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Bifunctional MetalЦLigand Catalysis Hydrogenations and New Reactions within the MetalЦ(Di)amine Scaffold.

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Highlights
DOI: 10.1002/anie.200501787
Asymmetric Catalysis
Bifunctional Metal–Ligand Catalysis: Hydrogenations
and New Reactions within the Metal–(Di)amine
Scaffold**
Kilian Muiz*
Keywords:
diamines · homogeneous catalysis · hydrogenation ·
reduction · transition metals
The observation of the ligand-associated heterolytic splitting of H2 dates back
to pioneering work by Fryzuk et al. on
organometallic amide complexes of rhodium and iridium.[1] The general importance of the reversible storage of dihydrogen within a transition metal/ligand
framework was later proposed by Crabtree but remained restricted to the
stoichiometric formation of hydrides of
iridium–amide complexes.[2] Starting in
1995, Noyori disclosed novel ruthenium
diamine complexes that enabled a conceptually new enantioselective hydrogenation process and thereby marked
that start of the rational design of chiral
catalysts with unprecedented activity in
the reduction of prochiral ketones and
imines.[3] In principle, the basic task of
the catalyst consists of the concerted
transfer of dihydrogen within a hydride–
ruthenium–diamine assembly. If the diamine or the remaining coordination
sphere of the metal center bears defined
stereochemical information, an enantio[*] Dr. K. Mu'iz
Kekul)-Institut f+r Organische Chemie
und Biochemie
Rheinische Friedrich-Wilhelms-Universit1t
Gerhard-Domagk-Strasse 1
53121 Bonn (Germany)
Fax: (+ 49) 228-735-813
E-mail: kilian.muniz@uni-bonn.de
[**] This work was supported by the Fonds der
Chemischen Industrie. Abbreviations
within this article: S/C = ratio of substrate
to catalyst, TON = total turnover number,
binap = 2,2’-bis(diphenylphosphanyl)-1,1’binaphthyl. For simplicity, in Schemes 1
and 3 the ligands 1,2-diphenylethylenediamine and h6-cymene are depicted as
ethylenediamine and benzene, respectively.
6622
Scheme 1. Catalytic enantioselective transfer hydrogenation of ketones.
selective process is possible (e.g., structure A, Scheme 1).
For this purpose, two types of reactions were envisioned. Transfer hydrogenation of both ketones and imines
used 2-propanol or formic acid/triethylamine as the hydrogen source.[3a,c,d] The
overall reaction pathway was determined in a seminal study in which all
active metal complexes involved in the
catalytic cycle were isolated.[4] Typically
the actual catalyst is generated in situ
through interaction of base which causes
elimination of HCl from precatalyst 1.
The resulting low-coordinate ruthenium–amide complex 2 interacts with 2propanol (A, R1 = R2 = Me) and within
a concerted process takes up hydride
and proton to generate the hydride
catalyst 3. Reduction of the carbonyl is
then accomplished via transition state A
(R1 ¼
6 R2) without metal–carbonyl interaction to yield the chiral alcohol or
amine product and 2. In principle, the
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
reaction turns endlessly at the Ru–NH
surface of the two catalytically active
species 2 and 3 (Scheme 1).
For transfer hydrogenation in formic
acid/triethylamine azeotrope, the working mode of catalyst formation was
addressed recently by Ikariya et al.,
who published an extensive investigation including the first structural elucidation of a transition-state analogue for
this step.[5] Reaction of the isolated
ruthenium–amide complex 2[4] with formic acid at 40 8C in THF generated the
formate complex 4 a as a single diastereoisomer. This confirms once again that
the formation of the actual hydride
catalyst proceeds with complete stereoselectivity.[4] As expected for an immediate catalyst precursor of high reactivity, 4 a was found to be rather unstable
and no crystal structure could be obtained. Instead, the related acetate 4 b
was characterized by X-ray structure
analysis. The structure is in accordance
Angew. Chem. Int. Ed. 2005, 44, 6622 – 6627
Angewandte
Chemie
with the expected absolute configuration at the Ru atom, and the distance
between the carbonyl group and the
NH2 entity is only 2.77 5. Kinetic NMR
studies on the decarboxylation of 4 a
revealed a first-order rate dependence
in substrate. Activation parameters of
DH° = 76 kJ mol1,
DS° =
1 1
°
38 J mol K , and DG = 87 kJ mol1
were obtained, and the negative entropy
value suggests that the active hydride
catalyst 3 is formed from 4 a by an
intramolecular process. Within this context, a remarkable study on the transfer
hydrogenation of arometic ketones in
water was reported recently, which suggested a dramatic pH dependence of
both catalyst performance and regeneration mode.[6]
recently led to an efficient catalyst class
for the enantioselective hydrogenation
of tert-alkyl ketones.[7c,d]
These extremely efficient catalyses
have stimulated ongoing mechanistic
investigations.[8] The exact working
mode of the uniquely enantioselective
catalyst derived from 5 was uncovered
by Noyori et al. in 2003.[9a] The structurally defined hydride–borohydride–
ruthenium precursor 6[9b] was found to
initiate catalysis even without addition
of base and therefore served as the
molecular basis for the unambiguous
determination of the underlying kinetics
and the influence of solvent, base, and
hydrogen pressure on the overall reaction profile. Two cycles turned out to be
of importance. Borohydride dissociation
from 6 gives the cationic complex 10
(Scheme 3). In protic solvents such as 2propanol this intermediate originates
from protonation of the basic imido
function in neutral 8. Complex 10 then
accomodates dihydrogen to yield the
cationic dihydrogen complex 11, which
represents the resting state of the catalyst and is the immediate catalyst precursor. Dihydride catalyst 7 is formed
from 11 by loss of a proton (Scheme 3,
cycle II).
In an elegant experiment Bergens
et al. showed that 11 can be generated
directly from the cationic binap–Ru–
hydride complex 12 (with tetrafluoroborate as the counterion) by addition of
the diamine ligand under 1 atmosphere
of hydrogen.[10] The general structure of
11 was determined by NMR spectroscopy as was the absence of deuterium
scrambling between the h2-bound dihydrogen and the hydride ligand. However, when generated this way 11 is not
an active catalyst precursor in the absence of base. This shows that 2-propanol itself is not sufficiently basic to
deprotonate the dihydrogen ligand in 11
and that the nature of the formal
counterion is of high importance. For
reactions in aprotic solvents, 6 generates
the dihydride catalyst 7, which reduces
the ketone substrate to give amide
complex 8. This complex splits dihydro-
Scheme 2. Enantioselective hydrogenation of ketones. R = phenyl, 4-tolyl, xylyl.
In contrast to transfer hydrogenation, the direct enantioselective hydrogenation of ketones requires the use of
ternary ruthenium complexes made up
of a bisphosphine, a diamine, and a
ruthenium(ii) center, which are usually
provided as preformed dichlorides such
as 5 (Scheme 2). In view of catalyst
efficiency, these compounds are unparalleled. Peak turnover rates reach 62 per
second, while enantioselectivities of up
to 99 % ee can generally be obtained.[3b,c]
Since the ternary composition allows for
selective replacement of the individual
ligands, the catalyst can be fine-tuned.[3b]
For example, a binap–RuII complex with
a 1,4-diamine ligand acts as a precatalyst
for the enantioselective hydrogenation
of tetralones,[7a,b] a substrate class which
proves problematic for the conventional
1,2-diamine-chelated catalysts such as 5.
Application of pyridinyl amines has
Angew. Chem. Int. Ed. 2005, 44, 6622 – 6627
Scheme 3. Catalytic cycles for ketone hydrogenation and the transition-state structure.
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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6623
Highlights
gen in a heterolytic manner (!9) to
regenerate catalyst 7 (Scheme 3, cycle I).
Thus, the mechanistic scenario for
the occurrence of peak turnover frequencies in bifunctional metal–ligand
hydrogenation is based on a well-balanced overall neutral environment with
only a local appearance of acidic and
basic species as provided by 6. Complexes 6 and 12 are the only direct
catalyst precursors isolated to date,
since the exact formation of 7 either
from 6 under nonprotic conditions or
from the dichloride 5 or related monohydrides through base interaction remains to be determined. Significant
contributions by Morris et al. include
the isolation of various ruthenium dihydrides related to 7, hydride chlorides,
and amide complexes which all show the
expected catalytic activity in ketone and
imine hydrogenation.[11] An extensive
comparative study on some dihydrides
from monophosphine complexes aimed
to clarify the preferential geometrical
arrangement of these catalysts.[12] In the
predominant catalysts the two hydrides
were found to adopt a trans arrangement, a result that in agreement with the
suggested geometry for 7.[9a]
The enantiofacial differentiation of
the substrate is accomplished kinetically
on the molecular surface of the chirally
modified RuH2 catalyst. The final hydrogen transfer proceeds through a sixmembered transition state and involves
simultaneous transfer of a proton from
an amino moiety of the chiral diamine
ligand to the carbonyl oxygen atom and
hydride transfer from ruthenium to the
carbonyl group. A molecular model of
the active trans-RuH2 species from the
hydrogenation is depicted in Scheme 3;
it shows how the hydride and the amine
proton (H-Ru-N-Hax) are involved in
the transition state. The prerequisite
diamine ligand plays a dual role: it
contributes to the chiral environment
of the catalyst and takes part actively in
hydrogen transfer.
Although the two catalytic systems
with hydride and dihydride catalysts (3
and 7, respectively) apparently share the
mode of hydrogen transfer, they differ
from each other in catalyst regeneration.
The decisive step of the hydrogen transfer from the polarized donor molecules
2-propanol or formic acid consists of a
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concerted addition across the polar Ru–
amide bond. The ionic character of the
hydrogen-transfer reagent is necessary,
since direct activation of Ru–amide 2
with hydrogen requires a pressure of
80 bar.[4] In contrast, cationic ruthenium
complexes such as 10 can activate dihydrogen heterolytically by h2 coordination. The subsequent deprotonation of
this dihydrogen ligand is rapid and
results in the high productivity of the
overall reaction, that is, the kinetic
preference of cycle II over cycle I.
In what represents a significant
structural alternative in this area of
hydrogenation catalysts, GrCtzmacher
et al. introduced a novel rhodium–
amide system for the heterolytic activation of hydrogen activation.[13] Starting
from the bistropylidenylamine 13, a twostep sequence led to the formation of a
defined rhodium complex in which the
bistropylidenylamine acts as a tridentate
ligand. The amino hydrogen displays the
expected acidity (pKa,DMSO = 15–20) and
can be removed upon addition of base
(Scheme 4). The resulting rhodium–
splitting at 78 8C under 1 atm of hydrogen and are transformed into the corresponding rhodium hydrides 15 a,b. Compound 15 b was again characterized by
X-ray structure analysis, and its structure was very similar to that of 14 b. This
result indicates that the accommodation
of hydrogen in this systm is readily
reversible; this was confirmed by the
observation of selective and reversible
deuterium incorporation under D2.
The structure and reactivity of the
resulting rhodium hydrides thus match
that required for bifunctional metal–
ligand hydrogenation catalysts. Comparative DFT calculations confirm a high
preference for the heterolytic dihydrogen cleavage, which yields the amino
rhodium hydride by an exothermic pathway (DHR = 67 kJ mol1), while the
conventional homolytic splitting to yield
a rhodium dihydride would proceed by
an endothermic pathway (DHR =
71 kJ mol1) and via an higher energy
transition state (DH° = 61 kJ mol1 for
heterolytic vs DH° = 75 kJ mol1 for
homolytic cleavage). Under 100 bar of
Scheme 4. A structually novel rhodium amide complex for the heterolytic cleavage of dihydrogen. cod = cyclooctadiene.
amide complexes 14 a,b are sufficiently
stable. Complex 14 b was characterized
by its solid-state structure, which revealed a RhN bond length of 2.0 5. In
addition, the observed sawhorse structure of the four-coordinate 14 b is unique
among this type of complexes and
appears to be a geometrical prerequisite
for dihydrogen splitting. Compounds
14 a,b readily induce heterolytic H2
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
hydrogen, both the crystalline amide
and the isolated hydride catalyze hydrogenation of ketones and imines with
TONs of up to 650. No further additives
are required; that is, the reaction proceeds under neutral conditions. Aside
from the borohydride complexes of type
6, this is a rare example of a base-free
bifunctional metal–ligand hydrogenation catalyst.[14]
Angew. Chem. Int. Ed. 2005, 44, 6622 – 6627
Angewandte
Chemie
Apart from their unmatched efficiency in the enantioselective hydrogenation and transfer hydrogenation of
carbonyl compounds, bifunctional metal–ligand complexes have been employed recently for other processes as
well. Sadler et al. described a structurally closely related cationic (h6-arene)Ru(en) unit for the recognition of nucleoside and nucleotide binding sites.[15]
In the area of transition-metal catalysis,
Ikariya et al. reported the catalytic hydrogenolysis of epoxides in 2-propanol
in the presence of an aminophosphine
ligand and a pentamethylcyclopentadienyl (Cp*) ruthenium precursor.[16]
The catalyst formed in situ was suggested to be the ruthenium hydride 17,
which at S/C = 100 gave yields from 79
to 99 % and a 99:1 ratio in favor of the
branched alcohol over the linear
(Scheme 5).
Scheme 6. Enantioselective Michael additions employing bifunctional ligand–metal catalysts.
Scheme 5. Ruthenium catalysts for the hydrogenolysis of epoxides and for the conversion of
diols to lactones.
Hartwig et al. employed catalysts of
ternary composition for lactone formation from the dehydrogenative cyclization of 1,4-butanediol. Among various
catalysts screened for this purpose, 18
performed best and converted the diol
quantitatively. The reaction could be
performed on a large scale; 22 g of diol
was converted into the lactone in 100 %
yield with as little as 5.4 mg of catalyst,
which corresponds to an overall TON of
17 000.[17]
The successful ruthenium diamine
motif was used to develop an enantioselective Michael addition.[18] Here, the
addition of dimethyl malonate across
prochiral cyclic enones such as cyclopentenone and cyclohexenone was catalyzed by 2 with selectivities of up to
99 % ee.[18a–c] The reaction is believed to
Angew. Chem. Int. Ed. 2005, 44, 6622 – 6627
start from an a-metalated structure 19
and to proceed through a highly organized transition state 20 with hydrogen
bonding toward the carbonyl oxygen
and olefin-face selection through the
imposing stereochemical environment
of the catalyst surface (Scheme 6).
The identical transformation was
accomplished by Morris et al. using
ternary catalysts of the binap–ruthenium–aminophosphine composition.[18d]
Here, the borohydride complex 21
proved to be the most efficient catalyst
precursor, leading to the Michael addition with 96 % ee at S/C = 100. Since
complexes derived from binap, ruthenium, and aminophosphines have proven
to be versatile hydrogenation catalysts
as well,[19] 21 can be used for a domino
catalysis.[18d] Thus, when the Michael
addition was complete, application of
28 bar of hydrogen pressure led to concomitant reduction of the ketone to the
corresponding alcohol with 30:1 selectivity in favor of the trans-configured
cyclohenanol. An identical sequence
with slightly lower selectivities (90 % ee
for the Michael addition and 10:1 selectivity at 99 % yield) was observed for
related ruthenium complexes based on
binol-derived bisphosphinites.[18e]
A ruthenium–amine moiety proved
successful for the isomerization of allylic
alcohols to ketones.[20] Again, the established Cp* ruthenium catalyst 17 from
epoxide hydrogenolysis was employed
and was generated in situ with KOtBu.
Various allylic alcohols could be isomerized to give the corresponding ketones
(Scheme 7).
Isotopic labeling provided the first
information about the mechanism. Apparently the reaction is initiated by
alcohol dehydration to give the unsaturated ketone together with a ruthenium
hydride or deuteride. Regioselective
conjugate addition of this complex furnishes the simple ketone product with
the observed deuterium incorporation
in the 3-position. This observation
matches the chemoselectivity observed
in related Michael addition processes
(Scheme 6). The fact that the reaction
proceeds with complete chemoselectivity for substrates containing further C=C
bonds led to the development of an
enantioselective allylic isomerization
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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6625
Highlights
Scheme 7. Bifunctional metal–ligand catalysts for the isomerization of allylic alcohols and the reduction of polarized C=C bonds.
with a proline-derived ligand. At
5 mol % loading, catalyst 22 selectively
converted the shown triene into the
corresponding enantioenriched ketone,
which within two subsequent steps gave
the natural product (S)-muscone.
While the original Noyori transferhydrogenation catalyst displayed high
chemoselectivity for the preferential
reduction of C=O over C=C bonds,
Deng et al. have now found that this
preference can be reversed for strongly
polarized olefinic substrates.[21] In addition, application of the triethylamine/
formic acid protocol led to an efficient
saturation of the C=C bonds for a
variety of unsaturated nitriles when a
slightly modified diamine ligand was
employed (precatalyst 23). Enantiomeric excesses of up to 89 % could be
obtained, and experiments with deuterated formic acid suggest that the reaction involving the polarized amine–
ruthenium hydride catalyst follows a
conjugate reduction pathway related to
the Michael additions from Scheme 6.
Bifunctional metal–ligand catalysis
had originally been conceived as a
synthetic methodology for the enantioselective hydrogenation of ketones and
imines. The first transition metal employed, ruthenium, is still the most
broadly applicable, but similar reactivity
has been shown for other metals as well.
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Certainly future interest in this area will
focus on the elucidation of the few
remaining mechanistic questions, for
example, the exact mode of catalyst
formation, and the development of
structurally new systems such as the
rhodium catalyst 15. Apart from this,
bifunctional metal–ligand catalysis may
develop into a more general concept for
enantioselective catalysis. In view of the
first examples of enantioselective Michael additions and the isomerization of
allylic alcohols, and considering the yet
unexplored hydrometalation[22] and the
recent detection of highly reactive
ruthenium and iridium alkoxides,[23]
one expects further exciting reactions
to emerge in due course.
Published online: September 27, 2005
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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