Schiff Base Cobalt(II) Complex-Catalyzed Highly Markovnikov-Selective Hydrosilylation of Alkynes

Article abstract of DOI:10.1021/acs.orglett.0c03721

A bench-stable cobalt(II) complex, with 3N-donor socket-type benzimidazole-imine-2H-imidazole ligand is reported as a precatalyst for regioselective hydrosilylation of terminal alkynes. Both aromatic and aliphatic alkynes could be effectively hydrosilylated with primary, secondary, and tertiary silane to give α-vinylsilanes in high yields with excellent Markovnikov selectivity and extensive functional-group tolerance. Catalyst loading varies within 0.5-0.05 mol %, which is one of the most efficient reported so far in the literature on cobalt-catalyzed alkyne hydrosilylation.

Full text of DOI:10.1021/acs.orglett.0c03721

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pubs.acs.org/OrgLett
Letter
Schiff Base Cobalt(II) Complex-Catalyzed Highly Markovnikov-
Selective Hydrosilylation of Alkynes
́
Maciej Skrodzki, Violetta Patroniak, and Piotr Pawluc*
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ABSTRACT: A bench-stable cobalt(II) complex, with 3N-donor socket-type benzimidazole-imine-2H-imidazole ligand is reported
as a precatalyst for regioselective hydrosilylation of terminal alkynes. Both aromatic and aliphatic alkynes could be effectively
hydrosilylated with primary, secondary, and tertiary silane to give α-vinylsilanes in high yields with excellent Markovnikov selectivity
and extensive functional-group tolerance. Catalyst loading varies within 0.5−0.05 mol %, which is one of the most efficient reported
so far in the literature on cobalt-catalyzed alkyne hydrosilylation.
challenging and rarely reported, and is usually achieved with
cobalt(II) precatalysts.24−30
rganosilicon compounds are particularly interesting and
Opowerful tools in organic chemistry. Vinylsilanes, which
belong to this group, are applied in the syntheses of natural
products,1 cross coupling reactions,2 and stereocontrolled
reactions,3 especially in hydrogenation processes.4 Among all
the synthetic pathways for obtaining vinylsilanes, the noble
metal-catalyzed hydrosilylation of alkynes and 1,3-dienes has
been generally accepted as the most effective route.5−10 One of
the main problems in hydrosilylation of internal and terminal
alkynes is the control of regio- and stereoselectivity. Develop-
ment of newer and more sophisticated catalysts, in terms of
structure, that would help to solve this problem has been the
challenge for a number of research groups.11
From the environmental and economic point of view,
compounds based on earth abundant metals are more desirable
as catalysts than those based on noble metals. On the other
hand, in the era of molecular-heavy and complex ligands, it is
essential to develop a rational and sustainable coordination
environment. New, simple ligands must be designed to
compete with multistep synthesized ones in terms of activity,
chemo- regio-, and stereoselectivity. Complexes of first row
transition metals, such as Fe or Co, have been proven to be
efficient and selective in the reaction of hydrosilylation of
alkynes,12−23 providing β-(E/Z)-vinylsilanes or α-vinylsilanes.
Although these catalysts show high catalytic activity, obtaining
α-vinylsilanes as a result of hydrosilylation of alkynes is still
For this purpose, as illustrated in Scheme 1, Huang’s and
Lu’s research groups have developed efficient systems based on
pyridine-bis(oxazoline) (PyBox)24 or iminopyridine-oxazoline
(IPO)25 3N tridentate pincer ligands, whereas Yang’s26 and
Jin’s27 groups have proposed structurally simpler (amine)-
pyridine-imine ligands. Regiocontrol in Markovnikov hydro-
silylation of terminal alkynes can be also achieved using
Co(OAc)2·4H2O/2,2′-bipyridine catalytic system.28 To the
best of our knowledge, all these catalytic systems are active in
reactions of terminal alkynes involving primary and secondary
silanes.
Recently we have synthesized a series of new cobalt(II)
chloride precatalysts coordinated to structurally similar
hydrazone Schiff base ligands, easily obtained in a two-step
synthetic protocol.31 These complexes are air- and moisture-
stable, and are operationally simple and easy to handle on the
laboratory bench. The synthesized cobalt(II) complexes have
been evaluated for their ability to act as olefin hydrosilylation
Received: November 8, 2020
Published: January 13, 2021
© 2021 The Authors. Published by
American Chemical Society
https://dx.doi.org/10.1021/acs.orglett.0c03721
Org. Lett. 2021, 23, 663−667
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increased selectivity toward Markovnikov product, while
maintaining high conversion of diphenylsilane (Table 1,
entry 2). Experiments with toluene and dioxane as solvents
(Table 1, entry 3−4) were successful, but resulted in yielding a
mixture of 95% of α-vinylsilane and 5% of β-vinylsilane.
Encouraged by these results we tried to lower the
concentrations of cobalt precatalyst and borohydride activator.
In our investigation, the catalyst loading was lowered from 3
mol % to 0.1 mol %, preserving its activity and selectivity
(Table 1, entry 7). Reaction of phenylacetylene with
diphenylsilane was completed in 1h at 40 °C. Further
decreases in the catalyst loading (0.05 mol % of 1 and 0.15
mol % LiHBEt3) resulted in a slightly lower diphenylsilane
conversion (85%), even at elevated temperatures. Having
optimized the conditions for hydrosilylation of phenylacetylene
with diphenylsilane, the reaction scope was explored by using a
variety of terminal and internal alkynes. In addition, the
influence of functional groups, i.e., electron-donating and
electron-withdrawing substituents to phenyl ring, was inves-
tigated. In most experiments, the yields of the reaction
products were quantitative, and selectivities toward α-addition
products exceeded 99% (Scheme 2).
Scheme 1. Cobalt(II) Based Precatalysts for α-Selective
Alkyne Hydrosilylation
catalysts in the presence of alkali metal trialkylborohydrides.
The cobalt(II) chloride complex containing benzimidazole-
imine-2H-imidazole ligand (complex 1, Scheme 1) has been
selected as the most efficient in terms of hydrosilylation activity
and selectivity toward β-addition. Successful use of complex 1
as a catalyst for hydrosilylation of olefins prompted us to
investigate its catalytic activity in the reaction with alkynes.
Herein we describe highly selective hydrosilylation of internal
and terminal alkynes with primary, secondary, and tertiary
silanes in the presence of Schiff base cobalt(II) complex.
For the optimization reaction (Table 1), phenylacetylene
and diphenylsilane were chosen as model substrates. To
Scheme 2. Range of Alkynes in Hydrosilylation with
a
Diphenylsilane
Table 1. Optimization Protocol for Hydrosilylation of
a
Phenylacetylene with Diphenylsilane
Ph2SiH
2 b
b
temp. cat. 1 loading
conversion
[%]
selectivity
(α:β)
entry solvent
[°C]
[mol %]
d
1
2
3
4
5
6
7
8
9
THF
THF
toluene
dioxane
THF
THF
THF
THF
THF
70
70
70
70
40
40
40
40
RT
60
3
3
1
1
1
1
0.5
0.1
0.1
0.05
>99
>99
>99
>99
>99
52
>99
>99
85
95:5
99:1
95:5
95:5
99:1
87:13
98:2
99:1
95:5
99:1
e
c
c
10
THF
85
a
Conditions: Ph2SiH2 (0.5 mmol), phenylacetylene (0.5 mmol),
precatalyst (0.05−3 mol %), LiHBEt3 (0.15−9 mol %), 20h, solvent
b
0.25 mL, unless stated otherwise. Calculated by GC with decane as
c
d
internal standard. 1 h. Reaction with NaHBEt3 (9 mol %).
e
Reaction with t-BuONa (2 mol %).
a
Conditions: alkyne (1 mmol), diphenylsilane (1 mmol), complex 1
(0.1 mol %), LiHBEt3 (0.3 mol %). Ratio of α to β-addition products
determined using GC. Isolation yields. Reaction performed at 60 °C
with 0.5% mol 1 and 1.5% mol LiHBEt3.
minimize the amount of wastes, the reaction setup was based
on an equimolar ratio of alkyne to silane. The initial reaction
was carried out in the presence of 3 mol % of precatalyst 1 with
9 mol % of sodium triethylborohydride in toluene as an
activator. The concentration of silane was set to 2 M in THF at
the temperature of boiling solvent (Table 1, entry 1). High
conversion of diphenylsilane and regioselectivity toward α-
addition were observed. To satisfy our aims we also
investigated the reaction with LiHBEt3 in THF as an activator.
Fortunately, the use of the alternative reducing agent led to
b
The reactions with alkynes effectively occurred in the
presence 0.1 mol % catalyst 1 and 0.3 mol % LiHBEt3
concentration at 40 °C but required a relatively long time
(20 h). Phenylacetylene reacted with diphenylsilane to give the
corresponding α-vinylsilane with 99% isolated yield (2a). To
check the impact of different substituents, the reactions
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involving para-substituted phenylacetylene were performed. 4-
tert-Butylphenylacetylene, 4-ethynylanisole, and 4-ethynylani-
line reacted with 99:1 selectivity (α/β-vinylsilane), with a 99%
isolated yield (2b, 2f, 2j).
Table 2. Scope of Alkynes in Hydrosilylation with
Phenylsilane
a
Halide substituents such as 4-bromophenylacetylene under-
went the reaction with excellent yield, employing increased
catalyst loading (2c). Cycloaliphatic alkynes, ethynylcyclohex-
ane, cyclohexenyleacetylene, and cyclopropylacetylene, were
also smoothly hydrosilylated according to our protocol (2d,
2g, 2h). However, despite excellent conversions of cyclo-
aliphatic alkynes, trace amounts of β-addition products were
observed (3−6%). Aliphatic alkynes, 3-phenylprop-1-yne and
oct-1-yne, were transformed easily to the corresponding α-
vinylsilanes, and no β-addition products were observed (2e,
2l). Symmetrical internal alkynes, 4-octyne and diphenylace-
tylene, underwent hydrosilylation with total conversion of
diphenylsilane (2k, 2t), giving selectively the corresponding
(E)-silylalkenes. Interestingly, the reaction with 1-phenylprop-
1-yne resulted in a mixture of α and β-addition products (in
favor of α), presumably due to a low steric hindrance of a
methyl group (2i). Heteroaromatic thiophene moiety was
tolerated, and the reaction proceeded smoothly to yield single
α-vinylsilane isomer (2s). It is worth noting that the reactive
groups such as nitrile and ester were compatible with the
hydrosilylation conditions (2r, 2u). We also examined the
impact of a methyl group substituted to phenyl ring. In
contrast to 4-ethynyltoluene, which required elevated temper-
ature and increased catalyst loading (2n), 2-ethynyltoluene and
3-ethynyltoluene were transformed easily (2o, 2p) to the
corresponding α-vinylsilanes. Masked alcohol, (but-3-yn-1-
yloxy)dimethyl(phenyl)silane, appeared to be less reactive and
gave the corresponding product with 72% yield (2m).
yield [%]
(isolated)
selectivity
selectivity
entry
R
α:β
α:α,α
1
2
3
4
5
6
7
8
9
Ph (3a)
4-t-BuC6H4
4-BrC6H4
4-MeOC6H4
4-NH2C6H4
4-MeC6H4
3-MeC6H4 (3g)
2-MeC6H4 (3h)
Cyclopropyl
PhCH2
99 (98)
99
38
99
99
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
79:21
100:0
97:3
>99:1
>99:1
97:3
91:9
91:9
91:9
97:3
99
99 (94)
99 (99)
99
10
93
a
Conditions: alkyne (1 mmol), phenylsilane (1 mmol), 1 (0.05 mol
%), LiHBEt3 (0.15 mol %), 40 °C, 20 h. Product ratio and reaction
yield determined by GC.
observed. Phenylacetylene derivatives with electron-donating
groups such as methoxy, amine, and methyl reacted smoothly
(99% yields), whereas the presence of electron-withdrawing 4-
bromo-substituent decreased the reaction yield (38%). Cyclo-
propylacetylene and 3-phenyl-prop-1-yne also underwent
successful hydrosilylation in good yields. In all cases, exclusive
formation of α-addition products was observed and no β-
vinylsilanes were detected.
Complex 1 was found to successfully catalyze hydrosilylation
of alkynes with tertiary silane−dimethylphenylsilane. To the
best of our knowledge, Markovnikov hydrosilylation of
terminal alkynes with tertiary silanes is very scarce using
cobalt catalysis. A single example of regioselective hydro-
silylation of phenylacetylene with PhMe2SiH (84% yield, α:β =
88:12) has been reported recently.25 In our experiments,
dimethylphenylsilane underwent quantitative reactions with
terminal alkynes; however, the reaction required higher
concentration of the catalyst (0.5 mol %) and elevated
temperature (60 °C) in comparison with the protocol
developed for primary and secondary silanes. Applying the
optimized conditions, we examined the impact of alkyne
structure on silane conversion and hydrosilylation selectivity
(Scheme 3). Phenylacetylene was hydrosilylated easily by
dimethylphenylsilane with 91% selectivity toward α-addition
product (4a). The presence of alkyl substituents at the phenyl
ring led to increased selectivity toward α-addition, irrespective
of their position (4b, 4f−h). Bromo, amino and methoxy
groups were tolerated, however, hydrosilylation of 4-
bromophenylacetylene and 4-aminophenylacetylene (4c and
4e) proceeded with lower yields (75−79%) than the other
compounds. Cyclopropylacetylene underwent hydrosilylation
with excellent yield and selectivity (4i), but 3-phenylprop-1-
yne was not reactive in the reaction with dimethylphenylsilane.
Our protocol seems to be the first effective cobalt complex-
catalyzed Markovnikov-selective hydrosilylation of aromatic
and cycloaliphatic alkynes with PhMe2SiH. Unfortunately,
increasing the reaction temperature and the precatalyst loading
did not afford positive results for the reaction of other tertiary
silanes such as triphenylsilane, triethoxysilane, or triethylsilane
with terminal alkynes, which implies that it is the inherent
To highlight the utility of this procedure for the synthesis of
α-vinylsilane, we conducted the synthesis on a gram-scale
under the optimized conditions. The hydrosilylation of
phenylacetylene (4 mmol) with Ph2SiH2 (4 mmol) in the
presence of 0.1 mol % 1 and 0.3 mol % LiHBEt3 afforded 1.12
g of diphenyl(1-phenylvinyl)silane (2a) in 98% isolated yield
with 99:1 α/β selectivity.
Our next aim was to investigate the reactivity of primary
silane in the reaction with terminal alkynes. The reaction of
phenylacetylene with phenylsilane catalyzed by 1 activated
with LiHBEt3 under standard conditions (0.1 mol % of 1, 0.3
mol % of LiHBEt3, 40 °C, 20 h) results in a mixture of
Markovnikov addition product, phenyl(1-phenylvinyl)silane,
and a product of subsequent hydrosilylation of the latter,
phenyldi(1-phenylvinyl)silane. During optimization of the
reaction conditions (SI), we found that the lowering of
catalyst 1 loading to 0.05 mol % resulted in the total
conversion of substrates and did not substantially affect
hydrosilylation selectivity. Further lowering of the catalyst 1
loading to 0.005 mol % resulted in a decrease in phenylsilane
conversion; however, the catalyst still remained very active
(TOF as high as 8000 h−1). In all cases only Markovnikov
addition products were detected, and no trace amounts of β-
addition products were observed. In the optimized conditions
(0.05 mol % of 1, 0.15 mol % of LiHBEt3, 40 °C, 20 h), the
substrate scope of the Markovnikov-selective hydrosilylation
was investigated with a range of electronically different
terminal alkynes (Table 2). Aromatic and aliphatic alkynes
underwent successful hydrosilylation with phenylsilane in
excellent yields and regioselectivity; however, small amounts
of double addition products (α,α-divinylsilanes) were
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selective hydrosilylation, recently proposed for 3N pincer−
cobalt complex on the basis of DFT calculations, can be
considered.32
Scheme 3. Range of Alkynes in Hydrosilylation with
Dimethylphenylsilane
a
In conclusion, a pentacoordinated cobalt(II) chloride
complex with benzimidazole/2H-imidazole-based ligand,
activated by lithium triethylborohydride, has been found to
act as a good catalyst for regioselective hydrosilylation of
alkynes by primary, secondary, and tertiary silanes. Mild
reaction conditions, low catalyst loading, functional group
tolerance (amine, halide, ester, nitrile, etc.), and extremely high
selectivity toward α-hydrosilylation products are unique
features of the developed methodology. We believe that the
possibility of using tertiary silane makes it more universal than
the previously reported catalytic systems based on cobalt
complexes with 3N-donor ligands.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03721.
a
Conditions: alkyne (1 mmol), dimethylphenylsilane (1 mmol), 1
Experimental details, characterization data, and copies of
1H NMR, 13C NMR, and IR spectra (PDF)
(0.5 mol %), LiHBEt3 (1.5 mol %), THF, 60 °C, 20 h. Isolated yields.
Regioselectivity α:β-(E) determined by H NMR.
1
AUTHOR INFORMATION
Corresponding Author
■
nature of the catalyst to be insufficiently active for this group of
compounds.
To understand the nature of silane addition to alkyne, NMR
studies with deuterium labeled phenylacetylene were per-
formed. Reaction of D1-phenylacetylene and diphenylsilane
afforded syn-addition product, and no anti-addition products
were observed (SI). On the basis of deuterium labeling
experiments and the previously reported data,30 we propose
the following simplified mechanism for cobalt-catalyzed α-
selective alkyne hydrosilylation (Scheme 4). Cobalt(II)
precatalyst reacts with borohydride and silane to form in situ
cobalt(I)-silyl intermediate. Following this, migratory insertion
of alkyne into Co−Si bond takes place to produce vinylcobalt
species, which can undergo reaction with silane to afford α-
vinylsilane and regenerate the catalytically active cobalt-silyl
intermediate. Alternatively, Co(0) pathway of Markovnikov-
́
Piotr Pawluc − Faculty of Chemistry, Adam Mickiewicz
University, 61-614 Poznań, Poland; Centre for Advanced
Technologies, Adam Mickiewicz University, 61-614 Poznań,
Poland; orcid.org/0000-0002-0622-6487;
Email: piotr.pawluc@amu.edu.pl
Authors
Maciej Skrodzki − Faculty of Chemistry, Adam Mickiewicz
University, 61-614 Poznań, Poland; Centre for Advanced
Technologies, Adam Mickiewicz University, 61-614 Poznań,
Poland
Violetta Patroniak − Faculty of Chemistry, Adam Mickiewicz
University, 61-614 Poznań, Poland
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c03721
Scheme 4. Proposed Mechanism for Cobalt-Catalyzed α-
Selective Alkyne Hydrosilylation
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This article is dedicated to Professor Bogdan Marciniec on the
occasion of his 80th birthday. This work was supported by the
National Science Centre, Poland grants no. UMO-2016/23/B/
ST5/00177. The research was supported by grant no.
POWR.03.02.00-00-I026/16 (M. Skrodzki), cofinanced by
the European Union through the European Social Fund under
the Operational Program Knowledge Education Development.
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