Cobalt-Catalyzed (E)-Selective anti-Markovnikov Hydrosilylation of Terminal Alkynes

Article abstract of DOI:10.1021/acscatal.8b01410

We report a regioselective and stereoselective hydrosilylation of terminal alkynes with catalysts generated from bench-stable Co(acac)₂ and bisphoshpine ligands. The cobalt catalyst precursors are activated by the reaction with hydrosilanes, and air-sensitive activators, such as Grignard reagents or NaBHEt₃, are not required for catalyst activation. A wide range of aromatic and aliphatic terminal alkynes underwent this cobalt-catalyzed hydrosilylation, affording the corresponding (E)-vinylsilanes in high yields with high regioselectivity and stereoselectivity. These reactions show good functional group compatibility and can be readily scaled up to gram-scales without using a drybox. Deuterium-labeling experiments suggest a cis-addition of hydrosilanes to alkynes.

Full text of DOI:10.1021/acscatal.8b01410

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Letter
Cobalt-Catalyzed (E)-Selective anti-Markovnikov
Hydrosi-lylation of Terminal Alkynes
Caizhi Wu, Wei Jie Teo, and Shaozhong Ge
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01410 • Publication Date (Web): 23 May 2018
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Cobalt-Catalyzed (
Terminal Alkynes
E
)-Selective anti-Markovnikov Hydrosilylation of
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Caizhi Wu, Wei Jie Teo, and Shaozhong Ge*
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
ABSTRACT: We report a regioꢀ and stereoselective hydrosilylation of terminal alkynes with catalysts generated from benchꢀstable
Co(acac)2 and bisphoshpine ligands. The cobalt catalyst precursors are activated by the reaction with hydrosilanes, and airꢀsensitive
activators, such as Grignard reagents or NaBHEt3, are not required for catalyst activation. A wide range of aromatic and aliphatic
terminal alkynes underwent this cobaltꢀcatalyzed hydrosilylation, affording the corresponding (E)ꢀvinylsilanes in high yields with
high regioꢀ and stereoselectivity. These reactions show good functional group compatibility and can be readily scaled up to gramꢀ
scales without using a dry box. Deuteriumꢀlabeling experiments suggest a cisꢀaddition of hydrosilanes to alkynes.
KEYWORDS: cobalt, hydrosilylation, alkynes, vinylsilanes, homogenous catalysis
Vinylsilanes are synthetically valuable building blocks in
organic synthesis due to their versatile reactivity, low toxicity,
high stability, and ease for manipulations.1 Metalꢀcatalyzed
hydrosilylation of alkynes will be one of the most atomꢀ
economic and straightforward approaches to prepare these
vinylsilanes.2 One challenge in developing the hydrosilylation
reaction of alkynes is to control both regioꢀ and stereoselectivꢀ
ity because this reaction can potentially produce multiple
β
ꢀvinylsilanes have been identified for hydrosilylation reacꢀ
tions of alkynes (Scheme 1).
products, such as
αꢀ, (Z)ꢀβꢀ, and (E)ꢀβꢀvinylsilanes. Over past
decades, however, a series of transition metal catalysts have
been successfully developed to address these selectivity isꢀ
sues.2c The other challenge is to identify catalysts that are seꢀ
lective for alkyne hydrosilylation with primary silanes (RSiH3)
to produce dihydrovinylsilanes, which are important monoꢀ
mers for the synthesis of poly(silylenes).3 However, selective
transition metal catalysts that can catalyze the hydrosilylation
of alkynes with primary silanes to produce stereodefined dihyꢀ
drovinylsilanes are still rare.
Catalysts based on noble metals have been extensively studꢀ
ied for alkyne hydrosilylation with tertiary hydrosilanes R3SiH
to produce α β β
ꢀ,4 (Z)ꢀ ꢀ,5 or (E)ꢀ ꢀvinylsilanes6 with high regioꢀ
and stereoselectivity. When primary silanes RSiH3 are emꢀ
ployed for the hydrosilylation of alkynes, however, these reacꢀ
tions tend to produce a mixture of multiple vinylsilane prodꢀ
ucts, such as mono(vinyl)silanes, di(vinyl)silanes, and
tri(vinyl)silanes.7 This is because mono(vinyl)silane and
di(vinyl)silane products are active enough to further react with
alkynes to form higher order vinylsilanes in the presence of
these noble metal catalysts.
Scheme 1. Regio- and Stereoselective Co-Catalyzed Hy-
drosilylation of Terminal Alkynes
In 2016, Lu and Huang independently developed a highly
selective cobaltꢀcatalyzed Markovnikov hydrosilylation of
alkynes with Ph2SiH2 to access αꢀvinylsilanes (Scheme 1A).11
In addition, highly Zꢀselective Coꢀcatalyzed hydrosilylation of
alkynes has also been developed, and both primary and secꢀ
ondary hydrosilanes could be used for this transformation
(Scheme 1B).12 In 2014, Deng reported a highly (E)ꢀselective
hydrosilylation of alkynes with Ph2SiH2 using a carbeneꢀ
supported cobalt alkyl complex (Scheme 1C).13 However, this
Recently, there has been a growing interest in replacing noꢀ
ble metal catalysts with base metal catalysts due to increasing
concerns on longꢀterm sustainability.8 With its low cost, high
natural abundance, and low toxicity, cobalt is a particularly
appealing alternative.9 Accordingly, the development of coꢀ
baltꢀcatalyzed hydrosilylation reactions of unsaturated organic
molecules is undergoing an explosive growth.10 Particularly,
cobalt catalysts for selective production of αꢀ, (Z)ꢀβꢀ, and (E)ꢀ
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Coꢀcatalyzed hydrosilylation of alkynes proceeds with low
selectivity when primary hydrosilane PhSiH3 is used, and proꢀ
duces a mixture of ꢀ, (Z)ꢀ ꢀ, and (E)ꢀ ꢀvinylsilanes. In 2017,
cause PhSiH3 can compete with hydrosilane 2a for the reaction
with phenylacetylene. Subsequently, we tested the hydrosilylaꢀ
tion of phenylacetylene with varied amounts of PhSiH3 in the
presence of 1 mol % of Co(acac)2 and dpephos (entries 6−8 in
Table 1). As expected, reactions with 1.5 or 2 equivalents of
PhSiH3, relative to phenylacetylene, afforded the product 2a
with excellent selectivity (entries 7 and 8 in Table 1). In addiꢀ
tion, we also tested the reactions with a 2:1 ratio of phenylaꢀ
cetylene and PhSiH3, and these reactions afforded the product
3a with a complete selectivity (entries 9 and 10 in Table 1).
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α
β
β
Thomas reported the hydrodrosilylation of 1ꢀoctyne with
PhSiH3 using (mesPDI)CoCl2/NaOtBu, and this reaction proꢀ
duces a mixture of (Z)ꢀβꢀ and (E)ꢀβꢀvinylsilanes with a modest
Eꢀselectivity (Scheme 1C).14 Therefore, it still remains a chalꢀ
lenge to develop catalytic hydrosilylation of alkynes with a
primary hydrosilane. Driven by our continuous interest in deꢀ
veloping selective base metal catalysis,10i,j,m,12a,15 we herein
report a Coꢀcatalyzed highly regioꢀ and stereoselective hydrosꢀ
ilylation of terminal alkynes with both primary and secondary
hydrosilanes PhR'SiH2 (R' = H, Me, Ph) to prepare (E)ꢀ
vinylsilanes (Scheme 1D).
9
Under the identified conditions (entry 7 in Table 1), we
studied the scope of terminal alkynes that undergo this cobaltꢀ
catalyzed Eꢀselective hydrosilylation and the results are sumꢀ
marized in Table 2. In general, a wide range of terminal alꢀ
kynes containing electronically varied aromatic (1b−1r) and
aliphatic (1s−1y) substituents reacted smoothly with 1 mol %
Co(acac)2/dpephos at room temperature, yielding the correꢀ
sponding (E)ꢀvinylsilanes (2b−2y) in high yields (59−91%)
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We started this cobaltꢀcatalyzed hydrosilylation of alkynes
by evaluating the conditions for the reaction between phenylaꢀ
cetylene and PhSiH3. Cobalt catalysts are generated from
benchꢀstable Co(acac)2 and bisphosphine ligands and activated
by the reaction with PhSiH3. These reactions were conducted
with 1 mol % cobalt catalyst at room temperature for 6 h, and
the selected examples are summarized in Table 1. In general,
these reactions occurred to full conversion of phenylacetylene
and afforded three products (1a, 2a, and 3a) with varied ratios,
and product 3a was generated by hydrosilylation of phenylaꢀ
cetylene with vinylsilane 2a.
with high regioselectivity ((E)-β:α up to >99:1). The GCꢀMS
analysis on the crude mixtures of these reactions indicated the
formation of trace amounts (< 3%) of the corresponding
bis(vinyl)silanes.
Table 2. The Scope of Terminal Alkynes for Hydrosilyla-
a
tion Reactions with PhSiH3
Table 1. Evaluation of the Reaction Conditions for Hy-
a
drosilylation of Phenylacetylene with PhSiH3
aConditions: phenylacetylene (0.500 mmol), PhSiH3 (0.500ꢀ1.00
mmol), Co(acac)2 (5.0 ꢀmol), ligand (5.5 ꢀmol), THF (1 mL),
b
room temperature, 6 h; Determined by GC analysis with dodecꢀ
c
d
ane as the internal standard; Isolated yield of 2a; Isolated yield
of 3a.
The reactions conducted with the combination of Co(acac)2
and various bisphosphine ligands afforded a mixture of three
vinylsilane products 1a−3a when a 1:1 ratio of phenylacetyꢀ
lene and PhSiH3 was used (entries 1−6 in Table 1). Monitoring
these reactions with GCꢀMS analysis revealed that product 3a
was generated only in the late stage of these reactions. Based
on these analyses, we rationalized that increasing the amounts
of PhSiH3 could improve the selectivity for product 2a beꢀ
aConditions: alkyne (0.500 mmol), PhSiH3 (0.750 mmol),
Co(acac)2 (5.0 ꢀmol), dpephos (5.5 ꢀmol), THF (1 mL), room
temperature, 6 h, and isolated yields after column chromatography.
c
bdppf ligand was used instead of dpephos ligand. nꢀC12H25SiH3
was used instead of PhSiH3.
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ligands are benchꢀstable (See the Supporting Information for
The data in Table 2 indicate that aliphatic alkynes reacted
with excellent regioselectivities ((E)ꢀ > 98:2) to afford the
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the detailed procedure). The reactions of 1ꢀoctyne or phenylaꢀ
cetylene and PhSiH3 on a 10 mmol scale in the presence of 0.5
mol % Co(acac)2 and dpephos proceeded to full conversion at
room temperature in 3 h and afforded the corresponding (E)ꢀ
vinylsilanes 2v and 2a in 87% and 85% isolated yields, reꢀ
spectively (eqs. 2 and 3). Similarly, a 10 mmol scale reaction
of phenylacetylene with secondary hydrosilane Ph2SiH2 cataꢀ
lyzed by 0.5 mol % Co(acac)2 and xantphos produced 2a' in a
90% isolated yield (eq. 4). In addition, we also showed that the
tertiary (E)ꢀvinylsilane 2v' smoothly underwent a Pdꢀcatalyzed
crossꢀcoupling with 4'ꢀiodoacetophenone at room temperature,
affording the internal (E)ꢀvinylarene 4 in a 85% isolated yield
(eq. 5).
β:α
corresponding (E)ꢀvinylsilanes in good yields (2s−2y). Howꢀ
ever, the electronic properties of aryl groups in aromatic alꢀ
kynes have noticeable effects on the regioselectivity of this
hydrosilylation reaction. For example, aromatic alkynes conꢀ
taining electroꢀrich aryl groups (2b−2g) reacted with higher
regioselectivities than those containing electroꢀdeficient aryl
groups (2h, 2o and 2p). This Coꢀcatalyzed hydrosilylation of
alkynes tolerates a range of functionalities, including ether
(2f), tertiary amine (2g), trifluoromethylether (2i), chloro (2k
and 2y), primary aniline (2l and 2m), carboxylic ester (2n),
ketone (2o), cyano (2w), and siloxy (2x) moieties. In addition,
nitrogenꢀ and sulfurꢀcontaining heteroaromatic alkynes reacted
with high yields and high regioꢀ and stereoselectivity (2p−2r).
Primary alkylsilanes also reacted termianl alkynes under the
identified conditions. For example, the hydrosilylation of pheꢀ
nylacetylene with nꢀdodecylsilane afforded the desired (E)ꢀ
vinylsilane 2z in 90% isolated yield with high regioselectivity.
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We also tested this Coꢀcatalyzed hydrosilylation with a secꢀ
ondary hydrosilane Ph2SiH2, and the reactions did not proceed
when conducted with the catalyst generated from Co(acac)2
and dpephos. However, when catalyzed by the combination of
Co(acac)2/xantphos, these reactions occurred smoothly at
room temperature in the presence of 1 mol % cobalt catalyst.
We chose several alkynes from Table 2 to study the scope of
this hydrosilylation with Ph2SiH2 and these examples are listed
in Table 3. Both aromatic and aliphatic alkynes reacted to
afford the corresponding (E)ꢀvinylsilanes in high isolated
yields and with excellent regioselectivity.
Table 3. Co-Catalyzed Hydrosilylation of Terminal Al-
a
kynes with Ph2SiH2
aConditions: alkyne (0.500 mmol), Ph2SiH2 (0.550 mmol),
Co(acac)2 (5.0 ꢀmol), dpephos (5.5 ꢀmol), THF (1 mL), room
temperature, 6 h, and isolated yields after column chromatography.
Scheme 2. Deuterium-Labeling Experiments
Internal alkynes were also studied for this cobaltꢀcatalyzed
hydrosilylation. For example, pentꢀ4ꢀenꢀ1ꢀynꢀ1ꢀylbenzene
(1aa), a 1,4ꢀenyne containing an internal alkyne group,
smoothly reacted with PhSiH3 in the presence of 1 mol %
Co(acac)2 and xantphos at room temperature, affording a 1,4ꢀ
dienylsilane product 2aa in 72% isolated yield with an excelꢀ
lent regioselectivity (eq. 1).
To get some insights on this Coꢀcatalyzed (E)ꢀselective hyꢀ
drosilylation of alkynes, we conducted a series of deuteriumꢀ
labeling experiments, and the results are summarized in
Scheme 2. The hydrosilylation of 4ꢀtertꢀbutylphenylacetyleneꢀ
d1 or 1ꢀdecyneꢀd1 with PhSiH3 under the standard conditions
produced 2eꢀd1 or 2vꢀd1 with high Eꢀselectivity, and the deuꢀ
terium atoms of these two vinylsilanes were exclusively locatꢀ
ed at the βꢀvinylic carbon (Scheme 2A and 2B). Similar reꢀ
sults were obtained for the hydrosilylation reaction of 4ꢀtertꢀ
butylphenylacetyleneꢀd1 with Ph2SiH2 (Scheme 2C). We also
studied the hydrosilylation of 4ꢀtertꢀbutylphenylacetylene or
1ꢀdecyne with Ph2SiD2 under the standard conditions. These
reactions afforded vinylsilanes 2e'ꢀd2 and 2v'ꢀd2 in high isolatꢀ
To highlight the utility of this practical protocol for the synꢀ
thesis of (E)ꢀvinylsilane, we conducted gramꢀscale reactions
without using a dry box because both Co(acac)2 and phosphine
ed yields with excellent Eꢀselectivity ((E)ꢀβ:α = 99:1), and the
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This work was supported by the Ministry of Education of Singaꢀ
pore (Rꢀ143ꢀ000ꢀA07ꢀ112). C. W. thanks Dr. Jiayu Liao for the
initial studies on the hydrosilylation reaction of 1,4ꢀenyne.
deuterium atoms of 2e'ꢀd2 and 2v'ꢀd2 are located at the αꢀ
vinylic carbon and silicon of these vinylsilanes (Scheme 2D
and 2E). The results of these deuteriumꢀlabeling experiments
suggest that this Coꢀcatalyzed hydrosilylation of alkynes proꢀ
ceed through a cisꢀaddition of SiꢀH to an alkyne group.
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REFERENCES
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Based on the precedent of cobaltꢀcatalyzed hydrosilylation
of unsaturated molecules9c,10i, and the results of the series of
deuterium labeling experiments, we propose a catalytic cycle
depicted in Scheme 3 for this Coꢀcatalyzed alkyne hydrosilylaꢀ
tion.16 The activation of Co(acac)2 with hydrosilane in the
presence of dpephos generates a cobalt hydride species (L)Coꢀ
H. 1,2ꢀMigratory insertion of alkynes into this CoꢀH intermeꢀ
diate produces a vinylcobalt species (I), which turns over with
hydrosilane to afford the (E)ꢀvinylsilane products and regenerꢀ
ate the catalytically active cobalt hydride (L)CoꢀH.
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(3) West, R. Comprehensive Organometallic Chemistry II;
Elsevier: Amsterdam, 1995; Vol. 2, p77.
(4) For selected examples: (a) Trost, B. M.; Ball, Z. T. Markovniꢀ
kov Alkyne Hydrosilylation Catalyzed by Ruthenium Complexes. J.
Am. Chem. Soc. 2001, 123, 12726ꢀ12727. (b) Kawanami, Y.; Sonoda,
Y.; Mori, T.; Yamamoto, K. RutheniumꢀCatalyzed Hydrosilylation of
1ꢀAlkynes with Novel Regioselectivity. Org. Lett. 2002, 4, 2825ꢀ
2827. (c) Trost, B. M.; Ball, Z. T. Alkyne Hydrosilylation Catalyzed
by a Cationic Ruthenium Complex:ꢁ Efficient and General Trans Adꢀ
dition. J. Am. Chem. Soc. 2005, 127, 17644ꢀ17655. (d) Menozzi, C.;
Dalko, P. I.; Cossy, J. Hydrosilylation of Terminal Alkynes with Alꢀ
kylidene Ruthenium Complexes and Silanes. J. Org. Chem. 2005, 70,
10717ꢀ10719. (e) Wang, P.; Yeo, X.ꢀL.; Loh, T.ꢀP. CopperꢀCatalyzed
Highly Regioselective Silylcupration of Terminal Alkynes to Form αꢀ
Vinylsilanes. J. Am. Chem. Soc. 2011, 133, 1254ꢀ1256.
(5) For selected examples: (a) Na, Y.; Chang, S. Highly Stereoseꢀ
lective and Efficient Hydrosilylation of Terminal Alkynes Catalyzed
by [RuCl2(pꢀcymene)]2. Org. Lett. 2000, 2, 1887ꢀ1889. (b) Mori, A.;
Takahisa, E.; Yamamura, Y.; Kato, T.; Mudalige, A. P.; Kajiro, H.;
Hirabayashi, K.; Nishihara, Y.; Hiyama, T. Stereodivergent Syntheses
of (Z)ꢀ and (E)ꢀAlkenylsilanes via Hydrosilylation of Terminal Alꢀ
kynes Catalyzed by Rhodium(I) Iodide Complexes and Application to
SiliconꢀContaining Polymer Syntheses. Organometallics 2004, 23,
1755ꢀ1765. (c) Trost, B. M.; Ball, Z. T. Alkyne Hydrosilylation Cataꢀ
lyzed by a Cationic Ruthenium Complex:ꢁ Efficient and General
Trans Addition. J. Am. Chem. Soc. 2005, 127, 17644ꢀ17655. (d)
Sridevi, V. S.; Fan, W. Y.; Leong, W. K. Stereoselective Hydrosilylaꢀ
tion of Terminal Alkynes Catalyzed by [Cp*IrCl2]2:ꢁ A Computational
and Experimental Study. Organometallics 2007, 26, 1157ꢀ1160. (e)
Iglesias, M.; PerezꢀNicolas, M.; Miguel, P. J. S.; Polo, V.; Fernandezꢀ
Alvarez, F. J.; PerezꢀTorrente, J. J.; Oro, L. A. A synthon for a 14ꢀ
Scheme 3. The Proposed Catalytic Cycle for this Co-
Catalyzed Hydrosilylation of Alkynes
In summary, we have developed a convenient and effective
protocol for the synthesis of (E)ꢀvinylsilanes through a cobalt
catalyzed antiꢀMarkovnikov hydrosilylation of terminal alꢀ
kynes. The cobalt catalysts are generated in situ from bench
stable Co(acac)2 and dpephos or xantphos ligands, and are
activated by the reaction with hydrosilane substrates. A broad
range of alkynes containing either aromatic or aliphatic subꢀ
stituents underwent this hydrosilylation reaction to afford (E)ꢀ
vinylsilanes in high isolated yields with high regioꢀ and stereꢀ
oselectivity. In addition, this reaction could be conducted on a
gram scale without the loss of selectivity with a reduced cataꢀ
lyst loading (0.5 mol %). Thus, this Coꢀcatalyzed hydrosilylaꢀ
tion provides a general and practical approach to prepare funcꢀ
tionalized (E)ꢀvinylsilanes with commercially available abunꢀ
dant baseꢀmetal catalysts.
ASSOCIATED CONTENT
Supporting Information
Experimental details, characterization data, and copies of NMR
spectra of all compounds. This material is available free of charge
on the ACS Publications website at http://pubs.acs.org.
electron Ir(iii) species: catalyst for highly selective
βꢀ(Z) hydrosilylaꢀ
tion of terminal alkynes. Chem. Commun. 2012, 48, 9480ꢀ9482. (f)
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Computational Studies of the RutheniumꢀCatalyzed Hydrosilylation
of Alkynes: Mechanistic Insights into the Regioꢀ and Stereoselective
Formation of Vinylsilanes. Organometallics 2014, 33, 6937ꢀ6944.
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AUTHOR INFORMATION
Corresponding Author
*chmgsh@nus.edu.sg
Note
The authors declare no competing financial interest.
ACKNOWLEDGMENT
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