Synthesis of the tantalum hydride complex (R,R)-[Ta(O2C 20H10 {SiMe3}2-3,3′) 2(H)] and reactivity with aldehydes, ketones, acetylenes, and related substrates: A reagent for the asymmetric hydrogenation of prochiral carbonyl species

Article abstract of DOI:10.1021/om050633s

The tantalum phenyl complex (R,R)-[Ta(O₂C₂₀H ₁₀{SiMe₃}₂-3,3′)₂(Ph)], (R,R)-1, has been prepared from [H₂NEt₂][Ta(O ₂C₂₀H₁₀{SiMe₃}₂-3, 3′}₂Cl₂] and has been employed for the synthesis of the hydride species (R,R)-[Ta(O₂C₂₀H ₁₀{SiMe₃}₂-3,3′)₂(H)], (R,R)-2, by reaction with diisobutylaluminum hydride (DIBAL-H). The reaction proceeds cleanly and in high yield. Compound (R,R)-2 undergoes clean reactions with acetophenone and benzaldehyde-d to give alkoxide complexes that are chiral at the α-carbon. The progress of these reactions was monitored by ¹³C NMR spectroscopy. An X-ray crystal structure of one of the products (R,R,R)/(R,R,S)-[Ta(O₂C₂₀H₁₀{SiMe ₃}₂-3,3′)₂(OCH{CH₃}{Ph})(O= C{CH₃}{Ph})] was obtained, and the catalytic conversion of benzaldehyde-d to benzyl alcohol-d was carried out using (R,R)-2. The hydride complex also reacts with acetylenes to give vinyl metal complexes, with 2,6-dimethylphenyl isocyanide to give an iminoformyl complex, and with allene to yield a labile η³-π-allyl species.

Full text of DOI:10.1021/om050633s

Organometallics 2005, 24, 5759-5766
5759
Synthesis of the Tantalum Hydride Complex
(R,R)-[Ta(O2C20H10{SiMe3}2-3,3′)2(H)] and Reactivity with
Aldehydes, Ketones, Acetylenes, and Related Substrates:
A Reagent for the Asymmetric Hydrogenation of
Prochiral Carbonyl Species†
Charles S. Weinert*
Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078
Phillip E. Fanwick and Ian P. Rothwell
Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907
Received July 26, 2005
The tantalum phenyl complex (R,R)-[Ta(O2C20H10{SiMe3}2-3,3′)2(Ph)], (R,R)-1, has been
prepared from [H2NEt2][Ta(O2C20H10{SiMe3}2-3,3′)2Cl2] and has been employed for the
synthesis of the hydride species (R,R)-[Ta(O2C20H10{SiMe3}2-3,3′)2(H)], (R,R)-2, by reaction
with diisobutylaluminum hydride (DIBAL-H). The reaction proceeds cleanly and in high
yield. Compound (R,R)-2 undergoes clean reactions with acetophenone and benzaldehyde-d
to give alkoxide complexes that are chiral at the R-carbon. The progress of these reactions
was monitored by 13C NMR spectroscopy. An X-ray crystal structure of one of the products
(R,R,R)/(R,R,S)-[Ta(O2C20H10{SiMe3}2-3,3′)2(OCH{CH3}{Ph})(OdC{CH3}{Ph})] was obtained,
and the catalytic conversion of benzaldehyde-d to benzyl alcohol-d was carried out using
(R,R)-2. The hydride complex also reacts with acetylenes to give vinyl metal complexes, with
2,6-dimethylphenyl isocyanide to give an iminoformyl complex, and with allene to yield a
labile η3-π-allyl species.
Introduction
(R,R)-[H2NEt2][Ta(O2C20H10{SiMe3}2-3,3′)2Cl2], which
serves as a precursor for various organometallic species
of the formula (R,R)-[Ta(O2C20H10{SiMe3}2-3,3′)2(R)] (R
) CH2SiMe3, CH2Ph, or c-C5H9).12 We now wish to
report the use of (R,R)-[H2NEt2][Ta(O2C20H10{SiMe3}2-
3,3′)2Cl2] for the preparation of a hydride complex that
contains two resolved binaphthoxide ligands and serves
as a hydrogenation agent for aldehydes, ketones, and
acetylenes.
Mono- and dihydride complexes of the heavy group 5
metals niobium and tantalum with ancilliary aryloxide
ligation have been shown to function as catalysts for
the hydrogenation of unsaturated hydrocarbons.1-5
Previous work has focused on hydrogenation reactions
using species containing achiral aryl ligands, although
(S)-[Ta(O2C20H10-3,3′-{SiMe3}2)(H)(Cl)2(PMe3)2], which
contains a resolved chiral binaphthoxide ligand, has
been recently prepared and has the potential to function
as an asymmetric hydrogenation catalyst.6 Although the
binaphthoxide ligand is an important chiral auxiliary
in complexes of titanium, substantially fewer examples
ofrelatedheavygroup5complexeshavebeenreported.7-11
We have prepared and characterized the salt complex
Results and Discussion
The phenyl complex (R,R)-[Ta(O2C20H10{SiMe3}2-
3,3′)2(Ph)], (R,R)-1, was prepared by the reaction of
PhMgCl with (R,R)-[H2NEt2][Ta(O2C20H10{SiMe3}2-
3,3′)2Cl2] via the method used for the preparation of the
alkyl complexes (R,R)-[Ta(O2C20H10{SiMe3}2-3,3′)2(R)]
(R ) CH2SiMe3, CH2Ph, or c-C5H9), as shown in Scheme
1.12 The identity of (R,R)-1 was confirmed by NMR
spectroscopy which indicates the presence of one phenyl
group attached to the tantalum metal center. Complex
(R,R)-1 was subsequently converted to the hydride
species (R,R)-[Ta(O2C20H10{SiMe3}2-3,3′)2(H)], (R,R)-2,
by reaction with diisobutylaluminum hydride (DIBAL-
H) as shown in Scheme 1. Other methods for the
† Dedicated to the memory of Professor Ian P. Rothwell.
* Corresponding author. E-mail: weinert@chem.okstate.edu.
(1) Rothwell, I. P. J. Chem. Soc., Chem. Commun. 1997, 1331.
(2) Bradley, D. C.; Mehrotra, R. C.; Rothwell, I. P.; Singh, A. Alkoxo
and Aryloxo Derivatives of Metals; Academic Press: New York, 2001.
(3) Mulford, D. R.; Clark, J. R.; Schweiger, S. W.; Fanwick, P. E.;
Rothwell, I. P. Organometallics 1999, 18, 4448.
(4) Parkin, B. C.; Clark, J. R.; Visciglio, V. M.; Fanwick, P. E.;
Rothwell, I. P. Organometallics 1995, 14, 3002.
(5) Visciglio, V. M.; Clark, J. R.; Nguyen, M. T.; Mulford, D. R.;
Fanwick, P. E.; Rothwell, I. P. J. Am. Chem. Soc. 1997, 119, 3490.
(6) Thorn, M. G.; Moses, J. E.; Fanwick, P. E.; Rothwell, I. P. J.
Chem. Soc., Dalton Trans. 2000, 2659.
(7) Brunkan, N. M.; White, P. S.; Gagne, M. R. Angew. Chem., Int.
Ed. 1998, 37, 1579.
(10) Zhu, S. S.; Cefalo, D. R.; La, D. S.; Jamieson, J. Y.; Davies, W.
M.; Hoveyda, A. H.; Schrock, R. R. J. Am. Chem. Soc. 1999, 121, 8251.
(11) Noyori, R. Asymmetric Catalysis in Organic Synthesis; John
Wiley and Sons: New York, 1994.
(12) Weinert, C. S.; Fanwick, P. E.; Rothwell, I. P. Organometallics
2002, 21, 484.
(8) Boyle, T. J.; Eilerts, N. W.; Heppert, J. A.; Takusagawa, F.
Organometallics 1992, 11, 1112.
(9) Totland, K. M.; Boyd, T. J.; Lavoie, G. G.; Davis, W. M.; Schrock,
R. R. Macromolecules 1996, 29, 6114.
10.1021/om050633s CCC: $30.25 © 2005 American Chemical Society
Publication on Web 10/13/2005

5760 Organometallics, Vol. 24, No. 23, 2005
Weinert et al.
Scheme 1
conversion of alkyl species to hydride species previously
employed in our research group, including reaction of
the alkyl with Bu3SnH or pressurization to 1200 psi
with hydrogen gas,4,6 were unsuccessful for the conver-
sion of (R,R)-1 to (R,R)-2. Furthermore, although the
phenyl derivative (R,R)-1 could be converted to (R,R)-
2, the other alkyl species (R,R)-[Ta(O2C20H10{SiMe3}2-
3,3′)2(R)] (R ) CH2SiMe3, CH2Ph, or c-C5H9) did not
yield the hydride under identical reaction conditions.
The fact that only (R,R)-1 can be used for the prepara-
tion (R,R)-2 is likely due to the increased lability of the
phenyl ligand versus the alkyl ligands in these tantalum
complexes, which was demonstrated for the elimination
of RH from (tBu3SiNH)2RTadNSitBu3 complexes.13
Complex (R,R)-2 was identified as a five-coordinate
undergo clean insertion reactions with the prochiral
substances acetophenone and benzaldehyde-d to give
alkoxides and also yielded organometallic species via
reaction with alkynes and isocyanides.
The reaction of (R,R)-2 with acetophenone leads to a
complex mixture of products as shown by 1H NMR
spectroscopy. There are nine sharp features in the
alkylsilyl region: four resonances of equal intensity
appearing between 0.51 and 0.42 ppm, a sharp peak at
0.32 ppm, and an additional set of four resonances of
equal intensity between 0.21 and 0.14 ppm. The peak
at 0.32 ppm is approximately twice the intensity of each
of the other eight peaks. The 13C NMR spectrum of the
product is also extremely complex. To identify the
species formed in this reaction, the progress of the
reaction of (R,R)-2 with acetophenone 13C-enriched at
the carbonyl carbon was monitored by 13C NMR spec-
troscopy, and the reactivity is summarized in Scheme
2.
When the ratio of acetophenone-13C to (R,R)-2 was
1.5:1, the initial spectrum indicated the presence of four
distinct complexes in solution: the six-coordinate spe-
cies (R,R,R)-3 and (R,R,S)-3 and the five-coordinate
species (R,R,R)-3a and (R,R,S)-3a. The two signals for
the coordinated ketone carbon appear downfield at δ
212.0 and 211.3 ppm, while those for the four alkoxide
carbons are observed at δ 83.6, 83.3, 82.7, and 80.3 ppm.
The alkoxide resonances for (R,R,R)-3 and(R,R,S)-3 at
δ 82.7 and 80.3 ppm are shifted slightly upfield relative
to the corresponding five-coordinate complexes due to
the presence of the coordinated ketone. A broad feature
at 200.4 ppm corresponding to free acetophenone-13C
was also observed. Addition of (R,R)-2 to the NMR tube
resulted in the consumption of the free acetophenone-
13C and the disappearance of complexes (R,R,R)-3 and
(R,R,S)-3. The resonances for the alkoxides (R,R,R)-3a
and (R,R,S)-3a shift downfield to δ 85.5 and 84.6 ppm
1
tantalum(V) monohydride complex by H NMR spec-
troscopy and elemental analysis. The singlet resonance
for the hydride proton appears very far downfield at δ
21.5 ppm due to the highly deshielding nature of the
electropositive tantalum(V) metal center. The related
complex [Ta(O2C20H10{SiMe3}2-3,3′)(H)(Cl)2(PMe3)2] ex-
hibits a similar chemical shift for the hydride proton at
δ 22.0 ppm.6 However, while the latter is six-coordinate
and coordinatively saturated, (R,R)-2 contains a vacant
coordination site. Introduction of trimethylphosphine to
a solution of (R,R)-2 results in coordination of the
1
phosphine as shown by H and 31P NMR spectroscopy,
but this ligand is labile and dissociates upon exposure
of a solution of this adduct to vacuum. Dimethylphen-
ylphosphine and other sterically unencumbering phos-
phines exhibit similar effects, but triphenylphosphine
does not coordinate to the metal center. The presence
of a vacant coordination site is advantageous since
complexation of unsaturated species can occur, which
could be followed by insertion of these substrates into
the Ta-H bond. Indeed, complex (R,R)-2 was found to
(13) Schaller, C. P.; Wolczanski, P. T. Inorg. Chem. 1993, 32, 131.

Synthesis of a Tantalum Hydride Complex
Organometallics, Vol. 24, No. 23, 2005 5761
Scheme 2
Table 1. Selected Bond Lengths (Å) and Angles
(deg) for (R,R,R)/(R,R,S)-[Ta(O2C20H10{SiMe3}2-
3,3′)2(OCH(Me)(Ph))(OdC(Me)(Ph)]‚2C6H6,
(R,R,R)-3/(R,R,S)-3
due to the absence of coordinating acetophenone. A new
peak at δ 160.6 ppm also appeared upon addition of
excess (R,R)-2, which corresponds to the tantalum
enolate complex (R,R)-3b. This chemical shift observed
for (R,R)-3b is similar to that observed for metal
enolates of titanium, zirconium, and ruthenium.14-19
A diastereomeric mixture of (R,R,R)-3 and (R,R,S)-3
was obtained on a preparative scale by reaction of
(R,R)-2 with an excess (4 equiv) of nonenriched ac-
etophenone followed by careful evaporation of the
solvent to avoid loss of coordinated acetophenone. An
X-ray crystal structure of (R,R,R)-3/(R,R,S)-3 was ob-
tained, and an ORTEP plot is shown in Figure 1.
Selected bond lengths and angles are collected in Table
1. Complex (R,R,R)-3/(R,R,S)-3 consists of a tantalum
atom surrounded by two resolved chelating binaphthox-
ide ligands as well as a 1-phenylethoxide ligand. A
molecule of acetophenone is coordinated to the tantalum
metal center in a trans-fashion relative to the alkoxide
ligand, and the environment about the tantalum atom
is approximately square bipyramidal. The Ta(1)-O(6)
bond length is 1.806(5) Å, which is typical for a
tantalum-alkoxide bond, while the Ta(1)-O(5) bond
length is 2.205(5) Å to the coordinated acetophenone
molecule.
Ta(1)-O(1)
Ta(1)-O(2)
Ta(1)-O(3)
O(5)-C(50)
1.988(5)
1.998(5)
1.981(5)
1.32(1)
Ta(1)-O(4)
Ta(1)-O(5)
Ta(1)-O(6)
O(6)-C(60)
1.936(6)
2.205(5)
1.806(5)
1.28(1)
O(1)-Ta(1)-O(2)
O(1)-Ta(1)-O(3)
O(1)-Ta(1)-O(4)
O(1)-Ta(1)-O(5)
O(1)-Ta(1)-O(6)
O(2)-Ta(1)-O(3)
O(2)-Ta(1)-O(4)
O(2)-Ta(1)-O(5)
O(5)-C(50)-C(57)
O(6)-C(60)-C(61)
89.4(2) O(2)-Ta(1)-O(6)
91.3(2) O(3)-Ta(1)-O(4)
159.2(2) O(3)-Ta(1)-O(5)
79.7(2) O(3)-Ta(1)-O(6)
101.1(2) O(4)-Ta(1)-O(5)
93.7(2) O(4)-Ta(1)-O(6)
174.3(2) O(5)-Ta(1)-O(6)
87.5(2) O(5)-C(50)-C(51)
89.9(2)
91.0(2)
79.9(2)
99.4(2)
90.2(2)
92.4(2)
177.3(3)
115.7(9)
118.3(9) C(51)-C(50)-C(57) 120.4(8)
118.0(9) O(6)-C(60)-C(67) 119.1(9)
C(61)-C(60)-C(67) 121.8(9)
none ligand are disordered with one another such that
two tantalum atoms with relative occupancies of 0.5 are
present. The C(61)-C(60)-C(67), O(6)-C(60)-C(61),
and O(6)-C(60)-C(67) angles of the alkoxide moiety are
121.8(9)°, 118.0(9)°, and 121.8(9)° (respectively), indicat-
ing the environment about the C(60) atom is ap-
proximately planar. Since the 1-phenylalkoxide ligand
is also disordered between the R- and S-configurations
in a 1:1 ratio, an average structure results with a planar
configuration at C(60) instead of the expected tetrahe-
dral environment. The proton attached to C(60) was not
detected crystallographically as a result of these two
distortions, as each hydrogen position has an occupancy
of 0.25. The geometry of the coordinated acetophenone
molecule is as expected, and the O(5)-Ta(1)-O(6) angle
of 177.3(3)° indicates the 1-phenylalkoxide ligand and
the acetophenone ligand are diposed in an approxi-
mately linear orientation.
There are two types of crystallographic disorder
present in the structure of (R,R,R)-3/(R,R,S)-3. The
1-phenylethoxide ligand and the coordinated acetophe-
(14) Hartwig, J. F.; Bergman, R. G.; Andersen, R. A. Organometallics
1991, 10, 3326.
(15) Gambarotta, S.; Floriani, C.; Chiesi-Villa, A.; Gustani, C. J. Am.
Chem. Soc. 1983, 105, 1690.
(16) Moore, E. J.; Straus, D. A.; Armantrout, J.; Santarsiero, B. D.;
Grubbs, R. H.; Bercaw, J. E. J. Am. Chem. Soc. 1983, 105, 2068.
(17) Straus, D. A.; Grubbs, R. H. J. Am. Chem. Soc. 1982, 104, 5499.
(18) Fagan, P. J.; Manriquez, J. M.; Marks, T. J.; Day, V. W.;
Vollmer, S. H.; Day, C. S. J. Am. Chem. Soc. 1980, 102, 5393.
(19) Sonnenberger, D. C.; Mintz, E. A.; Marks, T. J. J. Am. Chem.
Soc. 1984, 106, 3484.
The reaction of (R,R)-2 with benzaldehyde to give the
corresponding benzyl alkoxide was investigated by NMR

5762 Organometallics, Vol. 24, No. 23, 2005
Weinert et al.
species is favored over the other. When the enantiomer
of (R,R)-2 is used, the reverse pattern can be observed.
Reaction of (S,S)-2 with benzaldehyde-d results in a
similar pattern with two peaks at δ 4.93 and 4.83 in a
ratio of 1:9. Therefore, this process provides one dias-
tereomer in 80% excess, which is greater than that
obtained using Ru-, Os-, Rh-, and Ir-based systems20,21
but less than that obtained with a Ti-based system.22
Titration of (R,R)-2 with the 13C-enriched benzalde-
hyde indicates that a mixture of species is formed as
was found when acetophenone was employed as the
substrate. When the ratio of (R,R)-2 to the benzaldehyde
is large, one peak is observed at δ 78.2 ppm in the 13C
NMR spectrum, but when the ratio is slightly less than
1:1, a second feature is observed at δ 198.6 ppm,
indicating the formation of a six-coordinate species that
contains an alkoxide ligand and a coordinated benzal-
dehyde ligand. This peak increases in intensity and
becomes broadened as subsequent aliquots are added,
indicating that the coordinated benzaldehyde ligand is
in a state of dissociative equilibrium. This is also
Figure 1. ORTEP diagram of (R,R,R)-3/(R,R,S)-3. Ther-
mal ellipsoids are drawn at 50% probability. The 3,3′-SiMe3
groups are not shown for clarity.
1
manifested in the H NMR spectra, as all features are
very broad when even a slight excess of the benzalde-
hyde is employed.
spectroscopy using benzaldehyde-d, unlabeled material,
and material 13C-enriched at the carbonyl carbon. The
three 1H NMR spectra are shown in Figure 2. The
spectrum for the unlabeled material exhibits four lines
due to the diastereotopic protons attached to the R-car-
bon (J ) 8.1 Hz), which are further split into two sets
of four lines when the 13C-enriched benzaldehyde was
employed (JH-H ) 8.1 Hz, JC-H ) 32.7 and 16.8 Hz).
Reaction of (R,R)-2 with benzaldehyde-d resulted in a
1H NMR spectrum exhibiting two resonances at δ 4.93
and 4.83 ppm with an intensity ratio of 9:1. As the
alkoxide ligand is now chiral at the R-carbon, two
diastereomeric complexes are expected, and the greater
intensity for the peak at δ 4.93 ppm indicates one
Addition of 6 mol % of (R,R)-2 to a solution of
benzaldehyde-d in benzene followed by addition of 1
equiv (relative to benzaldehyde-d) of DIBAL-H and
subsequent hydrolysis after 12 h yields a mixture of the
free benzyl alcohol-d and 3,3′-bis(trimethylsilyl)-1,1′-bi-
2,2′-naphthol, which can be separated by preparative
TLC. The benzyl alcohol-d was isolated in 52% yield and
was characterized by mass spectrometry (EI), which
exhibited a peak for the parent molecular ion as well
as fragmentation of the CHO group. Thus, the conver-
sion of benzaldehyde-d to benzyl-1-d alcohol deuterated
at the R-carbon can be carried out catalytically. The
Figure 2. 1H NMR spectra for the reaction of (R,R)-2 with benzaldehyde (bottom), benzaldehyde 13C-enriched at the
carbonyl carbon (middle), and benzaldehyde-d (top).

Synthesis of a Tantalum Hydride Complex
Organometallics, Vol. 24, No. 23, 2005 5763
Scheme 3
product of this reaction was converted to the benzyl
ester of R-methoxy-R-(trifluoromethyl)phenylacetic acid
(MTPA, Mosher’s acid),21,23,24 and the 1H NMR spectrum
was recorded to determine the stereochemical outcome
of hydride transfer reaction.25 The ratio of the R-isomer
to the S-isomer is 93:7, and thus the R-isomer is present
in an 86% enantiomeric excess. This is superior to that
reported for the conversion of benzaldehyde-d to benzyl-
1-d alcohol using a catalyst system generated in situ
from (1R,2S)-(+)-cis-1-amino-2-indanol and (p-cyme-
neRuCl2)2, (p-cymeneOsCl2), (Cp*RuCl2)2, or (Cp*IrCl2)2,
which ranged from 54 to 63%.21 However, the reduction
of benzaldehyde to benzyl-1-d alcohol using Bu3SnD and
a resolved titanium binaphthoxide system resulted in
an ee of 94% of the S-isomer,22 and reduction of
benzaldehyde-d using a chiral ruthenium catalyst in the
presence of tBuOK gave an ee of 98% of the R-isomer.20
Compound (R,R)-2 also reacts with acetylenic species
to give vinyl metal complexes as shown in Scheme 3.
Reaction of (R,R)-2 with phenylacetylene cleanly gives
the phenylethylene tantalum complex (R,R)-4. Reso-
species. However, the resonance at δ 8.51 ppm is no
longer present and the doublet at δ 6.42 ppm has
collapsed to a singlet, indicating that the upfield reso-
nance must arise from the hydride of (R,R)-2.
The reaction of (R,R)-2 with propyne yields a mixture
of two products resulting from addition of the hydride
ligand to either the terminal carbon or the internal
carbon of propyne. In the first complex, (R,R)-5a, the
methyl group is distal to the tantalum and appears as
a doublet at 1.24 ppm (J ) 6.0 Hz). The proton proximal
to the tantalum metal center appears as a doublet of
quartets centered at 5.55 ppm with coupling constants
of 6.0 Hz, resulting from coupling to the methyl group
and 18.0 Hz resulting from coupling to the other proton.
In the second complex, (R,R)-5b, the methyl group is
proximal to the tantalum, giving rise to a singlet at 1.81
ppm. The vinylic protons appear at 5.91 and 4.99 ppm
with the downfield resonance corresponding to the
proton trans to the methyl group. The two complexes
are present in an approximate 4:1 ratio (for (R,R)-5a:
(R,R)-5b), indicating the addition of the hydride to the
internal carbon of propyne is favored.
1
nances in the H NMR spectrum of (R,R)-4 for the two
protons attached to the vinylic carbon atoms appear as
doublets (J ) 18.9 Hz) at δ 8.51 and 6.42 ppm. The
coupling constant of 18.9 Hz indicates the alkene moiety
is present in the E-configuration, and there is no
evidence for the presence of the Z-isomer. A similar
ligand disposition was observed in the reaction of [Ta-
In addition to reactions with acetylenic substrates,
complex (R,R)-2 also undergoes insertion of 2,6-dimeth-
ylphenyl isocyanide into the Ta-H bond to give the η2-
metal iminoformyl complex (R,R)-6 (Scheme 4). The 1H
NMR spectrum of (R,R)-6 exhibits a singlet appearing
far downfield at δ 11.49 ppm attributed to the proton
bound to the nitrogen atom of the iminoformyl moiety.
The dramatic downfield shift is likely attributable to
the interaction of the lone pair of electrons on nitrogen
with the tantalum metal center, resulting in a substan-
t
(OC6H3 Bu2-2,6)2(H)2Cl(PMePh2)] with phenylacetylene,
which yields a cyclometalated product also containing
an ethenyl group in the E-configuration.3
The downfield resonance in the 1H NMR spectrum of
(R,R)-4 can be attributed to the proton that is proximal
to the metal center, while the upfield resonance arises
from the terminal proton. The hydride ligand of (R,R)-2
is transferred from the tantalum atom to the terminal
position, as shown by a deuterium-labeling experiment.
Reaction of (R,R)-2 with phenylacetylene-d6 gives a 1H
NMR spectrum similar to that obtained for the protio
(20) Yamada, I.; Noyori, R. Org. Lett. 2000, 2, 3425.
(21) Faller, J. W.; Lavoie, A. R. Organometallics 2002, 21, 3493.
(22) Keck, G. E.; Krishnamurthy, D. J. Org. Chem. 1996, 61, 7638.
(23) Dale, J. A.; Mosher, H. S. J. Am. Chem. Soc. 1973, 95, 512.
(24) Dale, J. A.; Dull, D. L.; Mosher, H. S. J. Org. Chem. 1969, 34,
2543.
(25) Wolfe, S.; Yang, K.; Weinberg, N.; Shi, Z.; Hsieh, Y.-H.; Sharma,
R. D.; Ro, S.; Kim, C.-K. Chem. Eur. J. 1998, 4, 886.

5764 Organometallics, Vol. 24, No. 23, 2005
Weinert et al.
Scheme 4
1-d alcohol, giving the R-isomer in an 86% enantiomeric
excess. Complex (R,R)-2 also reacts with phenylacetyl-
ene and propyne to give the corresponding vinyl metal
complexes and with 2,6-dimethylphenyl isocyanide to
give an iminoformyl complex. This species also hydro-
genates allene, but the resulting product is labile and
cannot be obtained on a preparative scale. Although
(R,R)-2 reacts readily with these substrates, it is un-
fortantely unreactive toward alkenes. Reactions of
(R,R)-2 with styrene and butadiene occurred only very
slowly at room temperature.
Experimental Section
General Procedures. All manipulations were carried out
using standard Schlenk line, glovebox, and syringe tech-
niques.31 The compound [H2NEt2][Ta(O2C20H10{SiMe3}2-3,3′)2-
Cl2] was prepared according to a previously published proce-
dure.12 Reagents were purchased from Aldrich and used
without further purification. Solvents were purified using an
Innovative Technologies solvent purification system. 1H NMR
spectra were recorded at 300 MHz and referenced to residual
protio solvent. 13C NMR spectra were recorded at 125.9 MHz
and referenced to the solvent. Elemental analyses and EI mass
spectrometry were carried out in-house at Purdue University.
Preparation of [Ta(O2C20H10{SiMe3}2-3,3′)2(C6H5)], (R,R)-
1. To a suspension of [H2NEt2][Ta(O2C20H10{SiMe3}2-3,3′)2Cl2]
(4.95 g, 4.18 mmol) in benzene (50 mL) was added a solution
of PhMgCl (2.0 M in THF) via syringe. The reaction mixture
became homogeneous and was subsequently stirred for 3 h,
after which time a white precipitate was present. The volatiles
were removed in vacuo, and the resulting solid was suspended
in 20 mL of benzene. Filtration through Celite and evaporation
of the solvent afforded 3.41 g (74%) of (R,R)-1 as an orange
solid. 1H NMR (C6D6, 25 °C, 300 MHz): δ 8.18 (s, 2 H, 4,4′-H),
7.91 (s, 2 H, 4,4′-H), 7.64 (pseudo q, 4 H, 6,6′-H), 7.14-6.74
(m, 17 H, aromatics), 0.58 (s, 18 H, Si(CH3)3), 0.06 (s, 18 H,
Si(CH3)3) ppm. 13C NMR (C6D6, 25 °C, 125.7 MHz): δ 161.7,
161.4, 142.0, 138.2, 137.7, 137.5, 137.1, 131.4, 131.1, 130.3,
129.7, 129.4, 128.9, 127.8, 127.5, 127.0, 126.9, 126.0, 124.8,
124.2, 25.1, 21.8, 1.35, -0.3 ppm.
tial deshielding effect. The 13C NMR resonance for the
carbon bound to the tantalum metal center also appears
downfield at δ 239.5 ppm. This value is similar to that
found for the cyclometalated tantalum iminoacyl species
t
t
[Ta(OC6H3 Bu2-2,6)(OC6H3 BuCMe2CH2-η2-CdNxy)(Ph-
CHdCH-η2-CdNxy)Cl], which appears at δ 243.0 ppm,3
although other tantalum η2-iminoacyl complexes exhibit
a variety of chemical shifts for this carbon atom,
depending on the nature of the ligands attached to the
metal center.26,27
The reaction of (R,R)-2 with allene was also investi-
gated. An atmosphere of allene was introduced into a
NMR tube containing a solution of (R,R)-2 in C6D6, and
an immediate reaction was observed resulting in the
allyl complex (R,R)-7. A doublet at δ 3.48 ppm (J ) 10.8
Hz) and a pentet at δ 6.18 ppm (J ) 10.8 Hz) were
observed in the 1H NMR spectrum of (R,R)-7, corre-
sponding to the terminal protons and central proton,
respectively. The 1H NMR spectrum indicates that the
allyl ligand is η3-bound rather than η1-bound, which
would result in four distinct resonances.28 Furthermore,
the presence of one doublet for the terminal protons
indicates the allyl moiety undergoes free rotation at
room temperature. The process is extremely fast, and
two distinct doublets for the syn- and anti-protons do
not appear even at - 80 °C.29,30 Attempts to synthesize
(R,R)-7 on a preparative scale were not successful
because the complex is not stable in the absence of an
allene atmosphere. Introduction of a nitrogen atmo-
sphere results in rapid decomposition of the product.
This observation, in conjunction with the spectroscopic
evidence, suggests that the allyl ligand is only weakly
bound to the tantalum metal center.
Preparation of [Ta(O2C20H10{SiMe3}2-3,3′)2(H)], (R,R)-
2. To a solution of (R,R)-1 (1.15 g, 1.03 mmol) in benzene (10
mL) was added a solution of diisobutylaluminum hydride (1.27
mL, 1.0 M in hexane, 1.27 mmol). The solution was stirred
for 2 h, after which time the solvent was removed in vacuo.
The resulting solid was suspended in 10 mL of hexane, and
the suspension was filtered. The filter cake was washed with
5 × 5 mL of hexane and dried in vacuo to yield 0.85 g (79%) of
(R,R)-2 as a gray solid. IR (Nujol mull): 1633.8 cm-1 (νTa-H).
1H NMR (C6D6, 25 °C, 300 MHz): δ 21.5 (s, 1 H, Ta-H), 8.17
(s, 2 H, 4,4′-H), 8.02 (s, 2 H, 4,4′-H) 7.66 (pseudo q, 4 H, 6,6′-
H), 7.19-6.77 (m, 12 H, aromatics), 0.44 (s, 18 H, Si(CH3)3),
0.11 (s, 18 H, Si(CH3)3) ppm. 13C NMR (C6D6, 25 °C, 125.7
MHz): δ 159.9, 159.6, 138.5, 137.6, 136.0, 135.5, 131.7, 131.3,
130.8, 128.9, 127.7, 127.5, 125.4, 124.7, 119.1, 116.7, 23.1, 14.7,
1.3, -0.5 ppm. Anal. Calcd for C52H57O4Si4Ta: C, 60.09; H,
5.53. Found: C, 60.04; H, 5.85.
Conclusions
The tantalum bis(binaphthoxide) hydride complex
(R,R)-2 can be prepared in excellent yield in two steps
from [H2NEt2][Ta(O2C20H10{SiMe3}2-3,3′)2Cl2]. This ma-
terial reacts readily with prochiral ketones to give metal
complexes containing chiral alkoxide ligands and serves
for the catalytic conversion of benzaldehyde-d to benzyl-
Preparation of [Ta(O2C20H10{SiMe3}2-3,3′)2(OCH(Me)-
(Ph))(OdC(Me)(Ph)], (R,R,R)-3/(R,R,S)-3. To a solution of
(R,R)-2 (0.53 g, 0.51 mmol) in benzene (15 mL) was added
acetophenone (0.50 mL, 0.51 g, 4.2 mmol). The resulting deep
red solution was stirred for 18 h, and the solvent was removed
in vacuo. The resulting oily red solid was redissovled in
benzene (5 mL), and the solution was layered with hexane (15
mL), resulting in the formation of a brown precipitate. The
solution was filtered and the volatiles were removed from the
(26) Clark, J. R.; Fanwick, P. E.; Rothwell, I. P. Organometallics
1996, 15, 3232.
(27) Thorn, M. G.; Fanwick, P. E.; Rothwell, I. P. Organometallics
1999, 18, 4442.
(28) Palmer, G. T.; Basolo, F. J. Am. Chem. Soc. 1985, 107, 3122.
(29) Brisdon, B. J.; Edwards, D. A.; White, J. W.; Drew, M. G. B. J.
Chem. Soc., Dalton Trans. 1980, 2129.
(30) McClellan, W. R.; Hoehn, H. H.; Cripps, H. N.; Muetterties, E.
L.; Howk, B. W. J. Am. Chem. Soc. 1961, 83, 1601.
(31) Shriver, D. F.; Drezdzon, M. A. The Manipulation of Air-
Sensitive Comounds, 2nd ed.; John Wiley and Sons: New York, 1986.

Synthesis of a Tantalum Hydride Complex
Organometallics, Vol. 24, No. 23, 2005 5765
1
filtrate in vacuo to yield a viscous red liquid. To this was added
hexane (5 mL), and the mixture was heated until all of the oil
dissolved. The solution was allowed to cool overnight, after
which time a dark oil separated from the solution. The mother
liquor was decanted, and the volatiles were removed in vacuo,
resulting in a red oil, which slowly crystallized. The resulting
solid was identified to be a diasteromeric mixture of (R,R,R)-3
and (R,R,S)-3. Yield: 0.41 g (63%).
3 h. H NMR (C6D6, 25 °C, 300 MHz): δ 8.22 (s, 2 H, 4,4′-H),
8.09 (s, 2 H, 4,4′-H), 7.78-7.68 (m, aromatics, 6 H), 7.76 (d, J
) 8.1 Hz, 2 H, 6,6′-H), 7.70 (d, J ) 8.1 Hz, 2 H, 6,6′-H), 6.42
(s, 1 H, TaCDdCHPh), 0.33 (s, 18 H, Si(CH3)3), 0.20 (s, 18 H,
Si(CH3)3) ppm.
Preparation of [Ta(O2C20H10{SiMe3}2-3,3′)2(C(H)dC(H)-
CH3)], (R,R)-5a/(R,R)-5b. Compound (R,R)-2 (0.32 g, 0.31
mmol) was dissolved in benzene (15 mL) in a Schlenk flask.
The solution was freeze-thaw-degassed three times, then an
atmosphere (730 Torr) of propyne was introduced into the
flask. The solution was allowed to thaw and was subsequently
stirred for 3 h. The solution was filtered through Celite to
remove a small amount of precipitate, and the volatiles were
removed in vacuo to yield a yellow solid, which was redissolved
in benzene (5 mL). The solution was layered with hexane (10
mL) to give a yellow solid, which was isolated and washed with
hexane (3 × 5 mL) to yield (R,R)-5 as yellow crystals (0.16 g,
47%). Compound (R,R)-5 is present as a mixture of the E- and
Z-isomers. For (R,R)-5a: 1H NMR δ 8.23 (s, 2 H, 4,4′-H), 8.04
(s, 2 H, 4,4′-H), 7.77-7.65 (m, X H, aromatics), 7.19-6.78 (m,
X H, aromatics), 5.55 (doublet of quartets, Ta-C(H)dC(H)-
CH3), J ) 6.0 and 18.0 Hz), 1.23 (d, J ) 6.0 Hz, Ta-C(H)d
C(H)CH3), 0.44 (s, 18H, Si(CH3)3), 0.17 (s, 18H, Si(CH3)3) ppm.
For (R,R)-5b: 1H NMR δ 8.20 (s, 2 H, 4,4′-H), 7.98 (s, 2 H,
4,4′-H), 7.77-7.65 (m, X H, aromatics), 7.19-6.78 (m, X H,
aromatics), 5.91 (s, 1 H, Ta-CH3dCH2), 4.99 (s, 1 H, Ta-CH3d
CH2), 1.81 (s, 3H, Ta-CH3dCH2), 0.49 (s, 18H, Si(CH3)3), 0.14
(s, 18H, Si(CH3)3) ppm. Anal. Calcd for C55H61O4Si4Ta: C,
61.20; H, 5.70. Found: C, 60.93; H, 5.75.
Preparation of [Ta(O2C20H10{SiMe3}2-3,3′)2(C(H)dNC6-
H2Me2-2,6)], (R,R)-6. To a solution of 0.43 g (0.41 mmol) of
(R,R)-2 in benzene (15 mL) was added a solution of 2,6-
dimethylisocyanide (0.055 g, 0.42 mmol) in benzene (5 mL).
The solution became dark red and was stirred 14 h. The
volatiles were removed, and the resulting red solid was
redissolved in benzene (5 mL). This solution was layered with
hexane (10 mL), resulting in the precipitation of a gray solid.
The solution was filtered, and the volatiles were removed from
the filtrate, yielding 0.37 g (77%) of (R,R)-6 as a red solid. 1H
NMR (C6D6, 25 °C, 300 MHz): δ 11.5 (s, 1 H, Ta-C(H)dNxy),
8.14 (s, 2 H, 4,4′-H), 7.81 (s, 2 H, 4,4′-H), 7.74 (d, J ) 8.1 Hz,
2 H, 6,6′-H), 7.44 (d, J ) 8.1 Hz, 2 H, 6,6′-H), 7.18-6.29 (m,
15 H, aromatics), 2.21 (s, 6 H, o-CH3), 0.29 (s, 18 H, Si(CH3)3),
0.25 (s, 18 H, Si(CH3)3) ppm. 13C NMR (C6D6, 25 °C, 125.7
MHz): δ 239.5 (Ta-CdNxy), 163.2 (TaOC), 160.7 (TaOC),
152.8 (ipso-CN), 145.8, 141.7, 140.9, 137.0, 136.4, 132.5, 131.8,
130.6, 129.7, 129.5, 129.3, 129.1, 128.9, 128.3, 127.9, 127.4,
127.2, 126.7, 123.8, 123.4, 122.6, 117.8, 117.6, 36.9 (o-CH3),
36.3 (o-CH3), 1.0 (-Si(CH3)3), 0.8 (-Si(CH3)3), -0.9 (-Si(CH3)3),
-0.8 (-Si(CH3)3) ppm. Anal. Calcd for C61H66NO4Si4Ta: C,
62.59; H, 5.68. Found: C, 62.48; H, 6.34.
Reaction of (R,R)-2 with Benzaldehyde. A solution of
(R,R)-2 (0.060 g, 0.058 mmol) in C6D6 (0.5 mL) was prepared
and placed in an NMR tube. To this solution was added
benzaldehyde (5.9 µL, 0.059 mmol) via microliter syringe. The
tube was shaken, and the progress of the reaction was
1
monitored by H NMR spectroscopy.
Reaction of (R,R)-2 with Benzaldehyde-d. A solution of
(R,R)-2 (0.080 g, 0.076 mmol) in C6D6 (0.5 mL) was prepared
and placed in an NMR tube. To this solution was added
benzaldehyde-d (7.9 µL, 0.078 mmol) via microliter syringe.
The tube was shaken, and the progress of the reaction was
1
monitored by H NMR spectroscopy.
Titration of (R,R)-2 with Benzaldehyde-carbonyl-13C.
A solution of (R,R)-2 (0.15 g, 0.14 mmol) in C6D6 (0.5 mL) was
prepared and placed in an NMR tube. To this solution was
added 10 equiv of benzaldehyde-carbonyl-13C (140 µL, 1.37
mmol) in aliquots of 7.0 µL. The progress of the reaction was
monitored using 13C NMR spectroscopy.
Catalytic Preparation of Benzyl Alcohol-d Using (R,R)-
2. To a solution of benzaldehyde-d (0.50 mL, 4.92 mmol) in
benzene (15 mL) was added a solution of (R,R)-2 (0.32 g, 0.31
mmol) in benzene (5 mL) followed by a 1.0 M solution of
DIBAL-H (5.0 mL, 0.50 mmol) in hexane. The reaction mixture
was stirred for 12 h under N2. The flask was opened in air,
and deionized water (10 mL) was added, resulting in the
formation of a white solid. The suspension was filtered, and
the filter cake was washed with hexane (5 × 5 mL). The
organic layer was separated, and the aqueous phase was
washed with hexane (3 × 5 mL). The combined organic phases
were dried over anhydrous MgSO4 and filtered, and the
volatiles were removed in vacuo to yield a yellow oil. The
product was purified by preparative thin-layer chromatogra-
phy (95:5 hexane/ethyl acetate eluent). The band containing
the product was scraped off, and the product was extracted
from the silica using acetone, which was subsequently removed
in vacuo to yield benzyl alcohol-d (0.28 g, 54%) as a pale yellow
oil. 1H NMR (C6D6, 25 °C, 300 MHz): δ 7.17-7.07 (m, 5 H,
aromatics), 4.29 (s, 1 H, PhCHDOH), 3.12 (br s, 1 H, -OH).
13C NMR (C6D6, 25 °C, 125.7 MHz): δ 140.9 (ipso-C), 128.4
(meta-C), 127.4 (para-C), 126.7 (ortho-C), 65.2 (t, JC-D ) 26.1
Hz, Ph-CHD(OH)). MS (EI): m/z 108 (M+), 79 (M+ - CHO).
Preparation of [Ta(O2C20H10{SiMe3}2-3,3′)2(CHdCHC6-
H5)],(R,R)-4. To a solution of (R,R)-2 (0.20 g, 0.19 mmol) in
benzene (10 mL) was added phenylacetylene (0.030 mL, 0.028
g, 0.32 mmol) via syringe. The reaction mixture became
reddish in color and was stirred for 6 h. The volatiles were
removed in vacuo to yield 0.16 g (73%) of (R,R)-5 as an orange
Reaction of (R,R)-2 with Allene. A solution of (R,R)-2
(0.075 g, 0.072 mmol) in C6D6 (0.7 mL) was placed in a J.
Young NMR tube. The solution was freeze-thaw-degassed
three times, and then 740 Torr of allene was introduced into
the tube. The solution was allowed to thaw, and the tube was
shaken. 1H NMR: δ 8.23 (s, 2 H, 4,4′-H), 8.02 (s, 2 H, 4,4′-H),
7.70 (pseudo q, 4 H, 6,6′-H), 7.18-6.81 (m, 10 H, aromatics),
6.18 (p, J ) 10.8 Hz, 1 H CH2CHCH2), 3.48 (d, J ) 10.8 Hz,
4H, CH2CHCH2), 0.54 (s, 18 H, Si(CH3)3), 0.15 (s, 18 H, Si-
(CH3)3) ppm.
X-ray Structure Determination. Crystal data and data
collection parameters are contained in Table 2. A suitable
crystal was mounted on a glass fiber in a random orientation
under a cold stream of dry nitrogen. Preliminary examination
and final data collection were performed with Mo KR radiation
(λ ) 0.71073 Å) on a Nonius KappaCCD. Lorentz and
polarization corrections were applied to the data.32 An empiri-
1
solid. H NMR (C6D6, 25 °C, 300 MHz): δ 8.51 (d, J ) 18.9
Hz, 1 H, TaCHdCHPh) 8.22 (s, 2 H, 4,4′-H), 8.09 (s, 2 H, 4,4′-
H), 7.78-7.68 (m, aromatics, 6 H) 7.55-6.85 (m, aromatics,
15 H), 6.42 (d, J ) 18.9 Hz, 1 H, TaCHdCHPh), 0.33 (s, 18 H,
Si(CH3)3), 0.20 (s, 18 H, Si(CH3)3) ppm. 13C NMR (C6D6, 25
°C, 125.7 MHz): δ 189.9 Ta(CHdCHPh), 161.1 (TaOC), 160.3
(TaOC), 148.0, 137.5, 137.2, 136.6, 135.6, 135.5, 132.3, 131.2,
131.1, 130.9, 130.2, 129.7, 129.0, 128.8, 128.7, 128.5, 127.6,
127.6, 127.4, 127.2, 127.1, 127.0, 124.7, 124.3, 83.9, 77.8, -2.9,
-5.5 ppm. Anal. Calcd for C60H63O4Si4Ta: C, 63.13; H, 5.56.
Found: C, 62.35; H, 5.53.
Preparation of [Ta(O2C20H10{SiMe3}2-3,3′)2(CDdCHC6-
D5)]. To a solution of (R,R)-2 (0.070 g, 0.067 mmol) in benzene-
d6 (0.5 mL) in an NMR tube was added phenylacetylene-d6
(10 µL, 0.010 g, 0.094 mmol) via microliter syringe. The tube
was shaken and then allowed to sit at room temperature for
(32) McArdle, P. C. J. Appl. Crystallogr. 1996, 239, 306.

5766 Organometallics, Vol. 24, No. 23, 2005
Weinert et al.
Table 2. Crystallographic Data for Compound
cluded in the refinement but restrained to ride on the atom to
which they are bonded. The structure was refined in full-
matrix least-squares, where the function minimized was
(R,R,R)-3/(R,R,S)-3‚2C6H6
formula
space group
a, Å
C80H84O6Si4Ta
2
2
∑w(|Fo| - |Fc| )2 and the weight w is defined as w ) 1/{σ2-
P212121 (#19)
(Fo ) + (0.0585P)2 + 1.4064P} where P ) (Fo + 2Fc2)/3.
Scattering factors were taken from the International Tables
for Crystallography.35 Refinement was performed on a Al-
phaServer 2100 using SHELX-97.36 Crystallographic drawings
were done using the programs ORTEP-3.37
2
2
15.9939(7)
b, Å
17.5689(4)
c, Å
25.6582(9)
R, deg
â, deg
γ, deg
V, Å3
Z
90
90
90
7209.9(4)
Supporting Information Available: Crystallographic
data for (R,R,R-3)/(R,R,S-3) in .cif format. This material is
available free of charge via the Internet at http://pubs.acs.org.
4
F
calc, g cm-3
1.322
temperature, K
150
radiation (wavelength)
R
Mo KR (0.71073 Å)
0.090
OM050633S
Rw
0.118
(34) Beurskens, P. T.; Admirall, G.; Beurskens, G.; Bosman, W. P.;
Garcia-Granda, S.; Gould, R. O.; Smits, J. M. M.; Smykalla, C. The
DIRDIF92 Program System, Technical Report; Crystallography Labo-
ratory, University of Nijmegen: The Netherlands, 1992.
(35) International Tables for Crystallography, Vol. C; Kluwer
Academic Publishers: Dordrecht, The Netherlands, 1992; Tables
4.2.6.8 and 6.1.1.4.
(36) Sheldrick, G. M. SHELXS97, A Program for Crystal Structure
Refinement; University of Gottingen: Germany, 1997.
(37) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.
cal absorption correction using SCALEPACK was applied.33
Intensities of equivalent reflections were averaged. The struc-
ture was solved using the structure solution program PATTY
in DIRDIF92.34 The remaining atoms were located in succeed-
ing difference Fourier syntheses. Hydrogen atoms were in-
(33) Otwinowski, Z.; Minor, W. Methods Enzymol. 1996, 276.