slow substitution
rates. Thus their complexes with nitrogen donors are seldom
useful in homogeneous catalysis. This leaves the important class
of low-spin complexes with coordination numbers four and a
square-planar structure, or five and structures which can range
from square-pyramidal to trigonal-bipyramidal.
Another group of metal centers deserves special mention.
namely those with a d 5 electron configuration such as Mn". This
50 1
A. Togni and L. M . Venanzi
REVIEWS
Table 5 . Spin-pairing ( f , ) and lipand field ( A or f 1 parameters for somc metal ions
in hexaaqua complexes. and for various ligands [a]
M't
Cr'+
Mn3+
Mn"
Fe'
Fe2
co3+
coz+
Electron
config.
[kJmol-'1
d"
244.3
d'
301.6
2x5.0
357 4
+
+
fT
d"
229.1
d'
282.6
250
A (HzO)
[kJmol-'1 [a]
X or L [b]
f
100.3
150.1
0
0
47.6
1 XX
71.5
Br
CI
F
dmf
0.72
0 78
09
0.98
1 .00
1.17
H,O
MeNH?
MeCN
NH,
bipr
phen
CN
[a] A =,f (ligands) ',q (central ion): g : see ref. [20]. [b] X
neutral hgdnd.
=
1.22
1.25
I .33
1.34
cil. 1.7
anionic hgdnd.
L=
electron configuration is particularly stable and, although very
labile, the complexes of these metal centers exibit useful reactivity for homogeneously catalyzed reactions.
Finally, another group of metal centers particularly useful for
organometallic reactions is that with d t o electron configuration:
Cu', Ag', A d ; Zn", Cd", Hg"; Ga"', In"', TI"'; Sn'".
Summing up, complexes with nitrogen donors displaying useful reactivity for organometallic reactions will be found mainly
among low-spin complexes of transition elements having coordination numbers four and five or having metal centers with d 5
configuration, or even among complexes of the post-transition
elements. The considerably oxophilic character of the early transition elements will severely restrict their use.
3. Characteristics of Nitrogen-Donors
Before discussing in detail the coordination chemistry of nitrogen donors it is appropriate to mention the relative strength
of M-C, M-N, and M - 0 bonds. (The M - 0 bond is included,
because many of the ligands used for the reactions to be discussed later also contain this atom as donor.)
Some representative values of the M - Y bond energies are
listed in Table 6.12'] The M-C bonds are weaker than the M - N
T;+hle 6. Mean M L bond dissociation enthalpies D [kJmol-'] for hornoleptic
metal compounds ML, as %ell as M - X bond dissociation enthalpies for X = H, C.
N [kcalmol-i].
X-M'
ML,
L
TI
Zr
Hf
M
H
CH,
iYH,
CH,lBu
NE1,
OiPr
F
I88
227
340
518
648
224
367
534
665
Sc
57(2)
309
444
586
Ti
V
54(3)
48(2)
50(2)
47(2)
59(3)
54(2)
58(2)
49(4)
40(2)
450)
X5(2)
85(1)
73(2)
67(12)
62(2)
56(5)
Fe
Co
NI
SO(?)
bonds, which in turn are weaker than the M - 0 bonds.[221Although different types of complexes may show significant deviations from the data in Table 6, the values quoted can be taken
as a guideline.
502
The range of nitrogen donors is more extensive than that of
any other atom: It is enough to recall how varied the organic
chemistry of nitrogen is and that most of these compounds are
potential donors. By comparison, in inorganic chemistry only
very few molecules or ions bind to metals through a nitrogen
atom. Thus, only ligands of the former group will be discussed
here.
An appropriate classification of nitrogen donors can be based
on the hybridization of this atom. sp3, sp2, and sp. The types of
compounds containing sp3-hybridized nitrogen atoms are summarized in Scheme 7. Complexes with nitrogen donors contain-
Scheme 7. Some nitrogen-donors with tertlary sp3-hybridired h' atoms.
ing N - H bonds are generally not suitable for most
organometallic reactions as the H atom (or atoms) on coordinated nitrogen are sufficiently acidic to react with a nucleophile.
Therefore, the common nitrogen donors having N- H bonds
have not been included in Scheme 7. Furthermore, coordinated
nitrogen atoms containing nonbonding pairs of electrons are
susceptible to electrophilic attack by suitable reagents. However, coordinated "nitrogen anions", also listed in Scheme 7.
show an extensive and useful coordination chemistry, particularly when associated with sp2-hybridized carbon atoms.
Of special interest in the present context are many natural
products containing ternary nitrogen atoms. for example the
alkaloids. A number of them have been successfully used for the
enantioselective synthesis of organic compounds (see, for example, Sections 5.1 I and 5.12.2).
Ligands containing sp2-hybridized nitrogen atoms, particularly when the N-atom is part of an aromatic system (see, for
example. Scheme X), have a very extensive coordination chemistry. Also for donors of this type. a much more extensive and
useful coordination chemistry is possible when they are bi- or
terdentate ligands. However, the use of such ligands in asymmetric catalysis requires the presence of a stereogenic substituent.
The presence of C-N and C-C double bonds in these molecules renders them susceptible to further reactions, such as nucleophilic attack or, under appropriate conditions, hydrogenation. This is particularly true of Schiff s bases.
The only class of organic nitrogen donors with sp hybridization are the nitriles. Their main function is as labile ligands that
are replaced by appropriate reagents. Furthermore, once coordinated. nitriles are quite susceptible towards nucleophilic attack.
As might be expected, the donor properties of nitrogen ligands differ significantly from those of phosphorus. However, an
accurate assessment of their relative donor properties and of
A I Z , Chivm
~ ~ I n / . Ed. En,qI.1994, 33. 497 -526
REVIEWS
Nitrogen Donors
Tolman has also obtained a set of values for an “electronic
parameter”, vp, which expresses the relative donor capacity of a
phosphane ligand in the absence of distortions caused by steric
effects.[231Once again, a similar set of data does not appear to
have been obtained for nitrogen ligands. However, in this case
the corresponding nitrogen values cannot be estimated from the
phosphorus data because the nature of the M-L bonds in the
two cases are different. M - N bonds are mainly of cr type and
M-P bonds of r~ and x type.
Finally. H. C. Brown et al. obtained data useful in the context
of the stability of metal-nitrogen bonds and their relation to
steric effects (relevant in the context of reactions catalyzed by
oxazaborolidines; see Sections 5.2.2, 5.6, and 5.7).[241This is
summarized in Fable 8.
Table 8. Bond dissociation energies [kcdlmol- I ] for trimethylhorane adducts of
amines at 100 C.
Compound
AC
AH
AS
Me,B- NH,
Me,B- NH,Me
Me,B NHMe,
Me,B - NMe,
Me,B - NEt,
Me,B - quinuclidine
-1.134
2.412
2.885
0.557
13.75
17.64
19.26
11.62
ca. 10.0
19.94
0.040
0.041
0.044
0.046
-
Scheme 8 Some nitrogen-donors with sp2-hybndized N atoms.
related ligands with other donor atoms is difficult. The most
commonly used criterion to compare donor properties of nitrogen compounds is their pK value. However, these values refer to
aqueous solutions and meaningful comparisons are seldom possible, because solvation effects may mask true electronic
contributions. Furthermore, for compounds sparingly soluble
in water, the p K values are not available.
Another useful criterion for the comparison of the donor
ability of various ligands is the first ionization potential of the
donor atoms. Also in this case the values can only be used
qualitatively, as removal of an electron to infinity is conceptually and quantitatively different from the formation of a dative
bond, particularly as the former process is not influenced by
steric effects whereas the latter is. Therefore, any consideration
of relative stability of metal-ligand bonds in general and
metal -nitrogen bonds in particular must take into account steric interactions. These have been successfully parametrized for the
phosphorus donors by means of Tolman’s “cone-angle” values,
Op. Some of these values are shown in Fdbk7.[231The corresponding values for nitrogen donors, H,, have only been reported,
r e ~ e n t 1 y .h1I ~They
~ are in general larger than for the corresponding P-donors, the difference being greater for the larger ligands.
Table 7. Tolman-s cone angle values for phosphorus-donors (0, (23al) and the
correspondinp values 0, for nitrogen-donors (23 b].
87
118
132
145
Anjieii. Clioi~./I?!.
Ed. Engl 1994, 33. 497-526
94
132
150
166
2.9 16
0.046
In conclusion, if one considers only the nitrogen ligands relevant for organometallic chemistry, one reaches the conclusion
that nitrogen atoms in donor molecules in coordination chemistry, as well as in other branches of chemistry, can form strong
bonds to metal centers; the strength of the bonds depends largely on their CF covalency with a potentially significant contribution from the ionic character of the bond itself.
Nevertheless, x interactions are possible with ligands containing sp2-hybridized nitrogen donors, and x back-bonding effects,
particularly between nitrogen heterocycles and metal centers,
have been frequently invoked to explain a variety of observations. However, while their occurrence may be beyond dispute,
the extent to which they determine the chemical properties of
any given compound is controversial, because ground-state effects are likely to be small.
Thus the following gneralizations, which typically must be
used in this case probably with a rather large grain of salt( !), can
be made:
Donor -acceptor bonds formed by nitrogen donors are generally fairly strong and. in contrast to those formed by phosphorus donors, their strength is unlikely to show drastic
changes if the metal center, but not the ionic charge. is
changed. As a consequence, bonds of nitrogen donors to the
transition elements of the first transition series and the inner
transition series as well as some of the post-transition metals
will be stronger than those of the related phosphorus donors
(“A” vs. “B” character, “hard” vs. “soft” donors).
The strength of the M - N bonds will be affected much more
by steric effects than that of the corresponding M - P bonds.
The nitrogen donors, generally, will not be as effective in
producing low-spin complexes with the consequence that the
species produced are less thermodynamically stable and
more kinetically labile than their low-spin analogues with
P-donors.
503
A. Togni and L. M. Venanzi
REVIEWS
The consequences of these “boundary conditions” are that
nitrogen-donor ligands forming robust complexes are, in general. di-. ter-, or niultidentate ligands, often macrocyclic, and contain ligands with sp’-hybridized N-atoms. Thus, the organometallic complexes with nitrogen donors as coligands described
in the next section are likely to contain the above-mentioned
types of compounds.
active cyclometalated compound is useful for the separation of
racemic mixtures, generally of phosphanes (see Scheme 9)
Many organometallic compounds containing nitrogen
donors have been obtained by using polypyrdzolylborate liga n d ~ . ‘ *A~ ]representative selection of compounds of this type is
shown in Scheme 10.[”0-331
H
H
1
4. Organometallic Complexes Containing
Nitrogen- Donors
Many ligands of organometallic complexes contain double
( C = X ) or triple ( C s Y ) bonds (X and Y are atoms other than
nitrogen). and it is now well established that a significant contribution to their total M - L bond strength is provided by the
formation of TI back bonds. However, nitrogen donors d o not
form x bonds to any significant extent, and thus, the strength of
the M C bonds in the presence of these ligands is directly related to the electron density at the metal center. Thus the isolated
occurrence of organometallic compounds containing nitrogen
coligands is not surprising. There is, however, an increasing
body of evidence showing that organometallic complexes of the
type [MR,,(L’),,J (LN = nitrogen donor ligand) are readily
formed and. in many cases. can be isolated, even if the metal
center belongs to the second or third transition series. Particularly instructive examples are provided by the complex
[PdX(Me),(NN)] (X = halide; NN = N.N,N’.N’-tetramethylethylenediainine, or 2,2’-bipyridine) .I2’] and by the [Os”(NH,),]
system described in Section 5.13. Furthermore, nitrogen donors
are frequently used as part of chelating systems together with
phosphorus- or carbon-donors. The organometallic chemistry
of the P,N-donors does not significantly differ from that of the
P.P-ligands, but several interesting differences in organometallic
reactivity. including catalysis, have been reported.[’”] On the
other hand the C,N-donors have played a significant role in
organometallic chemistry because of their ready formation
through a cyclometalation
and because an optically
dF3
[30]
~311
H
H
~
Scheme 10. Examples of organometallic complexes containing the tris(pyra2olyl)bOrdto ligand (30 331.
Noteworthy is also the organometallic chemistry based
on complexes of the N,C,N-terdentate ligand shown in
Scheme 11, [ 3 4 1 and that of complexes containing 1,4-diaza-I ,3dienes, some of which are given in Scheme 12.[351
Me
,
Me
*2
1 ‘Me
Me
[34b]
L“
M
‘e\
PMePh
PMePh
ye
0.05 rnol %
&-Me
Ni-
CI
CI +
COOMe
Schrine 9. The I’urniutioii of optically actibe I’d” complcxes with C,N-donors and
tlirir 11\c i n (he wpnr;ition of i-acernic mixtures via diaatrromeric complexes [28a].
SO4
WfI
I
Me
Scheme 11. Somc complcxes with van Koten’s “pincer” ligand. and a reaction catalyzed by i t 1341
A n g m C‘hcm
Inf Ed Eiigi 1994. 33. 491- 526
Nitrogen Donors
REVIEWS
However, a common feature of organometallic compounds containing nitrogen
ligands is apparent: they usually exhibit
high reactivity. Examples of this type of
behavior are shown in Scheme 13.[361
The electron-withdrawing effect of coordinated alkenes is well illustrated by the
reaction of the complex [Ni(Et),(bpy)].
Upon addition of acrylonitrile, this complex gives a relatively stable adduct with
iPr
pentacoordinated Ni atoms, but exposure
to excess acrylonitrile leads to clean reductive elimination of butane to afford
ococ,
, R.u,'uR GO
the corresponding bis(o1efin) - Nio coinco
pIe~.[~"~l
Finally. even a very brief survey of the
organometallic chemistry with nitrogendonors would be grossly incomplete without a mention of the extensive and fascinating chemistry of Vitamin B,,, its coenzyme (Scheme 14), and the cobal0.5 mol % 2
amines. While reference to more specialized reviewsL"'"' will give for a deeper un0.7 rnol % Mg (C,HJ
derstanding of the chemistry of systems of
-17°C / 14d
this type. two general points of coordination chemistry emerge from published
work, which may be relevant in some catalytic systems used for organic syntheS ~ S . ' ~ ' ~The
'
first of them concerns the
mode of cleavage of Co- R bonds in conipounds of this type (Scheme 15).[381 All
Scheme 12. Some transition metal complexes with 1.4-diaaa-1.3-dienes and a reaction catalyzed by them [35].
the cleavage modes described in Section 2.1 are found to occur with comparable activation energies in the presence of
the appropriate reagents. The second point concerns the oxidaCONH,
tion state at the cobalt atom. A summary of the electron-trans\
CONH2
fer processes are shown in Scheme
Given the complex chemical nature of vitamin B,, and its
coenzyme, coordination chemists have invested much time in
developing and studying model systems. The simplest (and not
the least effective) models for B,, systems are the cobalamines,
shown schematically in Scheme 1 7.[3H1
L/
W
8
ivng
I
OH
/ N
\
+H,CCH,CH,CH,
Scheme 13 Reactions of organometallic (bpyJNi" complexes with olefins [36].
U
+vitamin
B,,
+ coenzymeB,2
Scheme 14. Vitamin B,? and its coenzyme.
505
A. Togni and L. M. Venanzi
REVIEWS
hvl A
____)
El''
+ kH3
-
CH4. C2Hs
OH2
I
Scheme 15. Course of the cleavage of Co- C bonds in the B,, system by nucleophilic
attack. heating or irradiation. and electrophilic attack.
Very often in such cases the N-ligands are derivatives of amino
acids and thus obtainable from the chiral pool.[3y1The following
discussion will focus on the type of reaction that is catalyzed by
complexes containing N-ligands, because a reaction-oriented
section should enable comparisons (for example. with phosphane ligands used in the same reaction), as well as the identification of new possibilities and trends. Since systematic studies
of the significance of N-ligands. for example. as opposed to
P-hgdnds, for a given reaction do not exist, and coordinationchemical rationalizations for the choice of such ligands have not
been reported, the N-ligands have been used on an ad hoc basis.
Studies concerning the influence of basicity. steric hindrance.
and electronic properties of a certain class of nitrogen ligands on
the catalytic properties of the corresponding complexes d o not
exist either. Such theoretical viewpoints are well established for
phosphane ligands (for example. the concept of ~ o n e - a n g l e , [ ' ~ ~
or the well-defined electronic influence of the substituents on the
phosphorus atoms upon reactivity) and are often applied in the
discussion of catalytic effects. Despite the relative infancy of
nitrogen ligands in homogeneous catalysis, the amount of published work is considerable. Therefore, this review cannot
present a comprehensive account. and only the most significant
or recent examples will be discussed.
5.2. Asymmetric Hydrogenation and Reduction
5.2.1. Hydrogen as Reducing Agent
Asymmetric hydrogenation reactions, catalyzed by rhodium
complexes containing chiral chelating diphosphanes. have been
very successful and constitute classic examples of well-established catalytic methodologies.[401Probably for this reason nitrogen ligands have been rather neglected. However, mixed P.Nferrocenyl ligands have been shown by Cullen et al. to give
enantioselectivities as high as 84 % in the rhodium-catalyzed
hydrogenation of acetamidocinnamic acidc4'](Scheme 18). An
Scheme 16. Changes of Co oxidation state in the B,, system.
X
COOH
1 mol % [Rh'], 1 atm H,
A
Me
..
...
Pi
+
\
NHCOCH3
JooH
Ph
EtOH, 30'C
04 YOee
N:
r
Me
Ph
5
B
NHCOCH,
p-Rh
[Rh'] =
c
Scheme 17. Cobaloximes, models for the B,, system (B is, for example, pyridine).
L
5. Homogeneous Catalysis with Nitrogen Ligands
Scheme 1% Asymmetric hydrogenation catalyzed by a Rh' complex [41]
5.1. Introductory Note
Nitrogen ligands in homogeneous catalysis have received increasing attention in recent years. In particular, the use of optically active, chelating. nitrogen-containing ligands made many
significant contributions to the field of asymmetric catalysis.
506
X-ray crystal structure determination of the catalyst precursor
[Rh'] as PF, salt shows that the ligand coordinates as a chelate
to the rhodium atom.[41a1However, selectivity and activity seem
to be dominated by the phosphorus ligand. Thus, replacement
of the PPh, group by a P(tBu), fragment leads to a more active
A n g e u . Chm. I n ! . Ed. Engl 1994. 33. 497-526
REVIEWS
Nitrogen Donors
cataiyst. which gives products of the opposite absolute configuration.14'b1 These catalysts are less active than those containing
chelating diphosphanes.
A completely different catalyst system for the hydrogenation
of benzil t o benzoin has been reported by Ohgo. Takeuchi
et al.'"' Chiral bis(g1yoximato)Co" complexes in the presence of
an optically active amino alcohol as cocatalyst (e.g., quinine)
were found to afford enantioselectivities up to 61 % ee
(Scheme 19). The same system also hydrogenates acetamido
Ph
Ph&
0
10 rnol % [Co(chd),L]
quinine, 1 atrn H,
C6H6, 30°C
5.1 1). Such monoanionic ligands are easily prepared from pyroglutamic acid and form six-membered conjugated chelate
rings which are Cz-symmetric.[431The reduction of a./i-unsaturated esters with NaBH, in ethanol/dimethylformamide (DMF)
is efficiently catalyzed by Co" salts in the presence of the ligand
shown in Scheme 20. The enantioselectivities reach 94 O/O. Iso-
OH
94% ee
1 rnol % CoCI, / L
ph+Ph
NaBH, f EtOH f DMF / ca. 20°C
0
4
61 YOee
? --'"'?
*
Ph
COOEt
Ph*
COOEt
94% ee
CN
\O . H p
H N
'CHZPh
Scheme 1 Y Asymmetric hydrogenation catalyzed by bis(g1yoximato) Co" complexes. with quinine as cocatalyst 1421.
acrylic acid derivatives, but with a much lower enantioselectivity
(up to 20% ee). The axial ligand in the catalyst precursor (typically an ainine or a tertiary phosphaiie) has only a minor influence on the efficiency of the catalyst, but increasing the hydrogen pressure had a beneficial effect on the reaction rate.
Although its details still remain obscure, this reaction raises
several interesting mechanistic aspects: The chiral cocatalyst
quinine does not appear to be directly bonded to cobalt; a
second-sphere interaction between the chiral amino alcohol and
the coordinated substrate has been invoked in the stereodifferentiating step; an electron-transfer step seems to be involved in
the course of substrate
The possibility that
the actual catalyst is a Co' complex does not appear to have been
considered.
5.2.2. Bovanes as Reducing Agents
In recent years, Pfaltz et al.1431have developed chiral chelating semicorrin ligands and used them for several asymmetric,
homogeneously catalyzed reactions (see also Sections 5.5 and
Ph
h
Scheme 20. (Semicorrinato)Co'-catalyzed asymmetric reduction o f x,/~-unsaturatrd
esters with NaBH, [44].
meric configurations at the double bonds yield products of opposite absolute configurations, and nonactivated olefins are not
reduced.[441The active catalyst, formed in situ from CoCI, and
the ligand, is thought to be a Co' species.
Chiral B-amino alcohols, mainly derived from a-amino acids,
react with BH, (mostly used as its T H F adduct) to form 1.3.2o x a z a b ~ r o l i d i n e s . These
[ ~ ~ ~ compounds are also efficient and
highly selective catalysts for the reduction of ketones with
H,B . L (L = THF, o r S(CH,),). Itsuno et al. were the first to
report, in 1981,[461the use of such borolidines as stoichiometric
reductants. Catalytic applications of oxazaborolidines were also
initiated by the Itsuno
and the methodology has
been exploited more recently by several other groups.[48- 5 3 1
Scheme 21 shows one specific example; a more detailed collection of data is found in a recent review by Wallbaum and
Martens.[451
5
HsCB(0H);
Ph
OH
,B-0
H3C
cat.
c
-
Ph
H3B
,B-0
H3C
OH
10 rnol % cat.
Scheme 21. Formation of a 1.3.2-oxazaborolidine and its use as catalyst for the enantioselective reduction of ketones with BH, . THF [54].
Bottom right: result o f t h e X-ray structure analysis.
H3B.THF
____t
c
PhACH3
2°C I2 min
96.5% ee
507
A. Togni and L. M. Venanzi
REVIEWS
Oxazaborolidines are unique in that they contain a Lewis
acidic and a Lewis basic center next to each other, and. according to the model developed by C ~ r e y . [ ’ ~are
] able to activate
both the carbonyl functionality of the substrate and the borane.
This model. which allows a prediction of the absolute configuration of the secondary alcohol formed, has recently been corroborated by the X-ray crystal structure of the BH, adduct shown
in Scheme 21 ,‘541 as well as by quantum mechanical calculations.[”]
The reduction of trichloromethyl ketones with catecholborane offers a general and highly selective method for the synthesis of x-amino acids[’”] (Scheme 22). Finally. the oxazaboro-
cylohexadiene),CI,]). as well as the rhodium-to-ligand ratio,
seem to influence the selectivity significantly. A pentacoordinated hydridorhodium complex is postulated as an intermediate in
the catalytic cycle[601(Scheme 23).
[Rh(cod)Cl],/ L
+
Ph
iPrOH
+H3CCOCH3
L
Ph
CH3
CH3
63 % ee
Scheine 23. Rh-catalyzed asymmetric transfer hydrogenation of acetophenone in
the presence of a chiral phenanthroline deribatlve [59] and the postulated Rh-hydrido intermediate [60]
‘ +
GCCi3
c
CHzCI, /-2O”C
98% ee
0
/
/
1) NaN3
2) H30+
3)H,/Pd/C
Alkylaminomethyl- and alkyliminomethylpyridines have
been used as ligands in the Ir-catalyzed hydrogenation reaction.
Selectivities are generally low, and only the bulky mt-butyl
phenyl ketone can be reduced according to Scheme 24 A in 84 %
~ e . [The
~ same type of ligand in a polymer-bound form yields
enaiitioselectivities up to 86 YO(Scheme 24 B) .Ih2]
[Ir(cod)CI],/ L
iPrOH / Nal
Scheme 22. Synthesis of r-amino acids b> ayqmmetric reduction of trichloromethyl
ketones [ 5 6 ] .
*
9
P
h
84% ee
U
L=
lidine methodology has recently been extended to the reduction
of ketimines.[”] The optical yields are generally lower than
those obtained for the reduction of ketones and also lower than
those of the corresponding stoichiometric reactions.
Ph
cat.
APh
iPrOH
*
LPh
86% ee
5.3. Asymmetric Transfer Hydrogenation
Chiral nitrogen ligands have been used in the asymmetric
hydrogen transfer more frequently and more successfully1581
than in hydrogenation reactions with H,. This reflects the compatibility of nitrogen donor atoms with the mechanistic peculiarities of this reaction. In the typical hydrogen-transfer reaction, the coordination sphere of the metal comprises hard donor
ligands (e.g., nitrogen and alkoxy), and no change in the formal
metal oxidation state need be invoked.[’*I
Rhodium catalysts containing chiral bipyridine or phenanthroline derivatives have been used by Mestroni et al.[591Because such ligdnds are planar, stereogenic groups can only be
introduced in peripheral regions of the molecule. where their
influence is rather modest. Accordingly. the selectivities obtained are rather low. The nature of the rhodium-containing
catalyst precursors ([Rh,(l.5-cycIoo~tadiene)~Cl~]
or [Rh,(l,3508
cat. =
Q
Scheme 24. Ir-catalyzed transfer hydrogenation in the presence of an alkyliminopqridine (A) [hl] and an immobilized alkylaminomethylpyridine(B) [62] as liganda.
For B the composition of the polymer is also giveii.
Pfaltz et al. have recently shown that C,-symmetric tetrahydrobioxazoles are effective ligands for this type of reaction
and reported that enantiomeric excesses reach 91 YO(Scheme
25) .[f’31
Nitrogen Donors
9
Ph
REVIEWS
0.5 rnol % [Ir(cod)CI],
1.3 rnol YOL
2 rnol YOKOH
iPrOH / 80°C
*
4
P
h
I
91 YOee
Scheme 25 TetrahydrobisoxazoleIr complexes in the asymmetric transfer hydroXenation of alkyl aryl ketones [63].
A completely different system for the hydrosilylation of 1.3dienes with HSiCI, was reported by Hayashi et aI.['*l
(Scheme 27). The catalyst precursor was a PdCI, complex containing a chiral P,N-ferrocenyl ligand. Good regioselectivities
5.4. Asymmetric Hydrosilylation
Brunner et al. have demonstrated that chelating nitrogen ligands containing a stereogenic fragment in a peripheral region of
the molecule are superior to conventional C,-symmetric chiral
diphosphanes in the Rh-catalyzed asymmetric hydrosilylation
of
The diphosphanes are characterized by the rather
rigid chiral array of the phenyl substituents on the phosphorus
atoms. This feature is one of the dominant selectivity-determining factors and was already recognized in the early days of
asymmetric Rh-catalyzed h y d r ~ g e n a t i o n . ' ~However,
~]
for hydrosilylation the key factor appears to be a stereogenic group
close to the free coordination sites at the metal center. This is
achieved by using pyridine derivatives such as those shown in
Scheme 26. The hydrosilylation of acetophenone with diphenylsilane affords 1 -phenylethanol with a maximum enantiomeric
excess of 86(!40.'~']
1) [Rh,(~od)~Cl,]
/L
(0.5-0.75 rnol % Rh)
H,SiPh,
phq
+
1) 0.02 rnol % cat.
HSiC13
2) EtOH / NEt3
3) H 2 0 2
66% ee
\Ph \CI
&
.
Scheme 27. Pd-catalyzed asymmetric hydrosilylation of a 1 J-diene 16x1
(up to 93 YO)as well as moderate enantioselectivities (up to 66%
ee) were obtained. The ligand contains a perfluorinated alkyl
substituent on the nitrogen atom, which ensures catalyst solubility in the solvent-free mixture of reactants.
5.5. Asymmetric Cyclopropanation
OH
5.5.1. Copper Catalysts
c
2) HO
,
-
4
3
PhACHB
83% ee (L = L')
86% ee (L = L2)
H
H
The synthesis of optically active cyclopropane derivatives[6y'
by the stereoselective addition of a carbenoid reagent to an
olefin is a most important reaction from both a historical as well
as from a practical point of view.[701The vast majority of the
known and successful catalysts for this reaction are complexes
of the late transition metals (Cu, Co, Rh) with nitrogen ligands.
Apparently the very first example of asymmetric catalysis by
a soluble metal complex is a cyclopropanation reaction reported
by Nozaki et al. in 1966. The Cu" complex shown in Scheme 28
Scheme 26. Asymmetric hydrosilylation of acetophenone with diphenylsilane catalyzed by Rh i i i the presence ofpyridyldihydrooxaroles and pyridylthiazolidines [66]
Ph%
Much higher enantioselectivities for the same reaction (up to
94 YO)have been reported by Nishiyama et al., who used terdentate ligands like 3.[671The corresponding trichlororhodium
complex, characterized by X-ray diffraction,L67b1
was used as a
catalyst precursor. F o r the catalytic reaction to take place, the
addition of a Ag' salt was required. This led to the formation of
a Rh"' cationic complex, which was then reduced by the silane
to the catalytically active Rh' species. A tetraaza ligand (4),
related to 3, which was recently reported by the same group,
gave slightly lower e n a n t i o s e l e c t i ~ i t i e s . [ ~ ~ ~ ~
+ N2CHCOOEt
-
pw
$
Ph
Ph
1
COOEt
H
b
COOEt
ca. 6% ee
"<Ph
Scheme 28. The first asymmetric reaction catalyzed by a soluhle h n s i t i o i i metal
complex [71 a]
509
REVIEWS
A. Togni and L. M. Venanzi
+ ’;w
- dcoon
ligands propanation. This problem
has not been completely solved as
yet. even by using catalysts bearing
+N,CHCOOR
PhNHNH,
C,-symmetric semicorrinato ligands.
first reported by Pfdtz et a[. in
94% ee
(93: 7)
46% ee
1986.[741or by employing bis(dihydrooxazole) ligands, described in
more recent years (Scheme 31).
R=
Me
Enantioselectivities up to 97 % are
obtained in thc cyclopropanation of
styrene with methyl diazoacetate
C , ~ C ~ C O O E ~
OOH
(S)-cat.
(Scheme31). A slight effect of
+ NZCHCOOEt
__t
double stereodifferentiation is obPhNHNH,
served by using the two enantiomeric
menthyl esters. Selectivity does not
85% cis, 91 % ee
depend upon the method of generation of the catalyst. Thus. either the
COOEt
(R)-cat.
bis(semicorrinato)Cu” complex as
+NzCHCOOEt
catalyst precursor, or the catalyst
PhNHNH,
generated in situ from equivalent
92 % ee
amounts of the ligand and [CuOrBu],
give identical stereochemical results.
More recently the Pfaltz group
(R)-cat. =
showed that very high enantioselectivities can be obtained by using the
conceptually similar (from a stereochemical and electronic point of
view), but synthetically more easily
accessible
bis(dihydrooxazole)[6”1
Scheme 29. Industrially important applications o f the Cu-catalyLed asymmetric cyclopropanation. Synthesis of
and 5 - a z a s e m i c o r r i n ~ . Both
[ ~ ~ ~ the
chrysanthemic acid esters ( R = u-menthyl). perinethrinic acid. and cilastatin [72]. Bottom left’ the catalyst precursor
Evans
and
Masamune
groups
have
in its R conliguration.
developed analogous systems.[76,771
Representative bis(dihydrooxazo1e)
contains a Schiff base ligand derived from salicylaldehyde and
ligands are shown in Scheme 32. With these ligands extremely
optically pure I-phenylethylamine and served as catalyst prehigh optical yields (up to 99% ee) can be obtained in the reaccursor for the “decomposition of diazoacetates in the presence
tion of styrene with ethyl d i a ~ o a c e t a t e . ~However,
’~]
high frun.7,’
of ole fin^".["^ The stereoselectivities were rather modest, but
cis ratios of the diastereomeric cyclopropanes are achieved only
intensive screening and improvement of the ligand system by the
when the diazo ester bears a very bulky group (e.g., 2,6 di-revibutyl-4-tolyl). The catalysts are best generated in situ from solAratani group finally led to the commercialization of the copuble Cu’ salts containing weakly coordinating anions (triflate,
per-catalyzed ~ y c l o p r o p a n a t i o n Industrially
.~~~~
important applications of this reaction are the synthesis of intermediates of
CIO;); [CuOtBu], seems to give erratic results.r751Evans et al.
have shown that the catalyst formed from the CMe,-bridged
pyrethroid insecticides and cilastatin. Some relevant examples
bis(dihydrooxazo1e) of Scheme 32 and CuOTf is polymeric in
with ee’s higher than 90% are summarized in Scheme 29. The
the solid state.[’”] An interesting helical structure results from
best ligands are terdentate dihydroxyl Schiff bases with two
the nearly linear coordination of two oxazole nitrogen atoms to
hydroxyl groups (O,N,O) derived from amino alcohols bearing
bulky orrho-alkoxyphenyl substituents. The dimeric, p-0-Cu”
complexes formed by such ligands are used as catalyst precursors. The catalytically active Cu’ species is generated in situ with
phenylhydrazine as a reducing agent.
Dauben et al. have shown the Aratani catalyst to be effective
44 - 77% ee
in the intramolecular version of the cyclopropanation reactions
too, although the resulting enantioselectivities were only moder(Scheme 30).
ate (up to 77%
Whereas nowadays high enantioselectivities can be obtained
with several catalysts (see Sections 5.5.2-5.5.4), relatively mod22 - 34 % ee
est diastereoselectivities (cis/fvunscyclopropane derivatives) still
constitute the major stereochemical problem of catalytic cyclopropanation. This problem has not been completely
as
Scheme 30. Intramolecular cyclopropanation of diazoketones catalyzed by
Ardtani’s complex (see Scheme 29) [73].
yet, even by Using catalysts bearing C,-SymmetriC SemiCOrrinato
R
(R)-cat.
‘13“7=(
-
1
9
__t
-
510
.4njiiw’. C ’ i w i i . hi.Ed.
tnyi. 1994. 33, 497 526
~
Nitrogen Donors
REVIEWS
alyst. but the reported enantioselectivities are
modest to moderate (up to 6 0 % w ) .
1 mol % cat.
+
fiph
NZCHCOOR
CICH,CH,CI
/ ca. 20°C
Ph
97%ee
(82:18)
5.5.2. Cobalt Catalysts
Parallel to the development of efficient
copper catalysts by Aratani et al., the Nakamura and Otsuka groups reported in the seventies the use of Co" complexes of type
6 bearing a-camphorquinonedioximato lig-
R'
R'
R'
CN
Scheme 31. Pl'xlt/'\ sernicorrinato hpinda in the Cu-catalyzed cyclopropanation
R' = CMr,OH.
I751
I761
each Cu' center. in which the ligands act as bridging units. The
triflate anions are not coordinated.
The CMe?-bridged bis(dihydrooxazo1e)s and the 5-azasemicorrins behave strictly as neutral ligands, whereas their
methylene-bridged counterparts, like the semicorrins. can be
dcprotonated, thus forming anionic ligands. This is an interesting feature which shows the flexibility of the copper catalysts,
because all classical ligands are anionic.
Finally. a C',-symmetric optically active ligand containing
stereogenic pyrazolyl units, recently described by Tolman
etal.,L78]was found to bind in a terdentate manner to Cu' to
form complcxes like 5. This complex is a cyclopropanation cat-
0
and^.['^]
These catalysts are somewhat unusual in
that they show good activity only for the conversion of conjugated olefins. Enantiomeric
1741. R = D-menthyl,
excesses up to 88 YOwere obtained in the reaction of styrene with neopentyl diazoacetata.
Despite the relatively high selectivity reported, no further development of these catalysts has appeared in the literature.
5.5.3. Rhodium Catalysts
Scheme 32. The novel ineth~lenehis(dihydroox~i~ole)
and 5-ara-sernicorrin ligands
used i n the C'dcatalyzed asymmetric cyclopropanarion.
r
9 5 % ~
7
The prototype of a catalyst for carbenoid reactions of diazocarbonyl compounds is [Rh,(OAc),] . Not only cyclopropanations, but also insertions into C-H and N - H bonds are effectively catalyzed by a number of tetracarboxylato dirhodium
complexes.[801Attempts to achieve stereoselectivity by using optically active amino acids in such complexes were not very successful.[811Doyle et al. used carboxamido complexes of the type
shown in Scheme 33[821after it was reported that acetamide can
displace acetate in [Rh,(OAc),] .[831 This discovery was the underlying idea for the development of. in part, highly enantioselective Rh" cyclopropanation catalysts, containing optically active, bidentate N,O ligands of this type. The most efficient of
them appears to be methyl 2-pyrrolidone-5-carbc~xylate ( 5 HMEPY). Its Rh" complex gives up to 86% ee in the cpclopropanation of styrene with menthyl diazoacetate,["j"] and
up to 94% C J ~in the intramolecular reaction of allylic diazoacetates.[84b.'] Furthermore. [Rh,(S-MEPY),] is a very effective catalyst for the cyclopropenation of alkynesCx5' (see
Scheme 33).
5.5.4. Miscellaneous
L
CH3
A procedure that does not employ diazo reagents for the
synthesis of cyclopropane derivatives from olefins is the
Simmons -Smith cyclopropanation.'861The carbene is typically
generated from CH,I, and mediated by a Zn-Cu couple.
Kobayashi et al. recently reported the first catalytic, enantioselective Simmons-Smith cyclopropanation of disubstituted allylic alcohols by a ZnEt,/CH21, system. Chiral disulfonamide
A. Togni and L. M. Venanzi
REVIEWS
0
0
NzCHCOOEt
c
- 40°C
94% ee
-/
Fi
-N,
&e
98 YOee
n
CH3
H+
+olefin
COOEt
cyclopropane
kh(l7P)
Scheme 35. Identification of intermediates in the addition of ethyl diazoacetate to
a Rh-porphyrin cyclopropanation catalyst [89]. TTP = tetra-p-tolylporphyrin. i.e.
R = p-tolyl.
U
CH3
5.6. Asymmetric Diels- Alder Reactions
+
[Rh,I(5S)-MEPYJ41
Scheme 33. Asymmetric intramolecular cyclopropanation of an allylic diaioacetace
[84b. c] and cyclopropenation of an acetylene ( R = 11-menthyl) [SS] catalyzed by a
carboxamidato- Rh" complex.
ligands were used as catalysts, and optical yields up to 82 YOwere
obtained.[*'] An example is given in Scheme 34.
L
82 YOee
(y
NHSO,( p-NO,-C,H,)
L =
'**a,
NHSO,( p-NO,-C,H,)
Scheme 34. The first example of asymmetric Simmons-Smith cyclopropanation
with a catalyst containing a chiral disulfonamide ligand 1871.
For the vast majority of the known cylopropanation reactions
no reliable experimental details on the mechanism are available.
The formation of carbene complexes is obviously the most probable route. but such complexes have never been observed in
catalytic systems. The formation of the cyclopropane ring can
occur either by a concerted addition of the olefin, or via a metallacyclobutane intermediate.[**^ A recent spectroscopic study
addressed these questions for the iodo(tetra-p-toly1porphyrin)rhodium system.[s91 The 1 -diazoalkyl complex resulting from
the addition of ethyl diazoacetate to the catalyst could be detected at low temperatures (Scheme 35). At temperatures higher
than - 20 "C this complex decomposed in the presence of olefins
to afford cyclopropanes. If no olefins were added, the corresponding 1 -iodoalkyl complex formed. Both reactions are taken
as indicative of the generation of a highly reactive carbene intermediate.
512
Chiral Lewis acids effectively catalyze [4 21 cycloaddition
reactions of various dienes and dien~philes.~"~
In recent years
several boron and aluminum catalysts bearing optically active
nitrogen ligands have been shown to impart very high enantioselectivities to Diels -Alder reactions. Whereas the vast majority
of the successful catalysts contain chelating oxygen ligands, recent developments have shown that optically active nitrogenligands can give superior results. In particular, with the nitrogen-ligands, fine-tuning the electronic and steric properties
seems to be much more flexible. Corey et al. have demonstrated
that the diazaaluminolidine represented in Scheme 36 is an effective catalyst for the cycloaddition of cyclopentadiene derivatives to activated dienophiles.["] The specific product shown in
the example was used as an intermediate for the synthesis of
prostaglandins. The ligand is a C,-symmetric bis(su1fonamide).
derived from 1.2-diamino-I ,2-diphenylethane. The catalyst,
which is easily formed in situ from the bis(su1fonamide) and, for
example, AlMe,, displays an interesting dimeric structure in the
solid state (Scheme 36, bottom right).['''] One of the two sulfonyl groups of each monomeric unit acts as a bridge through
one oxygen atom, to give a tetrahedral coordination environment at the aluminum centers.
An oxazaborolidine derived from tryptophane has recently
been shown by Corey et al. to impart extremely high enantioselectivity ( > 99 O/O re according to NMR) to the cycloaddition of
2-bromoacrolein and cyclopentadiene (see Scheme 37) .[921 The
high stereoselectivity is ascribed to an attractive interaction between the x-acidic dienophile and the x-basic indole moiety in
the transition state, which leads to a highly preferential exposure
of one enantioface of the dienophile to the attack by the diene.
Such an attractive interaction is similar in concept to secondary
interactions between ligands and substrates, which were postulated, for example. for several reactions catalyzed by functionalized ferrocenylphosphane complexes.[931 Very similar oxazaborolidines, also derived from amino acids but not able to
display such crucial interactions, were found to afford lower
optical yields (up to 8 6 % c ~ e ) . [ ~ ~ ]
AnEnL.. Chrm. Int. Ed. EngI. 1994. 33. 497--526
REVIEWS
Nitrogen Donors
"""3
-
Bis(dihydrooxazo1e) complexes of Fe"', Mg",
and Cu" have also been successfully employed as
catalysts for the Diels- Alder
The
10 rnol % cat.
exact nature of the iron catalyst, which is gener--prostaglandins
4ated in situ from the ligand L1 in Scheme 38 and
- 7 a ~
FeCl,, FeI,. o r Fe powder and I,. is rather obscure. This catalyst affords enantiomeric exU
cesses of up to 86% in the Diels-Alder conden94% ee
sation illustrated in Scheme 38. It is also one of
the still rare examples of non-d" transition metal
F3c\s.p
Ph
F3CSP
Lewis acids used as catalysts in asymmetric cy..'
0 4 \
cloaddition reactions."61 Slightly higher optical
H3C\ Al
cat. =
Ph
yields (90 YOee) were reported for the same reacN
&
C
N
'
'
C
H
/ bN\ Ph
F,CO,SHN\~~/
SOpCF3
tion when MgI, and the more bulky ligand L2
SO2CF3
\
formed the catalyst, whereas enantiomeric exI
OSS,
CH3
cesses of exceeding 98% were obtained in the
CF3
catalyst in
with
Of the
Scheme .36. At-catalyred asymmetric D i d - Alder reaction of a functionalired cyclopentadiene and
(bottom right) ii schematic representation o f t h e dimeric structure of the catalyst in the sol~dstate 1911.
ligand L3, which demonstrates that the choice
of metal and ligand plays a very important
5 mol % cat.
In all systems, cationic or dicationic complexes are assumed to be the catalytically active
species.
Bno%:Ao
JNA
uo
p'h
'
'--$
p",\d
20
96% ex0 (CHO)
> 99% ee
5.7. Asymmetric Aldol Condensations
ti
(pyo
0
H '0
ONB/
N4
\
\
nBu
S02T0l
nbu
The addition of silyl enol ethers to carbonyl compounds. the
Mukaiyama aldol reaction,["l is promoted by aeveral Lewis
acids. Studies by Mukaiyama et al. have in recent years shown
that the combination of Sn" triflate and a chiral diamine impart
very high stereoselectivities to this reaction.ry81This system is
commonly best used in a stoichiometric fashion with addition of
tributyltin fluoride or dibutyltin diacetate as promoters. However, a judicious choice of solvent and reaction conditions turns
this reaction into a catalytic process, albeit with modest
b%
b
CHzCI, / Me2CHN02
0
- 50°C
10rnol%Mgl,/I,/L2
+
CHzCIz /-7a0c
10 rnol% cu(oso,cF,),
99 YOendo
90% ee
97 YOendo
/ L3
b
> 98 YOee
Ph
L'
L2
L3
Scheme 38. Asymmetric Diels-Alder
reactions catalyzed by Fe"' and Mg"
halide as well as by Cu" triflate complexes containing bis(dihydrooxa7ole)
ligands [95].
A. Togni and L. M. Venanzi
REVIEWS
perature affords the aldol products in good yields and high
stereoselectivities. The condensation of cyclohexylcarbaldehyde
with (Z)-t-trimethylsilyloxy-I-(ethy1thio)propeneis shown in
Scheme 39. The .sjx-configurated aldol product is exclusively
formed with an optical yield that is higher than 98% ee.
Future developments of this important reaction will probably
focus on the improvement of the catalytic activity of the already
successful systems described above.
5.8. Asymmetric Alkylation of Aldehydes
20 mol
PSiMe3SdOTfl
EtCN / -78°C
100 YOsyn, > 98 YOee
L=
Scheme 39. Aaymmcti-ic Mukaiyaina :ildol condensation cat'ilyred by
plex containing ii chiral chclatins didiiiine [%].
:I
Sn" coin-
More recently. the Kiyookai""] and Masamunei'OO1groups
have shown that chiral oxazaborolidines similar to those described in Section 5.6 (Diels-Alder reactions) are very effective
catalysts for the Mukaiyama reaction. Optical yields up to
9 9 % C P were reported. The catalysts are formed in situ from
BH, . THF and an arenesulfonamide of an .*-amino acid. Two
examples are illustrated in Scheme 40. As was the case for the
Sn-catalyzed reactions. the choice of solvent and reaction conditions plays a crucial role; best results are obtained i n nitroethane, or propionitrile. and by slow addition of the aldehyde
compound to the reaction mixture.
PhCHO
The enantioselective nucleophilic addition of dialkylzinc
reagents to aldehydes. in the presence of catalytic amounts of a
chiral ligand (typically an amino alcohol), has been extensively
studied by several groups. Excellent review articles on this topic
have appeared in recent years.['0t1
Oguni and Omi first showed that the reaction of ZnEt, with
benzaldehyde is effectively catalyzed by chiral 2-amino-1-alcohols derived from simple amino acids.["'] Since then, this reaction has served as a standard test reaction in the development of
new catalysts. One of the best understood systems has been
described by Noyori et al.i1031
The main conclusions from that
study will be briefly discussed here. as they can be applied to
other systems. There. 3-exo-(dimethylaminojisoborneol(DAIBj
is used as catalyst in 1 -5 mol YO(to aldehyde). A virtually quantitative yield of the secondary alcohol is obtained if one equivalent of the ZnEt, reagent is employed (Scheme 41). Only one
1) 2 mol % (-)-DAB
PhCHO
+ ZnEt,
2)H20
v
Kt
Ph
98% ee
(-)-DAIB =
+
A
/
96 % ee
[ ( (-)-DAIB)ZnR],
[ { (-)-DAIB ) { (+)- DAIB } Zn 2RZ]
Scheme 41. The DAIB-ciitalyied asymmetric addition of ZiiEt, to benzaldehyde
and the t w o diastereomeric dimers [(DAIB)ZnR],, involved in chiral amplification
(cheracterued b) X-ray crystallography for R = Me). The less stable and inore
reactive homochiral form. m d the more stable and leas reactive mcvo form [103].
OSiMe,
PhCHO
+
4
OPh
-
20 mol % cat.
EtCN /-78"C
I
L C O O P h
Ph
93 Yo ee
cat. =
Scheme 40. Asymmetric Mukaiyamd aldol condeiiwtionr catdlyied by chiral oxazaborolidinones derivcd from ,b-sulfonqIainino acids 199. 1001
514
ethyl group i s transferred to the electrophile, and the actual
alkylatioii takes place via a dinuclear zinc species as illustrated
in Scheme 42.
One of the most interesting features of this reaction are the
nonlinear effects in the enantioselectivity. Thus, maximum
enantioface discriminations (95-98 O/O ee) in the ethylation of
benzaldehyde are still obtained when scalemic mixtures of
DAIB. for instance, in an optical purity as 10% as 15%. are used.
This has been explained by the formation of two diastereomeric
dimeric complexes (both characterized by X-ray diffraction),
one of them a hoinochiral and the other a centrosymmetric mexi
form (see Scheme 41). The former is both thermodynamically
and kinetically less stable and therefore exhibits much higher
turnover frequencies than the achiral counterpart. The possibilAngcmi~.Clirin. Inr E d En,?/. 1991. 33. 497 52h
REVIEWS
Nitrogen Donors
In recent years efforts have been made to extend the range of
zinc reagents from simple to functionalized alkyls.[llO]Progress
in this area is exemplified by the work of Knochel et al.[lloaland
Oppolzer et al.[llob-dl For example, alkenylzinc reagents are
generated in situ by transmetalation from ZnR, (R = Me, Et)
and a vinylborane, the latter being obtained by hydroboration
of the corresponding terminal alkyne. The addition of such nucleophiles is catalyzed, for example, by DAIB, which gives excellent enantioselectivities. This methodology was very recently
applied, in an intramolecular variation, to the total synthesis of
(R)-muscone[' lod1 (Scheme 44). Finally, dialkynylzinc and
alkynylbromozinc reagents can also be used in this type of reaction (catalyzed by, for instance, ephedrine derivatives) and give
moderate to good stereoselectivities.[' I
ZnEt2
___)
PhCHO
OH
Scheme 42. The mechanism of the DAIB-catalyzed addition of ZnEt, to benadldehyde [lo?]
ity of obtaining high enantioselectivities despite the relatively
low optical purity of the inducing ligand has been named asymmetric ampLification, and was first observed by Kagan
et al.,[104dlas well as by other groups more recently.1104"
Several other N,O-ligands catalyze the addition of dialkylzinc
to aldehydes in high optical yields. Scheme 43 illustrates the
structural variety of such ligands.[loS-
-. .
93% ee [lo51
97% ee [104a,b]
97% ee [lo61
95 % ee [lo71
1) HBCy, / 0°C
H
CHO 2) ZnEt,, 1 rnol % (+)-DAM
=
=(R)-muscone
4
1
1
2
3) NH,CI (as)
0
Scheme 44. Oppolzer's synthesis of muscone by the intramolecular reaction of the
alkenylzinc reagent (shown at the bottom) that arises in situ from ZnEt, and the
hydroborated 01-pentadecindl [IIOd].
Scattered reports have appeared on the use of boron and
titanium catalysts containing nitrogen-ligands. Thus, the
oxazaborolidine, formed in situ from (-)-ephedrine and H,B .
S(CH,), ,catalyzes the standard formation of I-phenylpropanol
with an enantiomeric excess of 9 5 Y 0 . [ " ~ ]The same reaction
is very efficiently catalyzed by the bistrifluoromethanesulfonamide of trans-I ,2-diaminocyclohexane in the presence of
equivalent amounts of titanium tetraisopropoxide, and affords
the product in 98% ee.[1131The actual catalyst is considered to
be the ethyltitanium complex shown in Scheme 43.
9 6 % e e [I081
5.9. Asymmetric Conjugate Addition Reactions
99 YOee [104c]
95%ee [112]
99 % ee [log]
98%ee [113]
Scheme 43 4 selection of ligands and complexes used a s catalysts in the addition of
ZnEt, to benzaldehyde (the ee values of the product I-phenylpropanol are indicated)
A n g w Clirnr Inr. Ed. Engl 1994. 33, 497-526
The conjugate addition reaction (this term includes any nucleophilic 1,Caddition to an a$-unsaturated system) is an extremely important transformation which finds wide application
in organic synthesis."
The number of recent publications
dealing with the asymmetric version of this reaction testifies to
the worldwide interest.["51 However, asymmetric catalysis still
plays a minor role in conjugate addition, since most studies have
employed chiral ligands (or complexes) in stoichiometric
amounts.["61 One of the first studies, carried out by Brunner
et at.,'"'] was the cobalt-catalyzed 1,4 addition of methyl 10x0-2-indanate to vinyl methyl ketone. By using 1,2-diphenyl1,2-ethanediamine as the chiral ligand, optical yields up to 66 %
ee were obtained (Scheme45). The catalyst precursor is the
octahedral Co" complex [Co(acac),(diamine)] , formed in situ
from [Co(acac),]. The same reaction is also catalyzed by a
515
*
REVIEWS
75
COOCH3 +
0
0
0
Ph
CH3
6 6 % e e (R)
-+
(S)
69%ee
5.10. Asymmetric Grignard Coupling
Scheme 45. Asymmetric Michael addition catalyzed by Co" [117]
and Cu" complexes [I 181.
dimeric Cu" complex bearing a tetradentate Schiff base as ligand) (Scheme 45). A slightly higher enantioselectivity (69 % ee)
was observed by Desimoni et al. with this complex.r1181
The use of [Ni(acac),] as a catalyst precursor in the addition
of diethylzinc to chalcone in the presence of various chelating,
optically active N,O-ligands has been reported by Soai et al.,['
Bolm et a1.,[1201and Feringa et al.['zll Simple ephedrine derivatives, novel pyridyl (or 2,2'-bipyridyl) alcohols. o r the wellknown DAIB afforded enantiomeric excesses of up to 9 0 % .
a
Ph
ZnEtp
[Ni(acac),] / L
(1-25mol%)
cat.
J,
Ph
M",
The use of nitrogen-containing ligands in the Ni- and Pd-catalyzed coupling of secondary Grignard reagents with alkenyl
halides has been pioneered by Hayashi and K ~ m a d a . [ ' The
~~]
most successful ligands used for this reaction are chelating P,Nsystems, derived either from amino
or from chiral
ferrocenyl systems.[1251The best characterized catalysts are the
NiCI,, o r PdCI, complexes with ppfa ((S*)-N,N-dimethyl-I[(R*)-2-diphenylphosphino)ferrocenyl]ethylamine).Enantiomeric excesses exceeding 90% were obtained in the reaction of [x(trimethylsilyl)benzyl]magnesium bromide with vinyl bromide,
which afforded an optically active allylsilane (Scheme 48). It
\
*
Ph
'hMi3r
Ph
+
f i ~ ~
L
____c
h
Me
+ 2,2'-bipyridine
90 % ee ( R )
~191
OH
Bu~N
r
Me,Si
Ph
A.
L=
Relevant examples are shown in Scheme 46. An interesting aspect of this reaction is the beneficial effect that the presence of
a second, achiral nitrogen ligand has on the enantioselectivity.
Thus, a variety of chelating diamines such as 2.2'bipyridine.
1,lo-phenanthroline, and 2,2'-biquinoline were found to improve selectivity significantly.[' 19, '' ' 1
Finally, Lippard et al. have shown that chiral aminotropone
imidates are effective ligands in the Cu-catalyzed conjugate addition of Grignard reagents to cylohexenone.['221This virtually
unique ligand system afforded enantiomeric excesses as high as
74 YO(Scheme 47).
10 mol % (CCI, /-2O"C)
ca. 5 rnol % (PhMe /-5O"C)
+
A. Togni and L. M. Venanzi
(30rnol%)
L=
Ph
86 % ee (R)
11201
M =Mg
.-t
95%
IX
(R)
M = Z n + 6l%ee(S)
OH
L=
(-)-DAB
+
85 YOee ( R )
2,2'-bipyridine
v211
Scheme 46. Enantioselective 1.4 addition of ZnEt, to chalcone catalyzed by Nil' in
the presence of chelating N,O-ligands [119- 1211.
Me2N
PPh,
(S)-valphos
0
1) 3-5 mol % CuBvSMe, /L
PhMe I HMPA f -78°C
0
+
0
Ph(tBu),SiCI
nBuMgCl
2)NH,CI (as)
Scheme 47. A chiral N.N'-dialkylaminotropone imidate ligand in the CU'-CdtdlyZed
1.4 addition of Grignard reagents to cyclohexenone [I221
516
Scheme 48. Synthesis of 0 , p n s a t u r a t e d allylsilanes by asymmetric Pd-catalyzed
Grignard coupling. Examples of catalysts with ferrocenyl [125], (@-arene)Cr [126],
and amino acid derived P.N-Iigdnds [124], and the postulated transition-state
assembly.
has been proposed that the dimethylamino group on the ligands fulfills an important role in the transition state of the
reaction. Its dissociation from palladium (or nickel) and temporary coordination to magnesium enables one of the enantiomers
ofthe Grignard reagent to be selected, which leads to the formation of the preferred diastereonieric transition-state assembly, as
illustrated in Scheme 48.112sc1Such an assembly immediately
.4ngiJ.ii-.C'heni. Ini. Ed. EngI. 1994. 33.
497-526
precedes the key transmetalation step of the alkyl fragment from
magnesium to palladium (or nickel). However, this model posp
tulates a coordinatively unsaturated palladium (or nickel) cen- Phd
ter, which seems rather unlikely.
Of the two elements of chirality contained in ferrocenyl ligands of the ppfa type, the planar chirality due to the 1,2-disubstitution of the ferrocene moiety plays the dominant role in
determining stereoselectivity in Grignard cross-coupling reactions." 2 5 c 1 Hayashi and Uemura recently reported the synthesis
of a chromium complex containing an analogous P,N-ligand
system in which the planar-chirality is part of a iT6-coordinated
cat.
arene." "'I The novel ligand provided enantioselectivities up to
61 YOec' (Scheme 48).
-
REVIEWS
Nitrogen Donors
H,C(COOMe),/
CH(COOMe)2
base
c
h
cat.
(or (Pd(q3-C,H,)CI], / L)
-
75% ee(R)
[1301
Me
Ph
Ph
L
-
83 % ee (S)
[I331
5.1 1. Asymmetric Allylic Alkylation
-
The allylic alkylation reaction consists of the substitution of
a suitable leaving group in allylic position by a carbon nucleoL + 99% ee (S)
L
95%&(R)
phile." l7I This transformation is typically catalyzed by phos[132a]
[751
phane --Pd complexes, and high degrees of enantioselection
have been obtained by using a variety of chiral chelating diphosphanes." 281 Reports on the use of optically active nitrogen-ligands did not appear until 1990 and are still very rare.L1291
In fact,
it is commonly accepted that Pd(o) species that are stabilized by
L + 77% ee (R)
L 4 81%ee(S)
phosphane ligands are involved in the catalytic
It is
1631
(132cl
also well-known that nitrogen-donors are generally rather poor
Scheme 49. Asymmetric allylic alkylation catalyzed by Pd coinplcxes containing
ligands for the stabilization of low oxidation states of late tranchelating nitrogen-ligands.
sition metals. A combination of these factors may have generated a prejudice against nitrogen-ligands as alternatives to phosA novel asymmetric bis(pyrazoly1)methane ligand derived
phorus-ligands.
from menthone was recently prepared in these laboratories.[' 3 3 1
One of us has recently shown that the alkaloid (-)-sparteine
Good enantioselectivities (up to 83% re) were obtained in
is an effective inducing ligand for aymmetric allylic alkylathe standard alkylation reaction of 1.3-diphenylallylacetate
tion." 3 0 . ' ' I Optical yields of up to 85 9'0 ee were obtained in the
(Scheme 49). It was found that one equivalent of chloride, for
alkylation of simple 1,3-disubstituted allylic acetates with the
example, present in solution when the catalyst was prepared in
anion of dimethyl malonate as a nucleophile. The Pd" complex
situ from the binuclear complex [Pd,(q3-C3H,),C1,] and the
[Pd(g3-C,H5)(sparteine)]PF, was used as catalyst precursor
ligand, had a beneficial effect on stereoselectivity. A halide effect
(Scheme 49). The corresponding g3-cyclohexenyl and q3-I ,I ,3of this type, which possibly also plays an important role in the
triphenylallyl complexes, postulated as intermediates in the catsystems of Pfaltz et al., is not yet understood, and holds out the
alytic alkylation of cyclohexenyl acetate and 1,1,3-triphenylallylacetate, respectively, were prepared and characterized by
hope that future mechanistic studies may lead to further imX-ray crystallography.'t30b1The catalytic activity of such sparprovements of this "old" reaction.
teine-Pd complexes was qualitatively estimated to be one to
two orders of magnitude lower than that of phosphane-Pd
systems['30a1(this rule of thumb seems to apply to other nitro5.12. Asymmetric Oxidation Reactions
gen-containing systems; see below).
Highly enantioselective catalysts containing bis(dihydrooxaBefore the advent of the Sharpless epoxidation of allylic alcozole), or 5-azasemicorrin ligands were used by Pfaltz et aI.1". "1
hols with rut-butylhydroperoxide (TBHP) catalyzed by titanium tartrate systems,11341several reports of molybdenum'' 351
Under conditions where the nucleophile is generated in situ
from dimethylmalonate, N,O-bis(trimethylsilyl)acetamide, and
and vanadium[1361catalysts had appeared. These systems contain chiral auxiliaries such as ephedrine and proline derivatives.
a cocatalytic amount of KOAc in CH,Cl,, alkylation products
The cases where the nitrogen-donors interact directly with the
were obtained in very high optical yields (up to 9 5 % ee). Recently Pfaltz and von
and Helmchen and S p r i n ~ , [ ' ~ ~ ~metal
]
are illustrated in Scheme 50. Modest to good enantioindependently. developed (phosphinoary1)dihydrooxazoles as
selectivities were obtained (up to 50% ee). It should be noted
P,N-ligands, which were shown to give more active and highly
that the dioxomolybdenum complexes shown there can give rise
selective catalysts in this reaction. Thienyl dihydrooxazoline ligto diastereomeric mixtures, because the metal atom becomes a
ands were used by Frost and
but were inferior,
stereogenic center.
both in terms of activity and selectivity, when compared to the
After the spectacular success of the Sharpless epoxidation
reaction, progress in this field followed very slowly, and effecabovementioned P,N-systems (Scheme 49).
Angru, Clioii Inr.
Ed. Etigl. 1994. 33, 491-526
517
doH
I
R
R
-
5 mot % cat.
H3C
CH3
3yc3d1
Phx o - O H
0
Me
0* M
II O0 : -,~ ~
cat. p h p , M e
0
Me
Me
Et
Me
R = P h -c 3 3 % e e
disubstituted and cyclic olefins. Thus, enantioface discriminations of up to 98 Yoee were obtained with chromene derivatives
as substrates.['
Mukaiyama et al. recently demonstrated
that chiral salen -manganese complexes are also suitable catalysts for the epoxidation of nonfunctionalized olefins with
molecular ~ x y g e n . " ~ "Since the system additionally requires
pivalaldehyde as reductant, the actual oxidant is possibly a hydroperoxide (peroxopivalic acid). These findings show the versatility of the manganese catalysts and their potential for future
developments and applications.
Moderate enantioselectivities (up to 62% ee) were recently
reported by Thornton et al. for the oxidation of silyl enol ethers
to 2-hydroxy ketones with PhIO, catalyzed by another salenmanganese ~ o m p l e x . ~Finally,
' ~ ~ ] efforts to develop chirally
modified porphyrin ligands are worth mentioning. The groups
of Groves,r1431K ~ d a d e k , " ~M~ ]a n ~ u y , ~ and
' ~ ~ Collman"
]
461
prepared peripherally functionalized porphyrins bearing stereogenic binaphthyl, amino acid, o r threitol units, as illustrated in
Scheme 52. Fe"' as well as Mn"' complexes of porphyrins of the
above type were shown to be efficient catalysts for the epoxidation of simple olefins in combination with either NaOCl or
PhIO as oxidants, and afforded eiiantioselectivities in the range
of 15-88 % re. The strategy common to the different approaches is apparent: Introduction of stereogenic substituents onto the
porphyrin moiety, either with bulky and conformationally rigid
groups or by bridging the meso positions, generates a "chiral
pocket" around the metal. The results obtained so far are encouraging and show scope for further developments.
R = M e --c 50%ee
Scheme 50. Early example of asymmetric epoxidation of an allylic alcohol with
cumene hydroperoxide catalyzed by Mo" complexes [135]
tive chiral catalysts capable of epoxidizing nonfunctionalized
olefins have been discovered only in recent years.
5.12.1. Asymmetric Expoxidation of Simple Olefins
Kochi et al. have shown that achiral Mn'I'and Cr"' complexes
containing salen type ligands (salen = N,N'-bis(salicy1idene)ethylenediamine) are active catalysts for the epoxidation of nonfunctionalized olefins with. for example, iodosylbenzene as oxidant.[1371In the case of chromium, an oxochromium(v) species
is an intermediate in the catalytic cycle. An analogous oxomanganese(v) could not be isolated and characterized, but sound
indirect evidence for its existence has been r e p ~ r t e d . ~ ' ~The
'"~
formation of such 0x0 species implies the successive oxygen
5.12.2. Osmium-Catalyzed Dihydvoxylations
atom transfer from the oxidant to the metal and hence to the
The development of the asymmetric 0s-catalyzed dihydroxysubstrate, the "oxygen rebound"." 381 This mechanism is fundalation of olefins by Sharpless et a]. is recognized to be one of the
mentally different from that operating with Sharpless's titanium
most significant advances in homogeneous catalysis in recent
catalysts.
Following up Kochi's pioneering work, Jacobsen et al.113y1 years. Scope, applications, and mechanistic aspects of this reaction have been studied in detail and excellent reviews have apand, independently, Katsuki et al.11401prepared chiral derivatives of the [Mn(salen)]+ complexes and,
using oxidants such as PhIO, or more eleNaOCl (aq)
gantly, aqueous NaOCl (commercial
5 rnol YOcat. 1
bleach) under phase-transfer conditions,
Ph
Me
achieved the catalytic epoxidation of simple
CHzCIz
Ph
92 YOee
olefins with enantiomeric excesses exceeding 90%. Some examples are shown in
Scheme 51. Chirality is introduced in theessentially planar salen-manganese system
QOH
\
by replacing the ethylenediamine backbone
I
H4'
by optically active 1,2-diphenylethylene
V
8
3
%
ee
diamine or trans-I ,2-diaminocylohexane.
Furthermore, it is crucial that the ligand
Ph
Ph
bears bulky substituents adjacent to the
cat.2
phenoxy oxygen atoms. Whereas Jacobsen
et a]. showed that a tert-butyl group is sufficient to achieve high selectivities in the
phase-transfer process (using NaOCl as oxidant), Katsuki et al. obtained better results
(with PhTO as oxidant) by introducing a
Et
Ph
stereogenic 1-phenylprop-I -yl group. These
systems seem to be particularly well suited
Scheme 51. Asymmetric epoxidation of unfunctionalized olefins catalyzed by Mn"l-salen complexes [139.
1401.
for the epoxidation of Z-configurated. 1,2-
AM^
m
QJ-J.0-
518
A n g e s . C'hem I n ! . Ed. Engl. 1994, 33, 491- 526
REVIRNS
Nitrogen Donors
peared.[l4'I Therefore, just the relevant
aspects related to the use of chiral nitrogen ligands will be highlighted.
It has been known for decades that
R
-N/ e
I N' R
,
pyridines or tertiary amines accelerate the
$
/ /
: addition of OsO, to olefins.['481Sharpless
et al. introduced cinchona alkaloids as inR'
\ \
ducing agents in the stoichiometric osmyR = R' [144]
In succeeding
lation of olefins in 1980.[1491
years several authors reported the use of
chelating diamines as chiral auxiliares in
this reaction.r15o1Some are shown in
Scheme 53. Enantiomeric excesses approaching 100% were obtained, but none
0
of these diamine systems could be turned
into a catalytic process. The fundamental
breakthrough came in 1988 when Sharpless reported the catalytic version of the
dihydroxylation with the OsO,/cinchona
system and N-methylmorpholine-N-oxide
-R-R'- [I451
as
a c ~ o x i d a n t . [ ' ~During
'~
the following
-R-R'- 11431
years, they further developed and optiScheme 52. Partial structures of some chiral porphyrins that in Fe"'comp1exes are used as catalysts in asymmetric
mized the process.[' 5 2 1 In particular, the
tion of simple olefins 1143-1451.
use of K,[Fe(CN),] as a co-oxidant in a
biphasic system and a broad screening of cinchona alkaloid
derivatives impressively extended the scope of the catalytic reaction. Thus, for a wide variety of olefins, excellent enantioselectivities were obtained (>95 OO/ ee). The new class of phthalazine
ligands, (DHQD),-PHAL and (DHQ),-PHAL, incorporating
two alkaloid units into the same molecule, are derivatives of
dihydroquinidine (DHQD) and dihydroquinine (DHQ) . As
D H Q D and D H Q are a pseudoenantiomeric pair, both optical
[150c.d]
isomers of the dihydroxylation products are accessible in remarkably similar stereoselectivities."
A very important aspect of cinchona alkaloid derivatives
(with respect to catalysis) is that they behave as monodentate
ligands, that is, only the quinuclidine nitrogen atom coordinates
to
the metal. The complexes characterized by X-ray diffraction
(1m g l
are shown in Scheme 54 (top left).['531
The dioxoosmium(v1)diolato complexes (osmate esters), reEt
sulting from the reaction of [OsO, . amine] with an olefin during
the catalytic cycle are coordinatively unsaturated species (16 e)
only when monodentate amine ligands are used (Scheme 54,
(DHQ)z-PHAL
coordinative
bottom). As pointed out by Sharpless et
unsaturation at this stage is required to achieve reoxidation of
Me0
OMe
the osmate ester and its hydrolysis. A coordinatively saturated
system (for example, when a chelating diamine is used) or one
formed in the presence of an excess of amine, is not susceptible
to fast reoxidation/hydrolysis. This offers an explanation why
chelating diamines, despite the excellent stereochemical results
obtained in stoichiometric reactions, cannot be employed in
Me0
OMe
Scheme 53. A selection of chirdl chelating diamines used in the stoichiometric osmylation of olefins (lSO] and the state-of-the-art dihydroquinine and dihydroquinidine
ligands for the catalytic dihydroxylation [I 521. C,,H, = naphthyl.
A n g m . C l i ~ mI n t . fid. Engl 1994, 33. 497-526
A speculative, postulated mechanism for the 0s-catalyzed
dihydroxylation has been reported by Corey et al." 55a. b1 They
suggested that the cinchona Iigands, which contain two alkaloid
moieties, are able to accommodate two OsO, units coordinated
to the quinuclidine nitrogen atoms. These complexes can exist as
the Open form in
mixture (Scheme "1.
an
which the OsO, fragments d o not interact with each other was
519
A. Togni and L. M. Venanzi
REVIEWS
ing CuOTf with bis(dihydrooxazo1e) ligands (these catalysts are
also very successful in cyclopropanation reactions; see Section 5.5.1), E v a k et al. obtained optical yields of up to 97% ce
for the aziridination of r,P-unsaturated esters (Scheme 56). The
aziridines themselves are suitable precursors for the synthesis of
a-amino acid derivatives.[' 57b1 A further, remarkably simple,
chiral, bidentate ligand has been reported by Jacobsen et al. (L2
in Scheme 56).[1591A common feature of these first two success0s"[l1,18 e [I 53a]
Osvl, 18 e [150d]
'
*
COOPh
Ts
I
5 MOIL% Cu'OTf / L'
Ph
+
Phl=NTs
COOPh
97 YOee
Osvl,16 e [I 53b]
+
Scheme 54. Schematic representation of results ofX-ray structural analyses ofoxoosmium complexes uith dihydroquinine derivatives as monodentate ligands 11531
and uith il clielating diamine [150d].
PhkNTs
87 Ya ee
isolated and characterized by X-ray diffraction, the more reactive 0x0-bridged form is proposed to be involved in the catalytic
cycle. Recently, however, Sharpless et al.[155c1have shown that
the kinetics of catalytic dehydroxylation is first order in osmium. This throuws doubt on the relevance of Corey's postulate.
5.12.3. Asymmetric Azividination
A topic closely related to epoxidation is aziridination (the
formation of an aziridine from an olefin and a suitable nitrenedelivering reagent) .'1561 An important discovery was made by
the Evans group in 1991,[157"1
when the catalytic activity of Cu'
salts was demonstrated. The simple compound [Cu(NCMe),]
CIO, effectively catalyzed the aziridination of various olefins
with ( N - (p-toluenesulfon y1)imino)phenyliodinane (PhI=NTs) .
More recently. other groups have shown that several salen-metal
complexes are able to catalyze this reaction,"581 but only very
recently were highly enantioselective Cu' catalysts disclosed. Us-
\ /
ICJ= (
cia'
\ /
ful approaches is the use of neutral, C,-symmetric N,N-ligands.
In analogy to cyclopropanation and to Mn-catalyzed epoxidation, the transfer of the nitrene fragment could occur via intermediate imido-metal complexes. Although such complexes are
known for several transition metals,[1601and play an important
role in ROMP catalysis (see Section 5.13), whether they are
involved in catalytic aziridination remains the subject of speculation.
5.13. Miscellanous Reactions
__c
.c-
:eom
I
Scheinc 55. Structure ol'a hinuclear oxoosimum complex (characterized by X - r a y analysis) and its postolated inore reactive. dioxo-bridged forin 11 551
520
L2
'Ts
Scheme 56. Asymmetric aziridination of olefins catalyLed by Cui complexes [I57 b.
1591.
I
I
Ar
CH,CI, / ca. 20°C
The oxidation of cyclohexane by air, which
gives a mixture of cyclohexanol and cyclohexanone, is a fundamentally important industrial
The reaction, in which
cyclohexyl hydroperoxide is an intermediate,
is catalyzed by a hydrocarbon-soluble Co"
carboxylate, whose role is to decompose the
hydroperoxide. Tolman, Ittel, et al. reported
in 1988 that bis(2-pyridylimino)dihydroindolatocobalt complexes (Scheme 57) are
very active and long-lived catalysts for this
An@,&! C h m . Ini Ed. Enxi 1994, 33. 497-526
Nitrogen Donors
oxidation process."621 The advantage of such complexes is that
they are active at lower temperatures and higher peroxide concentrations (higher conversions) than conventional catalysts.
Cobalt porphyrins and phthalocyanins were also found to be
efficient
In recent years well-defined catalysts for the ring-opening
metathesis polymerization (ROMP) reaction have been developed. Starting from (usually) strained cyclic olefins and using
ROMP it is now possible to prepare homopolymers and block
copolymers with a very narrow molecular weight distribution." "31 The best catalysts are pseudo-tetrahedral alkylidene
complexes of molybdenum and tungsten in the formal oxidation
state + VI. The coordination sphere of the metal in these complexes contains two bulky alkoxide ligands and one sterically
demanding imido ligand. This combination of ligands ensures
sufficient stability of the alkylidene complex by preventing, for
example, intermolecular ligand scrambling or other decomposition reactions. At the same time it still allows the coordination
of the substrate with formation of a five-coordinate complex,
and thus its transformation into the intermediate metallacyclobutane c ~ m p l e x . ~ 'One
~ ~ "of] two imido ligands also plays the
role of a protecting group in a precursor of the catalyst, allowing
the selective introduction of two alkyl l i g a n d ~ . [ ' The
~ ~ Isynthesis
and reactivity of such imido complexes are illustrated in
Scheme 57 Cobalt catalysts containing bis(pyridy1imino)dihydroisoindolato ligands for cyclohexane oxidation (L = RCOO- or second ligand) 11621.
Scheme 58. Several of these complexes have been characterized
by X-ray diffraction and show the peculiarity of the NAr ligand:
Because of the relative electron-deficiency of the metal center, a
strong donation of the nitrogen lone pair takes place. This causes an apparent sp hybridization at the nitrogen atom, resulting
in an M-N-C angle approaching 180". Correspondingly, the
M - N distance in the range of 1.70- 1.75 A is very short--indeed shorter than the M - C distance of the alkylidene hgands
(1.85 - 1.95 A) . L ' h 4 1
With R O M P catalysts the synthesis of, for example, small
block copolymers containing redox-active groups was recently
achieved (Scheme 59 A). The well-behaved solution electrochemistry of such copolymers has been reported by Schrock.
Wrighton, et al.[lbsl
Grubbs et al. have shown that the interesting polymer
poly(l.4-phenylenevinylene) is readily accessible by R O M P
(Scheme 59B)[lbh1and that the catalysts shown in Scheme 57
can be employed in the ringclosing olefin metathesis of
Aiigc'w. Chvrii.
/lit.
Ed. Eiigl. 1994. 33. 497-526
REVIEWS
Ar
1""
[MoO,CI,LJ
+ 2 ArNHSiMe,
*
!r
c,"=qo:l
I
Me
N
I
Ar
tBuCH,MgCI
Me
3Me
\
Me
Scheme 58. Synthesis of a Mo ROMP catalyst containing imido ligands [164a].
L = thf.
diolefins for the synthesis of oxygen- and nitrogen-heterocycles
(Scheme 59C and D).[1671
An al kylideneoxotungsten(v1) complex containing a tris(pyrazoly1)borato ligand has been reported by Boncella
et a1.['681as an exceptionally stable ROMP catalyst. Its synthesis is achieved from an alkylidynetrichloro complex by ligand
exchange and treatment with neutral alumina, as shown in
Scheme 60. It is found to be a very active R O M P catalyst, in the
presence of AICI, as a cocatalyst, for example, for cyclooctene
and norbornene polymerization. The role of the Lewis acid
AICI, is thought to involve abstraction of the chloride ligand,
thus generating a coordinatively unsaturated cationic alkylidene
species which acts as the actual catalyst.[t681
Although not directly relevant to homogeneous catalysis, the
unique reactivity patterns imparted to arenes by the fragment
[Os(NH,)J+, n = 2, 3, discovered by Harman and T a ~ b e " ~ ~ '
should be mentioned. When 0s"' but also Ru"' pentaammine
salts are reduced in the presence of benzene, exceptionally stable
[M"(NH,),(qZ-C,H6)]2+ complexes are formed. This behavior
is unprecedented in the coordination chemistry of such ammine
complexes. The arene derivatives undergo smooth arene substitution reactions, but most important, they are susceptible to
selective hydrogenation reactions and electrophilic substitution
processes. Thus, a coordinated, substituted arene can be
stereospecifically hydrogenated (for example, in the presence of
a heterogeneous catalyst) to the corresponding $-cyclohexene
derivative (Scheme 61 A). Furthermore, the electrophilic substitution a t a coordinated phenol derivative by, for example, maleic anhydride (a potentially acylating reagent) can be mechanistically viewed as a 1,4 addition (Scheme 61 B). This reaction has
been shown to be useful in the dearomatization of /I-estradiol by
direct stereospecific 1,4 alkylation (Scheme 61 C).
Finally, to return to asymmetric reactions. a recent report by
Inoue et al.[1701about the Ti-catalyzed formation of cyanohy521
A. Togni and L. M. Venanzi
REVIEWS
OC(0)OMe
0
C
Me
T
M
e
-
Me
drins is worth mentioning. Enantiomeric excesses up to 97%
have been obtained for the addition of HCN to aldehydes in the
presence of 10 mol% of ethyl or isopropyl orthotitanate and a
Schiff base of a peptide as chirai ligand (Scheme 62). MM2
calculations indicate that such compounds behave as terdentate
O,N,O ligands; the oxygen atom of the peptide linkage coordinates to the metal. A judicious construction of the ligand system
utilizing only natural amino acids allows the preparation of
both enantiomers of the products in high er’s.
6. Conclusions
Nitrogen-ligands play an important role in modern organometallic chemistry and homogeneous catalysis. The properties
Scheme 60. Synthesis ofan exceptionally stable W -alkylidene complex for applications as a ROMP catalyst [168].
522
Scheme 59. Examples of recent applications of the imidomolybdenum ROMP catalysts
and reactivities of the many complexes reported in the literature that contain such donors can be understood if one
takes into consideration the general rules of coordination
chemistry. Thus, in terms of their coordination behavior,
it is fundamental to recognize the most important differences between nitrogen- and phosphorus-ligands: 1. the x backbonding ability of N-donors is insignificant, thus making
them generally unsuited for the stabilization of low oxidation states of the transition metal center. 2. The trans effect
exerted by N-donors is negligible, when compared to that
of most other ligands encountered in organometallic
chemistry, and thus their own rates of substitution reactions
are usually high. 3. The reactivity of, for example, alkyl
complexes of transition metals containing N-donors is usually
high.
For the vast majority of the reported cases nitrogenligands have been used as part of a chelating system that also
involves other donor atoms such as C, 0, P, and S. This
is particularly true for the many applications in homogeneous catalysis described in Section 5, where nitrogen-ligands
are responsible for various important developments in the
field of asymmetric synthesis. Progress in this field in recent
years has been achieved in a very empirical manner, mainly by
synthetically oriented research groups, because chiral nitrogen ligands are often easily accessible from the “chiral pool”.
Thus, one is tempted to attribute the success of such ligands
to organic chemists. At the same time, the wealth of applications of nitrogen-ligands in a variety of catalytic homogeneous reactions should be an incentive for systematic studies
from the point of view of coordination chemistry in the near
future.
AngeJi Chem I n f Ed. Engl. 1994, 33, 491-526
Nitrogen Donors
REVIEWS
Hv
[l] J. E. Huheey. E. A. Keiter. R. L. Keiter. /nor,qmic
ChemLtry . Principles of Strwlurc, and Rcuctirrt I',
Harper Collins. New York. 1993. Chapter 11 13.
OCH3
H2rRh/C
%
,
,
.a**
[2] a ) J. P. Collman, L. S. Hegedus. J. R. Norton. R. G .
___c
OCH,
Finke, Prmciples and App/icutlon.\ of Or~~iriolrair.sition
H~CO""
Metul Chemistry, University Science Books, Mill ValA
ley. CA. USA. 1987; b) A. Yamamoto. Orjpnntrun?i(H,N)50s2'
(H3N)50~2+
f i ~ Mcvul
n
Chemirtr~,Wiley, New York. 1986: c ) R . H.
Crabtree. Th~-lirOrgunomr~ollicC'hiwirsrry of thv 7 h 7 s i rion Mcirrls, Wiley. New York. 1988; d ) C. Elschenhroich. A. Salzer. Orjinnc,mcrtrlli(..\. 2nd ed , VCH.
Weinheim. 1992: O r ~ u ~ i ~ i n i c ~ t a l l ~ h Teuhner.
e n i i c ~ . Stultgart. 1988; e) G. W Parshall, S. D. Ittel. Ilomo#tweous
Cutulrsis. Wiley, New York. 1992.
[3] a) C. A. McAuliffe, W. Levason. Pho.\phinr. .4r.>111i,
find
Sribine CornpL.ws of ihe Z-unsitwn Elcw~wt.~,
Elsevier,
B
H
(H,N),Os*+
O
T
MeCN I24 h
* O
Amsterdam, 1979; b) E. C. Alyea, Caiu/r'rir A . p w . 5 of
Merul Phosphine Compl~w,s(Adv. C'hmi. &r. 1982,
196): c ) Homngeneom C'utalvsi.\ with :Metal Phospliine
(H3N)50s2+
Comple-xer (Ed.: L. H. Pignolet). Plenum. New York.
1983; d) C. A . McAuhffe in Cimtprrhi~n.siwC'oordmrrtion Chemistry, l'iil. 2 (Eds.: G . Wilkinson. R . D.
Gillard. J. A. McCleverty). Pergamon. Oxford. 1987.
p. 989 - 1066.
[4] See, for example, a) W J. Evans. S. E. Foster, J.
(
H
3
N
)
p ____)
&Me
(H3N15OS2'
Orgunomet. Chrm. 1992. 433, 79-94. h) W. J. Evans.
Adv. Orpnamer. Chem. 1985. 24. 131 -177: c) T. J.
C
Marks. R. D. Ernst in C'ontprrhmwc Organomr,tallr<~
MeCN /-40°C
ChcmiFtrr. Ril. 3 (Ed.: G . Wilkinson. F G . A. Stone.
\
E. W. Abel). Pergamon, Oxford. 1982. pp, 173 .?79.
/
HO
0
[5] N. N. Greenwood. A . Earnshaw, Chcmir dcr Eimtcvne~,
VCH. Weinheim. 1990.
Scheme 61. Catalytic reactions involving the [Os(NH,),] system of Harman, YdUbe et al. [169].
[6] See, for example. a) S. J. Ashcroft, C . T. Mortimer.
Thhrrmochemi.?trjof Transitn)n Metal Compleur\, Academic Press, London. 1970; b) Bonding Energetic.\ m Oriymomerullic~.sChenihtrj (Ed.: T. J. Marks) (ACS Symp. Ser. 1990, 428).
[7] J. K . Kocbi. Orgunometullic Mechun;.ons and Catalysis, Academic Press, New
York, 1978.
[8] See, for example. ref. [2c], pp. 154-159.
[9] See, for example, ref. [2a], pp. 322-343.
10 rnol % Ti(OEt), / Lc @CN
[lo] See. for example. ref. [7]. pp. 293-310.
+ HCN
[ l l ] W Lau, J. C. Huffman, J. K. Kochi. OrgunometuNics 1982. 1. 155-169.
[I21 See, for example, ref. [7], Chapter 14.
[I31 a) C. M. Langford, H. B. Gray, Ligund Suhsriturion Proce.\ses. Benjamin.
I
A
New York. 1965; b) F. Basolo. R. G. Pearson, Mechuni.snt of /nor,yrmic
R
Reuctions, Wiley. New York, 1967: c) R. G. Wilkins, Kinetics arid MechuH
nism Qf Rruclions of' Transition Metul Comp1exe.v. 2nd ed.. VCH, Weinheim.
90 % ee (R)
1991.
R=H,L=
[I41 D. W. Margerum, G . R. Cayley, D. C . Weatherburn. G. K Papenkopf. Cumdinution Chemisrry ( A C S Monogr. 1978, 174).
[15] G. Geier. Hels. Chim. Acta 1968. 51, 94-105.
[16] F. Basolo, J. Chatt, H. B. Gray. R. G. Pearson, B. L. Shaw, J. Chem. Soc.
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~
Angew. Chem Int. Ed Engl. 1994, 33, 497-526
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