Cobalt-Catalyzed Z-Selective Hydrosilylation of Terminal Alkynes

Article abstract of DOI:10.1002/anie.201700868

A cobalt-catalyzed Z-selective hydrosilylation of alkynes has been developed relying on catalysts generated from bench-stable Co(OAc)₂ and pyridine-2,6-diimine (PDI) ligands. A variety of functionalized aromatic and aliphatic alkynes undergo this transformation, yielding Z-vinylsilanes in high yields with excellent selectivities (Z/E ratio ranges from 90:10 to >99:1). The addition of a catalytic amount of phenol effectively suppressed the Z/E-isomerization of the Z-vinylsilanes that formed under catalytic conditions.

Full text of DOI:10.1002/anie.201700868

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Communications
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International Edition: DOI: 10.1002/anie.201700868
German Edition: DOI: 10.1002/ange.201700868
Alkyne Hydrosilylation
Cobalt-Catalyzed Z-Selective Hydrosilylation of Terminal Alkynes
Wei Jie Teo, Chao Wang, Ye Wei Tan, and Shaozhong Ge*
Abstract: A cobalt-catalyzed Z-selective hydrosilylation of
alkynes has been developed relying on catalysts generated from
bench-stable Co(OAc)2 and pyridine-2,6-diimine (PDI)
ligands. A variety of functionalized aromatic and aliphatic
alkynes undergo this transformation, yielding Z-vinylsilanes in
high yields with excellent selectivities (Z/E ratio ranges from
90:10 to > 99:1). The addition of a catalytic amount of phenol
effectively suppressed the Z/E-isomerization of the Z-vinyl-
silanes that formed under catalytic conditions.
hydrosilylation of terminal alkynes has also been reported
and these reactions selectively produce either a- or (E)-b-
vinylsilanes (Scheme 1a,b).[11] However, the Co-catalyzed
alkyne hydrosilylation that can selectively produce (Z)-b-
vinylsilanes still remains unknown. Very recently, we have
shown that the combination of Co(acac)2 and a pyridine-2,6-
diimine ligand selectively catalyzes Markovnikov hydrosily-
lation of aliphatic alkenes.[12] In this study, we tested this
catalytic system for alkyne hydrosilylation. Herein, we report
the first Co-catalyzed Z-selective anti-Markovnikov hydro-
silylation of terminal aromatic and aliphatic alkynes (Scheme
1c). Furthermore, we found that the addition of phenol can
effectively suppress the isomerization of Z-vinylsilanes that
would result in the thermodynamically more stable E-vinyl-
silanes after full conversion of alkynes.
T
ransition-metal-catalyzed hydrosilylation of alkynes is the
most straightforward and atom-economical approach to
prepare synthetically valuable vinylsilanes.[1] The major
issue in the hydrosilylation of alkynes is the control of the
regio- and stereoselectivity because this reaction can afford
multiple products. For example, the hydrosilylation of termi-
nal alkynes can produce a-, (E)-b-, and (Z)-b-vinylsilanes.
Among these isomeric vinylsilanes, (E)-b-vinylsilanes are
usually produced with high selectivity in most of transition-
metal-catalyzed alkyne hydrosilylation reactions.[2] However,
the stereoselective formation of (Z)-b-vinylsilanes still
remains highly challenging for the hydrosilylation of terminal
alkynes. Furthermore, the isomerization of Z-vinylsilanes to
thermodynamically favorable E-vinylsilanes under conditions
for transition-metal-catalyzed alkyne hydrosilylation poses
extra difficulty to the development of the Z-selective alkyne
hydrosilylation.[3]
There are a few catalysts that can produce Z-vinylsilanes
through hydrosilylation of terminal alkynes, but catalysts for
this transformation have been limited to noble metals such as
Ru,[4] Rh,[5] and Ir.[3,6] In addition, these catalysts show either
low reactivity or substrate-dependent selectivity and often
produce a mixture of a-, (E)-b-, and (Z)-b-vinylsilanes. An
iron catalyst has been studied for the Z-selective hydro-
silylation of alkynes,[7] but the scope of alkynes has not been
well established and only two terminal alkynes were reported
for this reaction. Therefore, it is highly desirable to develop
a base metal catalyst that can effectively catalyze Z-selective
hydrosilylation of a wide range of functionalized terminal
alkynes.
Scheme 1. Cobalt-catalyzed hydrosilylation of terminal alkynes.
We chose the reaction of phenylacetylene with PhSiH3 to
evaluate the reaction conditions. Cobalt catalysts were
generated from bench-stable Co(acac)2 or Co(OAc)2 and
pyridine-2,6-diimine ligands and activated in situ by the
reaction with PhSiH3. Reactions were conducted with
1 mol% cobalt catalyst at room temperature with a 1:1 ratio
of phenylacetylene and PhSiH3. Selected experiments are
summarized in Table 1. The reaction catalyzed by Co(acac)2
and mesPDI (L1) proceeded to modest conversion in 12 h and
gave the vinylsilane (Z)-1a in 78% GC yield with good
selectivity (98%, entry 1). Extending the reaction time to 24 h
did not significantly improve the reaction (entry 2). However,
the reaction conducted with Co(OAc)2/L1 occurred in full
conversion in 12 h, but afforded a mixture of Z/E-isomers
with a ratio of 65:34 (entry 3). Considering the potential
isomerization of Z-vinylsilanes to E-vinylsilanes, we followed
the selectivity of the reaction by GC analysis and found that
the reaction selectively afforded (Z)-1a before phenylacety-
lene was fully consumed. Subsequently, the isomerization of
Recently, cobalt complexes have been emerging as active
catalysts for hydrogenation,[8] hydroboration,[9] and hydro-
silylation[10] of alkenes and alkynes. The cobalt-catalyzed
[*] W. J. Teo, Dr. C. Wang, Y. W. Tan, Prof. Dr. S. Ge
Department of Chemistry, National University of Singapore
3 Science Drive 3, Singapore 117543 (Singapore)
E-mail: chmgsh@nus.edu.sg
Homepage: http://www.geresearchgroup.com
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201700868.
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Table 1: Evaluation of conditions for the Co-catalyzed hydrosilylation of
ing Information). The reaction with the addition of 10 mol%
phenol afforded (Z)-1a in 95% yield with 98% selectivity
(entry 5).
phenylacetylene.[a]
To determine the parameter that governs the regioselec-
tivity of alkyne hydrosilylation, we conducted this Co-
catalyzed hydrosilylation of phenylacetylene (entries 5–11 in
Table 1) in the presence of a series of sterically diverse PDI
ligands L1–L3 and L7, oxazoline iminopyridine ligands L4
i
and L5, and Pr-PyBox ligand L6 (L4–L6 have been used for
Co-catalyzed a-hydrosilylation of alkynes[11b,c]). The results of
these experiments indicate that the steric properties of the
ligands significantly influence the regioselectivity of the
reaction, with sterically less hindered ligands (L3–L6:
entries 7–10) favoring a-selectivity and sterically more con-
gested ligands (L1 and L7: entries 5 and 11) favoring (Z)-b-
selectivity (see the Supporting Information for a detailed
discussion).
The scope of aromatic alkynes that undergo this Co-
catalyzed Z-selective hydrosilylation is summarized in
Table 2. In general, a wide range of ethynylarenes containing
Table 2: Scope of aromatic alkynes.[a]
[a] Reaction conditions: phenylacetylene (0.500 mmol), PhSiH3
(0.500 mmol), Co(OAc)2 (5.0 mmol), ligand (5.0 mmol), solvent (1 mL).
[b] Determined by GC analysis with dodecane as the internal standard.
[c] Co(acac)2 (5.0 mmol).
(Z)-1a to (E)-1a occurred when the reaction was not
quenched after full conversion of phenylacetylene was
reached. These results indicate that this Co-catalyzed alkyne
hydrosilylation is Z-selective and the E-vinylsilane is gener-
ated via the isomerization of the Z-vinylsilane. For this
reaction, it took 4.5 h to reach almost full conversion and (Z)-
1a was formed selectively (entry 4).
As the time needed to achieve full conversion of alkynes is
substrate-dependent, it is more desirable to develop a general
procedure for the hydrosilylation of a variety of alkyne
substrates. As such, we need to identify an additive that can
suppress the Z/E-isomerization after the full conversion of
alkynes is achieved. For this reason, we tested the hydro-
silylation (reaction time: 48 h) with a variety of terminal
alkynes that contain various functional groups. We found that
the reaction of 3-ethynylphenol afforded the corresponding
Z-vinylsilane with > 99% selectivity [Eq. (1)]. This result
suggested that addition of phenol could possibly suppress the
Z/E-isomerization of Z-vinylsilanes. To test this, we subse-
quently conducted the hydrosilylation of phenylacetylene in
the presence of various amounts of phenol (see the Support-
[a] Reaction conditions: alkyne (0.500 mmol), PhSiH3 (0.500 mmol),
Co(OAc)2 (5.0 mmol), mesPDI (5.0 mmol), phenol (0.050 mmol), THF
(1 mL), RT, 6 h; yield of isolated product. [b] 508C. [c] 3 mol% catalyst,
phenol (0.150 mmol). [d] (L1)CoMe (5.0 mmol), 1,4-dioxane (1 mL),
808C, 6 h.
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electronically and sterically varied aryl groups reacted
For a preliminary understanding of this Co-catalyzed Z-
smoothly in the presence of 1 mol% of Co(OAc)2 and
mesPDI (L1) at room temperature, affording the correspond-
ing Z-vinylsilanes (1a–1ab) in high yields with excellent
stereoselectivities (Z/E = 96:4 to > 99:1).
selective alkyne hydrosilylation, we conducted a series of
deuterium-labeling experiments (Scheme 2). Hydrosilylation
of 4-tert-butylphenylacetylene-d1 and 1-octyne-d1 with PhSiH3
in the presence of phenol proceeded with excellent stereose-
lectivity (Z/E = 99:1, Scheme 2A,B) and afforded vinylsilane
products with the deuterium atom located on the carbon
bearing the silyl group. The corresponding deuteriosilylation
of 4-tert-butylphenylacetylene and 1-octyne with PhSiD3
produced the opposite results (Scheme 2C,D). The results
of these deuterium-labeling experiments indicate an overall
This Z-selective alkyne hydrosilylation shows good func-
tional group tolerance and a wide range of reactive groups,
such as ether (1d, 1i, and 1k), trifluoromethyl ether (1e),
halogen (1l–1p), unprotected phenolic and alcoholic hydrox-
yl (1q and 1r), unprotected primary aniline (1s and 1t),
ketone (1u), ester (1v), amide (1w), aldehyde (1x), and imine
(1y) moieties, are compatible with the reaction conditions. In
addition, nitrogen- and sulfur-containing heteroaromatic
alkynes (1z, 1aa, and 1ab) reacted in good yields and
excellent selectivities. However, ethynylarenes containing
nitro or carboxylic acid moieties do not react under the
identified conditions. In addition, the alkyne hydrosilylation
was also tested with secondary hydrosilanes and the hydro-
silylation of 2-ethynylanisole with PhSiH2R (R = Me, Ph)
afforded the corresponding Z-vinylsilanes (1ac and 1ad) in
good yields and high selectivity. The results of hydrosilylation
reactions on a 5 mmol scale conducted without using a dry
box as well as further transformations of Z-vinylsilanes are
included in the Supporting Information.
ꢀ
trans-addition of Si–H to a C C bond.
Table 3 summarized the scope of aliphatic alkynes for this
Co-catalyzed Z-selective hydrosilylation (see the Supporting
Information for the evaluation of the reaction conditions).
These reactions were conducted with 3 mol% catalyst under
slightly modified conditions with iPrPDI ligand (L7 in Table 1)
at 708C. Z-Vinylsilanes (2a–2l) were obtained in modest to
high yields with good to excellent selectivities. A variety of
functional groups, such as silylether (2g), cyano (2h), chloride
(2i), bromide (2j), ketone (2k), and ester (2l), were tolerated.
Aliphatic alkynes containing aldehyde, aliphatic alcohol, and
nitro moieties do not react under the identified conditions.
Scheme 2. Deuterium-labeling experiments.
The kinetic behavior was assessed for phenylacetylene
hydrosilylation with PhSiH3 catalyzed by (mesPDI)CoMe in
the presence of phenol at room temperature. The plots of the
initial rate of the reaction vs. the concentration of
Table 3: Scope of aliphatic alkynes.[a]
(
mesPDI)CoMe, PhSiH3, and phenylacetylene are shown in
Figures S1–S3 in the Supporting Information. The kinetic
analysis showed that this catalytic reaction is first order in the
cobalt catalyst and zero order in phenylacetylene and silane.
Based on the results of the deuterium-labeling experi-
ments and the kinetic experiments and the precedent for the
cobalt-catalyzed hydrosilylation and hydrogenation of
alkenes and alkynes, we proposed a silylmetalation pathway
with a CoI silyl intermediate for this Co-catalyzed Z-selective
alkyne hydrosilylation (Scheme 3).[13] Migratory insertion of
the alkyne into the Co–Si bond forms a cobalt vinyl species
(Z)-I, followed by the rate-limiting isomerization to a cobalt
vinyl species (E)-I through a zwitterionic or metallacyclopro-
pene intermediate.[14] Subsequent turnover with PhSiH3
releases Z-vinylsilane and regenerates the Co-silyl species.
The relief of steric repulsion between the ligand and the silyl
group in (Z)-I may account for the isomerization of the cobalt
species (Z)-I to (E)-I.
[a] Reaction conditions: alkyne (0.500 mmol), PhSiH3 (0.500 mmol),
Co(OAc)2 (15.0 mmol), iPrPDI (15.0 mmol), phenol (0.150 mmol), THF
(1 mL), 708C; yield of isolated product.
We subsequently studied the Z/E-isomerization of (Z)-1a
under two sets of conditions that can generate cobalt hydride
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Scheme 3. Proposed catalytic cycle for this Z-selective hydrosilylation.
species because cobalt hydride species have been shown to
catalyze the isomerization of alkenes.[8c,15] Indeed, the Z/E-
isomerization of (Z)-1a occurred under both sets of condi-
tions [Eq. (2)]. The isomerization of aliphatic vinylsilane (Z)-
2b was also tested [Eq. (3)]. In addition to the Z/E-isomer-
These results suggest that phenol suppresses the isomeriza-
tion of Z-vinylsilanes by quenching the Co–H species to form
a cobalt phenolate and is not directly involved in the catalytic
production of Z-vinylsilanes.
In summary, we have developed the first Co-catalyzed Z-
selective anti-Markovnikov hydrosilylation of terminal
alkynes with catalysts generated from bench-stable Co(OAc)2
and pyridine-2,6-diimine ligands and activated in situ by the
reaction with PhSiH3. A broad range of ethynylarenes and
aliphatic alkynes underwent this reaction to afford (Z)-b-
vinylsilanes in high yields with excellent stereoselectivities.
The addition of a catalytic amount of phenol is crucial to
achieve high Z-selectivities because phenol suppresses the
isomerization of Z-vinylsilane products that would form E-
vinylsilanes after the full conversion of alkynes has been
achieved. Further studies to determine the detailed mecha-
nism of this transformation and to apply this transformation
to the synthesis of complex molecules will be the subject of
further work.
ization, the formation of E-allylsilane was detected. These
results suggest the Z/E-isomerization of Z-vinylsilanes occurs
À
via alkene insertion into the Co H bond followed by b-H
Acknowledgements
elimination.[8c,15]
As the addition of phenol can effectively suppress the
isomerization of Z-vinylsilanes, we propose that phenol can
quench the Co–H species generated from the reaction
mixture after the alkyne substrates are fully consumed. To
test this hypothesis, we conducted the reaction of 4-methox-
yphenol with the Co–H species “(mesPDI)CoH”, generated in
situ from the reaction of (mesPDI)CoMe with 1 equivalent of
pinacolborane (HBpin), and this reaction formed a cobalt
phenolate complex (mesPDI)Co(OC6H4-4-OMe) (3).[16] Alter-
natively, compound 3 can be prepared by the reaction of
This work was supported by the National University of
Singapore (No. R-143-000-614-133 and No. R-143-000-630-
133) and the Ministry of Education (MOE) of Singapore (No.
R-143-000-615-112). We thank Bruno Donnadieu and Geok
Kheng Tan for single-crystal X-ray diffraction analysis.
Conflict of interest
The authors declare no conflict of interest.
(
mesPDI)CoMe with 4-methoxyphenol [Eq. (4)]. As a refer-
ence reaction, the addition of 4-methoxyphenol can also
suppress the Z/E-isomerization of Z-vinylsilane [Eq. (5)].
With complex 3 in hand, we tested it as a cobalt precursor for
the hydrosilylation of phenylacetylene with PhSiH3 and found
that it can effectively catalyze this hydrosilylation reaction
with high Z-selectivity [Eq. (6)]. As expected, compound 3
does not catalyze the Z/E-isomerization of Z-vinylsilanes.
Keywords: alkynes · cobalt · homogeneous catalysis ·
hydrosilylation · vinylsilanes
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Manuscript received: January 25, 2017
Final Article published: && &&, &&&&
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Communications
Alkyne Hydrosilylation
W. J. Teo, C. Wang, Y. W. Tan,
S. Ge*
&&&& — &&&&
Catching some Z’s: A cobalt catalyst,
which was generated from bench-stable
Co(OAc)2 and a pyridine-2,6-diimine
derivative and activated in situ by PhSiH3,
promotes the highly regio- and Z-selec-
tive hydrosilylation of terminal aromatic
and aliphatic alkynes. This reaction tol-
erates a variety of functional groups such
as halide, aldehyde, ketone, ester, amide,
cyano, and free hydroxy and amine func-
tionalities.
Cobalt-Catalyzed Z-Selective
Hydrosilylation of Terminal Alkynes
6
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