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Hydrozirconation A New Transition Metal Reagent for Organic Synthesis.

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[I701 J . F. Arens, L. Brandsma, P . J . W Schuije, and H . E. Wijers, Q.
Rep. Sulfur Chem. 5 , 1 (1970).
[I711 U . Schollkopfand I . Hoppe, Angew. Chem. 87, 814 (1975); Angew.
Chem. Int. Ed. Engl. 14, 765 (1975).
[172] a) R. Gompper and R. Weiss, Angew. Chem. 80. 277 (1968); Angew.
Chem. Int. Ed. Engl. 7 , 296 (1968); b) R. Weiss and R. Gompper,
Tetrahedron Lett. 1970, 481.
[I731 u! Steglich and G. Hofle, Chem. Ber. 102, 883, 889, 1129 (1969).
Hydrozirconation : A New Transition Metal Reagent for Organic
New synthetic
methods (16)
By Jeffrey Schwartz and Jay A. Labinger[*I
Hydrozirconation has recently been developed as a procedure for functionalizing alkenes,
alkynes, and 1,3-dienes via organozirconium(1v) intermediates. These intermediates react with
a variety of electrophilic reagents to give organic products in high yield. Mechanisms of reactions
involved in these sequences are discussed.
1. Introduction
Because of the scope of reactions found in transition metal
organic chemistry, there is much current interest in the development of transition metal complexes as reagents for organic
synthesis. Transition metal hydrides constitute one class of
compounds presently under study, since by selective hydrometalation readily available starting materials, olefins or acetylenes, can be converted specifically into reactive a-bonded
organometallic intermediates, at least in theory; upon C-M
cleavage these intermediates give the desired organic product.
Hydrides of main group elements (e.g., boron['], aluminum[21,
or tinr3])have become commonly used stoichiometric reagents
for accomplishing this type of synthetic transformation, and
transition metal hydrides find use as catalysts for reactions
of unsaturated hydrocarbons such as hydrogenation, hydroformylation, hydrosilation, and isomerizationr4].
To develop transition metal hydrides as useful reagents
of general applicability, it is necessary to find such complexes
which will react with ordinary "unactivated"["] alkenes or
alkynes to give alkyl- or alkenyl-metal derivatives in high
yield. The presence of electron-withdrawing substituents on
the unsaturated organic molecule seems to favor this reaction.
For this reason the study of the formation of transition metalcarbon a-bonds by P-hydride addition to unsaturated organic
molecules has heretofore centered around the chemistry of
transition metal hydrides and olefins or acetylenes containing
electron-withdrawing substituents such as tetrafluoroethylene[5.h1,hexafluor0-2-butyne[~~,
dimethyl acetylenedicarboxylate'8], acrylonitrile", 'I, and f~maronitrile[~].
whereas these substrates d o provide important mechanistic
information, they are not generally useful precursors of desired
organic products.
Prof. Dr. J. Schwartz ['I and Dr. J. A. Labinger
Department of Chemistry, Princeton University
Princeton, New Jersey 08540 (USA)
['I To whom correspondence should be addressed.
By "unactivated" we mean acetylenes or olefins which are substituted
with alkyl or proton substituents only.
Angeu. Cheni. Int.
Ed. Engl. f Vol. 15 (1976) No. 6
The conversion of an unactivated alkene or alkyne into
a transition metal alkyl or alkenyl involves two steps (Scheme
1) Coordination of the olefin or acetylene to the transition
metal atom;
2) "P-M-H addition" to the coordinated unsaturated molecule.
Scheme 1
For a variety of transition metal complexes the magnitude
of the equilibrium constant K I has been found["] to depend
on the number and size of alkyl substituents attached to
the double bond; as these increase, K I decreases rapidly.
P-Hydride addition or elimination in transition metal complexes ( K 2 )has been studied in numerous cases[6.l 1 - l41 , and
Hoffmann has recently
that the position for this
equilibrium ( K 2 )should depend on the availability of electron
density on the metal to stabilize the alkene complex; its
increase favors the hydrido-alkene species. Similar arguments
are applicable to alkyne chemistry.
The reaction between ethylene and certain transition metal
hydrides which are catalytically active can result in the formation of an isolable ethyl derivative'" 16-201.Although alkylmetal derivatives cannot usually be observed in reactions between higher alkenes and transition metal hydrides used as homogeneous catalysts, there is evidence for their formation as
intermediatesC6.'1. Even though the concentration of an alkyltransition metal compound thus formed is too low to be
detected directly, several types of catalytic sequences involving
such intermediates are known. In catalytic hydrogenation,
for example, even though K and K , can be relatively unfavorable, the reagent that ultimately effects C-M bond cleavage
in the intermediary alkylmetal species, viz. Ha, does not destroy
the metal hydride catalyst; it can react reversibly with H2
to give a metal trihydrideC4."I. A different situation exists
when the cleavage reagent chosen is not H, (or is any other
species that reacts irreversibly with the metal hydride). Here,
consumption of the metal hydride reagent can occur to give
a low yield of desired product unless
- a) The alkyl compound is disfavored relative to either hydride species at equilibrium (Scheme I),
but the cleavage
reagent attacks it preferentially;
- b) P-hydride addition is fostered by attack of the cleavage
reagent on a metal hydride species;
- c) the alkyl compound is favored relative to both hydride
species at equilibrium (Scheme 1) and the cleavage reagent
can react with any of these complexes with comparable facility.
Hydrozirconation, a process permitting the selective and
high-yield conversion of unactivated alkenes and alkynes into
a variety of types of organic products uia isolable o-bonded
organozirconium intermediates, was developed as a result
both of the mechanistic considerations discussed above and
of synthetic studies using hydrides of various other transition
metals. This evolution is traced in brief below.
2. Hydroacylation
Preliminary studies indicated practical limitations for
sequences involving condition (a) described in Section 1. Ethylene can be converted into an ethyl ketone via an ethylrhodium
or ethylcobalt complex generated by reaction with
HRh(CO)L3 or HCo(N2)L3(L =PPh3)[221,In this sequence
ketone is formed by oxidative addition of acyl halide to the
alkyl-transition metal intermediate followed by reductive elimination of the p r ~ d u c t [ ’ ~ . ’ However,
the use of higher
olefins results in diminished yield of ketone. Meager success
with these substrates can be attributed to unfavorable factors
associated with the equilibrium constant K1 (Scheme 1) and
the observation that many acyl halides react irreversibly with
HRh(CO)L3 or HCo(N2)L3 to give halometal complexes.
Although the equilibrium concentration of alkyl is not high
even for ethylene, the reaction of ethylrhodium or ethylcobalt
is faster than that of the hydride with acyl halide; hence
for ethylene the concentration of alkylmetal is sufficiently
large to give yields of ketone as high as 86% (based on
starting metal hydride). With long chain alkyl substituents
on the double bond the concentration of the alkylmetal drops
so that most of the starting hydride is consumed by the
acyl chloride.
S c h e m e 2.
Scheme 3.
- &\/
(la), M = Nb
(Ib), M = Ta
NaBHdM = Nb)
LiAIH.,(M = Ta)
on Cp2Nb(C2H4)C2HS
that Nb is pseudotetrahedrally coordinated and that the two
carbon atoms of the ethylene ligand lie in a plane which
also contains the niobium atom and the a-carbon atom of
the ethyl chain. NMR evidence obtained for the related species
(3) indicates that the acetylene unit in these complexes is
deployed in the same fashion : the hydrido ligand, the metal
atom, and both carbons of the acetylenic unit all lie in one
plane. In this geometry one substituent on the triple bond
is in a relatively uncrowded environment near the sterically
small hydrido ligand while the other substituent on the triple
bond resides in a more congested surrounding near the two
cyclopentadienyl rings (Scheme 3). Two consequences of this
mode of coordination are notable:
1) Coordination of a symmetrically substituted acetylene
renders the two ends of the alkyne ligand non-equivalent;
2) coordination of an unsymmetrically substituted acetylene
gives rise to two isomers which differ only in the orientation
of the alkyne ligand in the plane containing the M-H bond.
If the substituents on the triple bond differ in steric size,
that isomer should predominate in which the largest substituent lies nearer the small hydrido ligand.
Scheme 4
Yield [yo]
(M = C o )
(M = Rh)
3. Alkenyl-tantalum(m) and -niobium(irI) Complexes
Parshall et al. have reported[”’ that Cp2NbH3 ( l a )
(Cp=q5-C5H5) coordinates ethylene with loss of Hz to give
CpzNbH(C2H4) (2) and that further treatment of (2) with
ethylene results in the formation of an isolable ethylniobium( i n ) species. Study of the reaction between Cp2MH, ( I )
(M=Nb, Ta) and dialkylacetylenes was initiated in light of
these r e s ~ l t s [ 261.
’ ~ ~Few complexes of dialkyl-substituted acetylenes are k n ~ w n [ ~ ’ . ’ however,
it was found that treatment
of (1) in benzene with such alkynes gave in high yield the
first known examples of hydrido-alkyne complexes,
CpzMH(C2R2)(3) (Scheme 3), compounds which can be
used as models to explain the selectivity of the hydrozirconation process (see below).These compounds (3) (M = Nb) could
not be prepared by displacement of the phosphane ligand
from Cp2NbH(L)[291.
Protonation of (3) (M=Nb, Ta) yields cis olefins as does
its reaction with H2[261[see condition (b) in Section I]. Treatment of (3) (M = Nb) with CO as shown in Scheme 4 converts
it into the alkenylcarbonyl species (4). This o-bonded organometallic species is formed with (C=C) cis stereochemistry[81
by P-Nb-H addition to the triple bond. It is interesting
to note that for 4-methyl-2-pentyne greater regioselectivity
is observed in ( 4 ) than was found in its precursor (3): quantitative NMR analysis demonstrates that all of (3) is converted
into (4). This observation can be explained by a process
in which the alkyne ligand is free to rotate about the metalligand bond axis, as has been demonstrated in another sysSince the steric requirements of the alkenyl ligand
Angew. Chrm. I n t . Ed. Engl.
Vol. 15 (1976) N o . 6
are different from those of the alkyne one, the selectivity
displayed by the starting material is different from that shown
by the o-bonded product. Carbon monoxide traps the coordinatively unsaturated alkenyl complex. Note that in both
(3) and (4) a lone pair of electrons is present which is
formally associated with the Nb”’ (dz) metal center. Such
a pair of electrons associated with a metal center can be
protonated or alkylated[32*331.The formation of cis olefin
upon protonation of (4)[”1 can be envisioned as proceeding
through such a protonation of niobium(rr1)followed by reductive elimination of olefin from the resulting cationic niobium(v)
intermediate (Scheme5). Labeling studies with DzSO4 support
the proposed mechanism for protonation : in the olefinic product deuterium replaces the niobium. A parallel synthesis
of a trialkyl-substituted olefin from ( 4 ) was therefore
attempted. However, although a trialkyl olefin is formed upon
treatment of (4) with CH30SO2F,the product
was not the one predicted by this mechanism. Rather, a mixture
of isomeric olefins was formed. This mixture probably arises
from alkylation at the p-carbon of the alkenyl ligand followed
by proton migration and reductive elimination of olefin
(Scheme 5). The cationic carbene complex intermediate contains a single C-C bond and rotation about this bond prior
to proton transfer leads to the cis-trans mixture of product
observed. This pathway is not unprecedented: Casey has
that deprotonation of a transition metal alkyl carbene complex occurs at the p-carbon to yield a species best
described as an alkenyl complex, and that protonation or
alkylation of this alkenyl compound also occurs at the p-carbon to regenerate a carbene complex.
4. Organozirconium(1v) Chemistry
Results with the niobium(lr1)and tantalum(ir1)systems described in Section 3 indicate that the system CpzMHX can
be used to selectively functionalize unsymmetrically substituted acetylenes. However, with these dZ metals cleavage of
the organometallic intermediate was found to be non-specific.
Using a do metal in CpzMHX ( e . g . M=Zr”), specific C-M
cleavage as well as specific activation of an olefin or an acetylene could be possible. Arguments described earlier also suggest that for a formally do metal the alkyl complex should
predominate over the hydrido (alkene) species at equilibrium
( K z in Scheme 1).
Angrw. Chem. Int. Ed. Engl.
A variety of stable zirconium(1v)alkyls have been prepared
by conventional alkylation of ZrCI4 with Grignard, lithium,
or aluminum reagentsL3’1. Several of these compounds demonstrate appreciable thermal stability, including tetraneopentylzirconium(1~)[~~1,
and tetra(trimethylsilylmethyl)zirconiurn(~v)~~*~.
Tetrabenzylzirconium(IV) is also knowd3’] and has shown limited use as a hydrogenation catalyst. However, none of these organozirconium
compounds has seen extensive use as a reagent in organic
synthesis.Alkylchlorobis(cyclopentadienyl)zirconium(w)complexes have been prepared from CpzZrC1, and lithium[401
or Grignard[41]reagents. These compounds have been briefly
investigated with regard to
and SOz~401
insertion into
the C-Zr b ~ n d [ ~ ~ their
, ~ ’ ]hydrolytic
lability has also been
The preparation of alkylzirconium compounds has now
been accomplished by reaction of CpzZrHC1 with a variety
of unsaturated organic molecules. This reaction, named “hydrozirconation”. is described in detail below.
4.2. Hydrozirconation
The zirconium hydride CpzZr(H)C1 ( 5 ) was first prepared
by Wailes et a/.[4z1from CpZZrC1, and LiAIH,. It is also
easily prepared by treatment of the dichloride in tetrahydrofuran with a stoichiometric amount of NaAIHZ(OR)2.Both
procedures result in the high yield precipitation of (5)[431.
4.2.1. Hydrozirconation of Alkenes
Scheme 5
4.1. Background
Vol. 15 (1976) N o . 6
Wailes briefly studied the reactions of ( 5 ) with several olethe resulting alkyl complexes were not identified. This
hydride was later
to react under mild conditions
with a variety of olefins to generate isolable alkylzirconium(rv)
complexes CpzZr(R)C1(6). Hydrozirconation of olefins proceeds to place the zirconium moiety at the sterically least
hindered position of the olefin chain as a
(cf. Scheme
6). Formation of this product involves either the regiospecific
addition of Zr-H to a terminal double bond or Zr-H addition to an internal double bond followed by rapid rearrangement via Zr-H elimination and readdition to place the metal
in each case at the less hindered position of the alkyl chain.
In contrast to ( 5 ) other transition metal hydrides are
known[”] to catalyze the conversion of a-olefins into thermodynamically more favored isomers having an internal double
bond but do not form isolable alkyl complexes. Although
this observation represents the first instance in which a stable
alkylzirconium compound is formed by rearrangement of an
unisolated precursor, the migration of a Zr moiety from a
secondary carbon to the terminal position of an internally
functionalized starting material is not totally unprecedented.
Thus Zr” and Ti” salts are known to catalyze the isomerization of secondary Grignard[441 and
to give primary ones and these isomerization procedures most
likely involve the intermediacy of Zr”’ or TiIValkyl derivatives.
Isomerization here too probably occurs through reversible
0-hydride elimination and readdition. In the case of alkylzirconium(rv) complexes prepared from ( 5 ) , the migration of
these metallic moieties proceeds rapidly at room temperature
in contrast to analogous o r g a n o b o r ~ n [or
~ ~-aluminum
which rearrange only slowly at elevated temperature.
Scheme 6.
tives in which the relative steric bulk of the alkyl substituents
on the CGC bond dictates the preferred direction of Zr-H
cis+ addition. The composition of such a mixture of vinylzirconium(1v) compounds does not appreciably change as a function of time if ( 5 ) is not present in excess. However, with
added ( 5 ) , isomerization to an equilibrium mixture takes
place at room temperature (Table 1). The composition of
this mixture displays higher regioselectivity than that observed
in hydroboration of unsymmetrical acetylenes with hindered
b~ranes[~'].In no case were products derived from allylic
rearrangements detected[5'1.
Isomerization probably occurs through the intermediacy
of doubly metalated alkyl derivatives ( 7 a ) analogous to the
one isolated (7L1)[~'1 from the reaction between ( 5 ) and
diphenylacetylene. Since metal hydride addition and elimination both occur stereospecifically cis, there is no loss of (C=C)
stereochemistry in the isomerization process.
(7a). R, R' = A l k y l
(7b). R = R'= P h e n y l
4.2.3. Hydrozirconation of 1,3-Dienes
for hydrozirconation of alkenes with ( 5 )
(at room temperature) are a-olefin > cis internal olefin z
trans internal olefin > exocyclically functionalized olefin >
cyclic olefin; terminal olefin > disubstituted olefin > trisubstituted olefin. Tetrasubstituted olefins such as tetramethylethylene fail to react with the hydride after many hours at room
temperature, as d o trisubstituted cyclic olefins such as l-methylcyclohexene. Competition studies indicate that an a-olefin
(e.g., 1-octene) reacts about seventy times faster than do either
cis or trans disubstituted olefins (e.g., 2-octene).
4.2.2. Hydrozirconation of Alkynes
that hydrozirconation of
WaiIes et al. have
terminal alkynes proceeds with (C=C) cis stereochemistry
to place the metal at the terminal carbon atom. Hydrozirconation of various unsymmetrically disubstituted acetylenes
occurs[49)readily to give mixtures of alkenylzirconium deriva-
While the overall mode of addition of ( 5 ) to alkenes and
alkynes parallels reactions known for several main group
or transition metal hydrides, the course of addition of ( 5 )
to 1,3-dienes is significantly different. In contrast to boron-[52]
or aluminum h y d r i d e ~ r ~which
~ ] , often doubly metalate 1,3dienes or give a mixture of products, or to most transition
metal hydrides which undergo 1,4- or 1,2-addition to yield
(5) reacts with a variety of 1,3-dienes
by 1,2-addition to the sterically less hindered double bond
to give y,&unsaturated alkyl complexes in high yield (see
Scheme 7)[55'.
However, 2,4-dienes d o not behave analogously.
Rather, they react with three equivalents of ( 5 ) (as confirmed
by [D]-labeling studies) to give saturated alkylzirconium compounds[61! The mechanism of this unusual transformation
has not yet been established.
S c h e m e 7.
Table 1. Hydrozirconation of unsymmetrically disubstituted acetylenes.
Cp,Zr(H)CI + RC-CR'
Product ratio ( A :€3) [a]
after treatment
with ( 5 )
55 :45
84: 16
>95: < 5
>98: < 2
[a] Determined by NMR analysis 1491
Angew. Chem. Int. Ed. Engl. f Vol. 15 (1976) No. 6
4.3. Cleavage of the Carbon-Zirconium Bond
4.3.1. Alkyl and Alkenyl Halides
Electrophilic halogenation reagents (e.g., Br2, 12, N-bromoor N-chloro-succinimide, or iodobenzene dichloride) react
with alkyl-(43.5 5 1 or alkenylzirconium(~v)~~~]
complexes to
yield the corresponding organic halides (Scheme 8). In contrast
to reactions involving electrophilic attack upon alkenylniobium(nr) complexes[251(see Section 3), the isomeric composition
ofthe vinyl halide product closely duplicates that of its alkenylzirconium precursor and no cis-trans isomerization in the
vinyl halide obtained is detectable.
Scheme 8.
the alcohol. Reaction with chromyl chloride is complicated
by the formation of some by-product alkyl chloride. The
reaction of alkylzirconium(1v)complexes with O2 is very slow
and requires several hours to attain completion. y,&Unsaturated alkyl derivatives can be oxidized using O2to give y,&unsaturated alcohols (Scheme 10). However, attempts to oxidize
alkenylzirconium(1v)compounds to give ketones (after hydrolysis) have met with only moderate success; oxidation with
O2 fails completely and only a low yield of ketone is produced
with tert-butyl hydroperoxide.
96 %
Br, -RBr
91 %
12 -RI
65 %
95 %
61 %
(R = n-Hexyl)
For y,&unsaturated alkylzirconium(1v) complexes[55] yet
another mode of carbon-metal bond cleavage is also possible.
For the compounds shown in which there is a methyl group
on the 3-position of the original 1,3-diene, electrophilic attack
on the remote double bond can occur to eventually generate
(1-methylcyclopropyl)methyl halide (Scheme 9). Noteworthy
here is the observation that only properly alkyl-substituted
y,&unsaturated alkylzirconium precursors undergo cyclization; the alkyl derivative prepared from 2-methyl-I ,5-hexadiene reacts with NBS to yield only the acyclic compound,
6-bromo-2-meth ylhexene.
Scheme 9 .
(3 7%)
[a] s o m e I-chlorooctane is a l s o f o r m e d .
4.3.3. Insertion Reactions
Carbon monoxide does not insert into the benzyl-zirconium
bond of Cp2Zr(CH2C6H5)2
even under forcing reaction conditions (lOO°C, 40atm)[571,and C P ~ Z ~ ( C reacts
H ~ ) ~with CO
under pressure (20°C, 40-80atm) to give an unstable acylalkyl species which rapidly loses CO at 1 atm[58J.In contrast
to these, CO undergoes clean and high-yield insertion (20"C,
1.5 atm) into the C-Zr bond of many alkyl, alkenyl, or y,6unsaturated alkylzirconium(1v)complexes to generate isolable
acylzirconium(1v)species which do not lose CO even under
vacuum[591.Relative rates for CO insertion[601into the C-Zr
bond depend on the nature of the organic substituent : cyclohexyl > n-alkyl
alkenyl. These acyls can in turn be converted, depending on subsequent procedures, into aldehydes,
carboxylic acids, esters, or acyl halides (Scheme 11). An aldehyde is produced by hydrolysis of the RC(0)-Zr bond with
dilute aqueous acid. Treatment of the zirconium acyl with
Scheme 11.
Oxidation of the alkyl carbon-zirconium bond can be
effected by 0, or by various electrophilic oxidizing reagents
such as hydrogen peroxide, tert-butyl hydroperoxide, m-chloroperbenzoic acid, or chromyl chloride. When protic reagents
are used, reaction with the alkylzirconium species yields the
corresponding alcohol directly. For O2 or chromyl chloride
an alkoxide species is produced which on hydrolysis gives
Angew. Chem. Int. Ed. Engl.
1 Vol. 15 ( 1 9 7 6 ) N o . 6
Cleavage of an alkyl-transition metal bond by bromine
NBS gives the acyl bromide. The acyl-zirconium bond can
proceeding with retention of configuration at carbon has been
be cleaved by other oxidative procedures as well. Reaction
when the metal atom can undergo oxidative addiwith aqueous H 2 0 2followed by acidification gives the carboxtion. Cleavage of the C-M bond by Br2 with inversion of
ylic acid, and with Br2 in methanol yields the methyl ester.
configuration at carbon has been noted as we11[64~651,
Alkylchlorobis(cyclopentadienyl)zirconium(tv) complexes
have not been observed to react with carbon d i o ~ i d e [ ~ ~ ~has
~ ~been
] interpreted in terms of initial oxidation of the metal
complex followed by nucleophilic attack at carbon (with inverin contrast to tetrabenzylzirconium(tv)[621,which undergoes
sion) by Br- to give the alkyl bromide. Reaction of bromine
facile reaction with C 0 2 in the molar ratio 1 :2. Ethylene
with (8a) or (8b) gives the alkyl bromide with retention
oxide inserts slowly into the C-Zr bond of linear alkylzirof configuration at carbon. In the complexes ( 8 a ) or (8b)
conium(iv) complexes. This reaction is facilitated by treatment
of the organometallic compound with Ag' in CH,C12 solution
the metal is formally do: hence oxidative addition is not
prior to addition of the ethylene oxide. Nitric oxide[401and
expected and further oxidation will be extremely difficult.
sulfur dioxider4'] react with Cp,ZrMe, to form insertion prodA closed transition state such as that proposed for halogenaucts comprising two equivalents of the electrophilic molecule.
tion of organomercurials[661reasonably accounts for retention
at carbon as observed for electrophilic carbon-metal bond
Scheme 12.
cleavage in these alkyl zirconium derivatives (see Fig. 1). Here
the electrophilicreagent coordinates to zirconium by donation
of a pair ofelectrons to its vacant low-lying
C p , Z r k
I . AgI/CH,CI,
3 H,O+
Fig. 1 . Probable mechanism of C-Zr
4.3.4.Mechanism of C-Zr
Bond Cleavage in (6)
The mechanism of C-Zr bond cleavage in compounds
prepared by hydrozirconation [(6)] has been
determining the stereochemistry of these processes, employing
the NMR technique developed by Whitesides et a1.[641.Diastereomerically pure dideuterated 3,3-dimethylbutylzirconium(1~)
complexes (8 a) and (8 b) were prepared by successive hydrozirconation sequences as shown in Scheme 13. Treatment
of 3,3-dimethyl-l-butyne with ( 5 ) yields the alkenyl derivative
as shown, which with dilute aqueous D2S04 gives the olefin
indicated (no scrambling of the D label was detectable by
NMR). Treatment of this olefin with Cp2Zr(D)C1 (prepared
from CpZZrCl2and LiAID4 in THF) gives the erythro alkyl
derivative (8 a) directly; formation of this diastereomer proves
that Zr-H addition to an olefin proceeds stereospecifically
Scheme 1 3 .
bond cleavage, for instance by Br2.
Carbon monoxide inserts into the C-Zr bond with retention at
as has been observed for all other alkyl-transition metal systems studied thus far[64].Sulfur dioxide also
inserts into the C-Zr bond with retention of stereochemistry[631.It has been reported[401that Cp2Zr(Cl)CH3adds two
equivalents of SO2 to give an insoluble product. However,
careful addition of one equivalent of SO2 to a solution of
( 8 a ) or ( 8 b ) gives a monoinsertion product
as Cp2Zr(Cl)[02S-CHD-CHD-C(CH3)3]with retention
of stereochemistry. This result is in contrast to those[64.681
obtained for other alkyl-transition metal compounds in which
SO2 insertion proceeds with inversion of configuration at
carbon. Nitric oxide also causes electrophilic cleavage of the
C-Zr bond with retention of configuration to give the N nitrosylhydroxylamine
Electrophilic oxygen
reagents such as H z 0 2 or tert-C4H900H transform ( 8 a )
or (8b) into the alcohol with retention of configuration at
carbon. These processes too can be accommodated by the
closed transition state shown in Figure 1. The postulate that
a nucleophilic interaction between the attacking electrophilic
molecule and the Zr atom is necessary to effect C-Zr cleavage
is supported by the observation that whereas poorly electrophilic (yet mildly nucleophilic) CO readily inserts into the C-Zr
bond, the stronger electrophile (but poorer nucleophile) COz
does not react with (6) even under more stringent conditions.
Treatment of (6) with O2 also gives the corresponding
Zr a l k o ~ i d e [ ~Here
~ ] . stereochemical results indicate that one
half of the product is formed with retention at C, while the
other half is formed with racemization. One possible
to account for 0 2 oxidation is shown in Scheme
14, analogous to those proposed for the autoxidation of
or lithium
Angew. Chern. Int. Ed. Engl. 1 Vol. 15 (1976) No. 6
S c h e m e 14.
+ 0,
+ R*[Zr]
R* + . O O [ Z r ] - + R O O [ Z r ]
Table 2. Compatibility of functional groups with the hydrozirconation reagent
(5) ; [Zr] =(q5-C5H,),ZrC1. (An exhaustive study of functional group or
protecting group compatibility has not yet been done.)
Product with H[Zr]
RCHzOH [a]
R-CH20H [a]
R-CHO [a]
+ R*O[Zr]
Re = c h i r a l alkyl g r o u p
R = racemized alkyl group
[ Z r ] = (95-C5H5)2ZrC1
4.3.5.Recovery of an Olefin from (6)
Hydrozirconation of an alkene having an internal double
bond by ( 5 ) yields the terminally functionalized alkyl complex
(6) from which the terminal alkene can be obtained. Isomers
of the starting olefins can be prepared from rearranged alkylboranes by treatment with a high-boiling terminal olefin and
removal by distillation of the desired organic product[71?
This procedure has not yet been successful for (6); the isomerized olefin can be freed neither by treatment with ethylene
(even at high temperature and pressure) nor by donor ligands
(such as pyridine or alkylphosphanes). However, it is possible
to obtain isomerized olefin from (6) through P-hydride
abstraction as has been observed in
Thus treatment[741of a solution of the rearranged alkylzirconium derivative with either trityl chloride or trityl tetrafluoroborate in methylene chloride or benzene at room temperature
gives the corresponding terminal olefin (Scheme 15). The pattern of D labeling in olefins recovered from ( 8 a ) indicates
that this reaction proceeds through P-hydride abstraction by
Ph3C+ with trans stereochemistry.
S c h e m a 15.
[a] Upon hydrolysis
5. Conclusion
The development of ( 5 ) as a reagent for organic synthesis
represents only one entry into organozirconium chemistry.
For example, sterically congested hydride ( 5 ) reacts only
slowly or not at all with hindered olefins; and, in (6), migration
of the metal moiety past a tertiary carbon center is very
slow. Therefore, the development of sterically less-crowded
zirconium hydrides is a worthwhile aim. Direct alkylation
of (6) is not yet possible and the means to accomplish this
transformation also represents an important goal of future
research. Of course, finding new means to effect electrophilic
cleavage of the C-Zr bond in (6) will continue to be of
interest to demonstrate the versatility of these organometallic
Apart from its utility as a technique, the development of
hydrozirconation illustrates two important points :
1) That synthetically useful transition metal based organometallic reactions can be devised through consideration of
organometallic mechanisms.
2) That the introduction of new such procedures will continue to be an exciting aspect of organic synthesis because
of the broad scope and high reactivity of organometallic complexes.
Received: January 19, 1976 [A 112 IE]
German version: Angew. Chem. 88.402 (1976)
4.4. Functional Group Compatibility
Hydride ( 5 ) can reduce several types of functional groups
at a rate competitive with its reaction with an olefin. However,
several common and convenient protecting groups have been
found to be stable toward the new transition metal reagent.
A brief tabulation of functional group reactivity is indicated
in Table 2.
It has been
that CpzZrHz reacts with acetone
to give CpzZr[OCH(CH3)z]z. Hydride ( 5 ) also reduces free
carbonyl groups to Zr alkoxides and nitrile compounds to
zirconium imine complexes (similarspecies have been prepared
from the corresponding lithium
These imine salts
can be hydrolyzed with dilute aqueous acid to give the corresponding aldehydes[77?
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