Stereoselective hydrosilylation of terminal alkynes catalyzed by [Cp*IrCl2]2: A computational and experimental study

Article abstract of DOI:10.1021/om0609158

The hydrosilylation of terminal alkynes is catalyzed by [Cp*IrCl ₂]₂ to afford selectively the β-(Z)-vinylsilanes in high yields. A catalytic cycle based on an Ir(III)-Ir(V) redox process is proposed.

Full text of DOI:10.1021/om0609158

Organometallics 2007, 26, 1157-1160
1157
Stereoselective Hydrosilylation of Terminal Alkynes Catalyzed by
[Cp*IrCl2]2: A Computational and Experimental Study
Venugopal Shanmugham Sridevi, Wai Yip Fan, and Weng Kee Leong*
Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Singapore 117543
ReceiVed October 6, 2006
The hydrosilylation of terminal alkynes is catalyzed by [Cp*IrCl2]2 to afford selectively the â-(Z)-
vinylsilanes in high yields. A catalytic cycle based on an Ir(III)-Ir(V) redox process is proposed.
Scheme 1
Introduction
Transition metal-catalyzed hydrosilylation of alkynes remains
an area of intense research interest, as it provides a simple and
direct means of producing vinylsilanes, which are widely used
intermediates in organic synthesis.1 With terminal alkynes, there
are three possible products (Scheme 1).2
The thermodynamically more stable â-(E) vinylsilane is
usually formed as the major product in most of the transition
metal-catalyzed reactions. The most active and widely used
catalysts for the selective formation of â-(E) vinylsilanes are
platinum-based.3 The selective formation of the â-(Z) vinylsilane
is regarded as much more challenging, and it has considerable
utility to synthetic organic chemistry.4 Rhodium catalysts have
been reported for the selective formation of both E- and
Z-vinylsilanes.5,6 High selectivity for â-(Z)-vinylsilanes has also
been reported with ruthenium4,7 and iridium catalysts.8
One of the most interesting catalysts is [Cp*RhCl2]2, 1a,
which was reported to afford very high stereoselectivity for the
â-(Z) vinylsilanes in the hydrosilylation of phenyl acetylene,6
as it is fairly easily obtainable. In the course of our investigations
into the chemistry of organoiridium complexes, we have
discovered that [Cp*IrCl2]2, 1, is a very efficient catalyst for
the hydrosilylation of terminal alkynes. We thought that
investigations into the catalytic efficiency of the iridium
analogue may be useful for mechanistic studies. Furthermore,
there is also the possibility of different or improved efficiency
on replacement of Rh with Ir, as exemplified by the Cativa
versus the Monsanto processes.9 We report our findings here,
together with the mechanistic studies that we have carried out.
Results and Discussion
The catalytic efficiency of 1 for the hydrosilylation of terminal
alkynes is shown in Table 1. Catalyst 1 exhibits remarkably
high â-(Z)-selectivity under mild reaction conditions; neither R
nor â-(E) isomers were observed. This result was quite similar
to that reported for 1a, although we did not observe any activity
for the reaction of Et3SiH with Me3SiCCH or that between the
silane (EtO)3SiH and the alkynes PhCCH or nBuCCH. Although
a trace of the other isomers was also reported in the hydrosi-
lylation of phenyl acetylene with 1a, we have found that this
was due to subsequent isomerization of the initial product. Thus
we have found that for short reaction times (<0.5 h) the initial
product of the reaction between PhCCH and Et3SiH is the â-(Z)
vinylsilane, but over 24 h a mixture of the â-(Z) and â-(E)
isomers (12% and 71%, respectively) together with the R isomer
(<1%) was obtained. If the reaction was carried out at 80 °C,
the main product was the â-(E) isomer (99%), with a trace of
the other two isomers. This isomerization was not observed in
all cases however; for instance, even at 80 °C the reaction
between CyCCH and Et3SiH afforded only the corresponding
â-(Z) vinylsilane after 2.5 h.
(1) Marciniec, B. ComprehensiVe Handbook on Hydrosilylation; Per-
gamon: Oxford, 1992; p 130.
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10.1021/om0609158 CCC: $37.00 © 2007 American Chemical Society
Publication on Web 01/27/2007

1158 Organometallics, Vol. 26, No. 5, 2007
SrideVi et al.
Table 1. Hydrosilylation of Terminal Alkynes
Catalyzed by 1
Scheme 3
S/no.
alkyne
silane
yield of 2 (%)a
isomerizationb
1
2
3
Ph-CtC-H
Cy-CtC-H
nBu-CtC-H
tBu-CtC-H
Ph-CtC-H
Cy-CtC-H
nBu-CtC-H
HSiEt3
HSiEt3
HSiEt3
HSiEt3
HSiPh3
HSiPh3
HSiPh3
96 (92)
93 (91)
92 (89)
13 (11)
97 (94)
97 (93)
91 (89)
y
n
y
y
n
n
n
4
5c
6c
7c
a Determined by NMR; isolated yields in parentheses. b A mixture of
isomers was formed on prolonged standing. c Comparable yields were
obtained with 1:2 alkyne:silane ratio.
Scheme 2
The equilibrium between 1 and its solvent-stabilized mono-
meric species has been suggested by earlier workers.11 Although
not shown in Scheme 3, the dimer to monomer reaction from
[CpIrCl2]2 to CpIrCl2 has a large ∆G° (118 kJ mol-1), consistent
with an equilibrium that normally lies toward the dimeric
species.
The first step of the catalytic cycle probably involves metal
insertion into the Si-H bond; we have found no apparent
reaction between 1 and PhCCH under scrupulously dry condi-
tions, while 1 reacted with an equimolar amount of Ph3SiH to
afford the Ir(V) species [Cp*Ir(Cl)(H)2(SiPh3)], 3; this com-
pound and its SiEt3 analogue have been reported previously.11c,12
Compound 3 is related to the catalytic intermediate [Cp*Ir(Cl)2-
(H)(SiPh3)], B; addition of an alkyne to 3 (formed in situ)
regenerated 1 together with the formation of 2. The conversion
of an analogue of B into an analogue of 3 has recently been
reported,11c so 3 is probably the observable form of B. We have
ruled out 3 being the intermediate B, as formation of 3 requires
elimination of one of the chloride ligands, but 1 is recoverable
after the reaction.
What is more uncertain is how the alkyne is involved in the
subsequent step (from B to D). The possibility that HCl
elimination occurred prior to binding of the alkyne to the metal
center was ruled out since carrying out the reaction in the
presence of D2O did not lead to any incorporation of deuterium
in the product; the eliminated HCl would have to be reincor-
porated in subsequent steps of the cycle. That there is no HCl
elimination is further corroborated by the observation that the
reaction was unaffected by the presence of [PPN]Cl (5 equiv
with respect to 1). It has been suggested that for the rhodium
We have also sought to investigate the mechanism for the
reaction. Direct monitoring of the reaction by 1H NMR
spectroscopy was futile; the only observable resonances were
those assignable to the reactants and the final vinylsilane
product. We have thus resorted to studying the catalytic cycle
indirectly and by computational means. The proposed mecha-
nism is shown in Scheme 2, and our computational results, with
the computed reaction free energies given in kJ mol-1, are
summarized in Scheme 3. The computational study was carried
out at the Moeller-Plesset MP2 level of theory together with
the LANL2DZ basis set. This level of theory has been shown
to be of utility in studies of catalysis by organometallic
systems.10 To minimize computation time, we have chosen a
model system that comprised CpIrCl2 as the catalytically active
species, with Me3SiH and MeCCH as the reacting silane and
alkyne, respectively. All except one step are associated with
negative free energy changes.
(10) (a) Martin, M.; Sola, E.; Tejero, S.; Andres, J. L.; Oro, L. A. Chem.-
Eur. J. 2006, 12, 4043. (b) Chung, L. W.; Wu, Y.-D. J. Theor. Comput.
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ciniec, B.; Chadyniak, D. J. Mol. Catal. A 2005, 237, 17. (d) Branchadell,
V.; Crevisy, C.; Gree, R. Chem.-Eur. J. 2004, 10, 5795. (e) Sakaki,
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1978, 617. (b) Mirabelli, M. G. L.; Sneddon, L. G. J. Am. Chem. Soc. 1998,
110, 449. (c) Esteruelas, M. A.; Ferna´ndez-Alvarez, F. J.; Lo´pez, A. M.;
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2063.

StereoselectiVe Hydrosilylation of Terminal Alkynes
Organometallics, Vol. 26, No. 5, 2007 1159
analogue6 the reaction involved the formation of a vinylidene,
i.e., a RhdCdC species, in accord with that proposed for a
related iridium species.8c However, we deem this unlikely, as
we have found that the reaction of PhCCD with Et3SiH afforded
PhCHdCDSiEt3 only, indicating that there was no tC-H bond
cleavage. The reaction of PhCCH with Et3SiD afforded only
PhCDdCHSiEt3, indicating clearly the source of the benzylic
hydrogen as the silane. We would therefore like to propose an
intermediate such as C, involving ring slippage of the Cp*.
Although relatively uncommon, ring slippage of the Cp* has
been proposed previously.13 This is also the most endoergic step;
the positive free energy computed for this step is consistent with
ring slippage.
The path from C to D is critical, as it accounts for the
stereochemistry of the final product. It has been suggested for
a related iridium system that the initial product is the isomer of
D in which the R and SiR′3 groups are trans, which then
isomerized to the cis isomer.8c However, in our case, we have
found by modeling that it is not possible to form a stable trans
isomer for D. Instead, we believe that C leads directly to the
proposed cis isomer. This proposal is essentially the same as
that of Trost for the ruthenium system [Cp*Ru(NCCH3)3]+ 14
and is in better agreement than the alternative involving an
isomerization step (see above).15 In his computational study,14b
Trost has found that silyl or hydrogen migration may occur. In
our case, hydrogen migration to form the corresponding anti
addition product (in which the migrated hydrogen is trans to
the iridium) [Cp*Ir(Cl)2(SiR′3)(CHdCHR)], D′, gave rise to a
steric problem similar to that for the trans isomer of D
mentioned above. We therefore believe that in our system silyl
migration is favored.
The transition state for this step has been computed and is
shown in Figure 1. It lies ∼75 kJ mol-1 above C. The vinyl
group is not planar, with the Ir-C-C-H dihedral angle at 110°.
Part of the reaction coordinate is also depicted, and it corre-
sponds to the hydrogen atom swaying (vibrating) perpendicularly
to the C-H bond. There is therefore the possibility of the
transition state leading to either isomer. As presented above,
the trans product is sterically hindered; thus the reaction should
proceed to form the cis isomer.
Figure 1. Structure of the computed transition state for C to D,
with selected bond lengths indicated. Part of the reaction coordinate
is depicted by the arrow.
purified, dried, distilled, and stored under nitrogen prior to use,
1
except for catalytic runs, which were used as supplied. H NMR
spectra were recorded on a Bruker ACF300, DPX 300, or AV300
NMR spectrometer as CDCl3 solutions unless otherwise stated. 1H
chemical shifts reported are referenced against the residual proton
signals of the solvents. Mass spectra were obtained on a Finnigan
MAT95XL-T spectrometer in an NBA matrix (FAB) or a Macro-
mass VG7035 at 70 eV (EI). [Cp*IrCl2]2 was prepared by the
literature method.16 PhCCD (99 atom % D) and Et3SiD (97 atom
% D) were purchased from Sigma Aldrich and used as such. All
other reagents are commercially available and used without further
purification.
Procedure for Catalytic Runs. In a typical reaction, a 10 mL
stock solution of the reactants was prepared from phenylacetylene
(1.0 mL, 9.1 mmol), triethylsilane (1.5 mL, 9.4 mmol), and nonane
as internal standard (0.41 mL, 2.3 mmol) and made up to the mark
with dichloroethane. A stock solution of the catalyst 1 (72.5 mg,
91.2 µmol) was also prepared in dichloroethane (10.0 mL). The
appropriate amounts of catalyst and reactant stock solutions (1.0
mL each for a 1.0 mol % reaction) were mixed and stirred at room
temperature for 0.5 h. Aliquots (∼0.5 mL) were drawn, and CDCl3
(0.1 mL) was added for NMR quantification. The yields were
calculated on the basis of the limiting reagent. For reactions in which
the products were isolated, the solvent and volatiles were re-
moved under reduced pressure after the catalytic run and the
residues chromatographed on silica gel TLC plates, with hexane
as eluant. The products were identified by comparison with literature
data.
The final step, a reductive elimination of the vinylsilane to
re-form the monomer A, is also energetically favorable.
Conclusion
In this study, we have reported the hydrosilylation of terminal
alkynes that was efficiently catalyzed by [Cp*IrCl2]2 to afford
selectively the â-(Z) vinylsilanes in high yields. In some cases,
the E isomers were also observed, which have been found to
be due to subsequent isomerization. The catalytic cycle was
studied with a combination of experimental and computational
techniques, and we have proposed a cycle that is based on an
Ir(III)-Ir(V) redox process and a direct anti addition of the
silane.
Reaction of [Cp*IrCl2]2 with Ph3SiH. Compound 1 (36.8 mg,
46.2 µmol) was reacted with Ph3SiH (24.1 mg, 92.6 µmol) in CD2-
Experimental Section
1
Cl2 (0.5 mL) for 0.5 h. H NMR analyses of the crude mixture
General Procedures. All reactions were performed under argon
using Schlenk techniques. Solvent such as dichloroethane was
showed unreacted starting materials and [Cp*Ir(Cl)(H)2(SiPh3)]
1
together with some unidentified products. H NMR (δ, CD2Cl2):
1.65 (s, Cp*, 15H), 7.31-7.48 (m, 15H, aromatic), -11.64 (s, 2H,
IrH). (lit. values:12b 1H NMR (δ, CD2Cl2): 1.62 (s, Cp*, 15H), 7.2-
7.4 (m, 15H, aromatic), 7.6 (dd, 6H, aromatic), -11.65 (s, 2H,
IrH). Based on the integration ratio against unreacted 1, yield )
(13) (a) Boag, N. M.; Quyoum, R.; Mohan Rao, K. J. Chem. Soc., Chem.
Commun. 1992, 114. (b) Pedersen, A.; Tilset, M. Organometallics 1993,
12, 3064.
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Chung, L. W.; Wu, Y.-D.; Trost, B. M.; Ball, Z. T. J. Am. Chem. Soc.
2003, 125, 11578. (c) Trost, B. M.; Ball, Z. T. J. Am. Chem. Soc. 2001,
123, 12726.
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1160 Organometallics, Vol. 26, No. 5, 2007
SrideVi et al.
24%. A FAB-MS spectrum showed a peak at m/z 775 [M + NBA]+.
Deuterium Labeling Studies. Catalyst 1 (0.905 µmol), Et3SiD
(15 µL, 93.1 µmol), phenylacetylene (10 µL, 91.1 µmol), and
deuterated dichloroethane (0.5 mL) were placed in an NMR tube.
The reaction mixture was allowed to stand for 0.5 h. Analysis of
Acknowledgment. This work was supported by an A*STAR
grant (Research Grant No. 012 101 0035) and one of us (V.S.S.)
thanks ICES for a Research Scholarship.
Supporting Information Available: 1H NMR and mass
spectroscopic data for all the vinylsilanes in this article and tables
for the catalytic run with a 2:1 molar ratio of PhCtCH:HSiEt3
and for the computational study.
1
the H NMR mixture showed the formation of [(â-Z)-Ph-(D)Cd
C(SiEt3)H]. 1H NMR (δ, ClCD2CD2Cl): 7.36-7.24 (m, 5H,
aromatic), 5.77 (s, 1H, dCHSiEt3), 0.88 (t, 3JHH ) 8 Hz, 9H, CH3),
0.58 (q, 6H, CH2).
OM0609158
A similar reaction using 1 (0.905 µmol), Et3SiH (15 µL, 93.1
µmol), PhCtCD (10 µL, 91.1 µmol), and deuterated dichloroethane
(0.5 mL) afforded [(â-Z)-Ph-(H)CdC(SiEt3)D]. 1H NMR (δ,
ClCD2CD2Cl): 7.45 (s, 1H, dCHPh), 7.36-7.25 (m, 5H, aromatic),
(17) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
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Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
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Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
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3
0.87 (t, JHH ) 8 Hz, 9H, CH3), 0.58 (q, 6H, CH2).
Computational Studies. The reaction pathways for the hydrosi-
lylation catalysis were studied using Moeller-Plesset MP2 theory
together with the LANL2DZ basis set. Spin-restricted calculations
were used for determining the structures of the organic and
organometallic reactants, intermediates, and products, fully opti-
mized at the MP2/LANL2DZ level. Harmonic frequencies were
calculated at the optimized geometries to characterize stationary
points as equilibrium structures, with all real frequencies, and to
evaluate zero-point energy (ZPE) correction. All calculations were
performed using the Gaussian 03 suite of programs.17