An NNN-Pincer-Cobalt Complex Catalyzed Highly Markovnikov-Selective Alkyne Hydrosilylation

Article abstract of DOI:10.1021/acs.orglett.8b02746

A new NNN pincer (amine-pyridine-imine, API) cobalt complex, which is bench-stable and is applicable for the highly efficient and regioselective hydrosilylation of terminal alkynes, is developed. A broad set of α-vinylsilanes was successfully synthesized in good to high yields with up to 98/2 Markovnikov regioselectivity. This protocol can be readily scaled up for gram-scale synthesis and demonstrates the most efficient cobalt-catalyzed hydrosilylation of alkynes with turnover frequencies as high as 126720 h^-1 to date.

Full text of DOI:10.1021/acs.orglett.8b02746

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
An NNN-Pincer-Cobalt Complex Catalyzed Highly Markovnikov-
Selective Alkyne Hydrosilylation
Shaochun Zhang, Jessica Juweriah Ibrahim, and Yong Yang*,†
†
†,‡
†CAS Key Laboratory of Bio-Based Materials, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of
Sciences, Qingdao 266101, China
‡
University of Chinese Academy of Sciences, Beijing 100049, China
*
S Supporting Information
ABSTRACT: A new NNN pincer (amine−pyridine−imine,
API) cobalt complex, which is bench-stable and is applicable
for the highly efficient and regioselective hydrosilylation of
terminal alkynes, is developed. A broad set of α-vinylsilanes
was successfully synthesized in good to high yields with up to
9
8/2 Markovnikov regioselectivity. This protocol can be
readily scaled up for gram-scale synthesis and demonstrates
the most efficient cobalt-catalyzed hydrosilylation of alkynes
with turnover frequencies as high as 126 720 h−1 to date.
inylsilanes are versatile building blocks in organic
synthesis and material chemistry, because of their ease
Scheme 1. Non-Noble Base Metal Complexes for
Hydrosilylation of Terminal Alkynes
V
of handling, nontoxicity, and utility in a broad range of
1
transformations. Among the various preparative methods for
vinylsilanes, transition-metal-catalyzed hydrosilylation of al-
kynes has been regarded as one of the most straightforward
and atom-economic approaches to obtain synthetically
2
valuable vinylsilanes. One key issue in hydrosilylation of
terminal alkynes is the control of the regio- and stereo-
selectivity, because the reaction usually results in a mixture of
regioisomers and/or stereoisomers, e.g., β-(E)-, β-(Z)-, and α-
vinylsilane isomers operated via either an anti-Markovnikov or
1e,2a
Markovnikov pathway (see Scheme 1).
Over the past few decades, great achievements have been
made and many expensive and scarce noble-metal-catalyst
3
4
5
3g,6
7
systems, such as Pt, Rh, Ru, Pd,
and Ir, for selective
synthesis of vinylsilanes have been developed. However, from
an economical and environmental point of view, the develop-
ment of alternative catalysts based on Earth-abundant and low-
2
d,8
toxicity non-noble metals is more desirable.
As a
consequence, several well-defined base-metal complexes (e.g.,
9
10
11
2d,12−15
Fe, Ni, Mn, Co,
) have been reported for highly
selective hydrosilylation of alkynes (Scheme 1). In this context,
various cobalt complexes coordinated with structurally varying
ligands generally outperformed in terms of activity and
regioselectivity and stereoselectivity. For example, Deng’s
group explored a three-coordinated carbene-phosphane
Co(I) complex, which provided high β-(E) selectivity in the
1
5
terminal alkynes with primary silanes. In addition, Huang’s
and Lu’s research groups have independently developed a
Pybox cobalt complex and asymmetry oxazoline iminopyridine
12
(
OIP) pincer cobalt complex that exhibit excellent accessibility
to α-vinylsilanes for the hydrosilylation of terminal alkynes in
016. Nonetheless, to date, the use of cobalt complexes to
hydrosilylation of terminal alkynes. Research groups led by
9
b
14a
Thomas and Ge demonstrated that pyridine-2,6-diimine
PDI), as an efficient ligand, is able to promote cobalt or iron
13
2
(
to form β-(Z)-vinylsilane isomers. Ge’s group reported
Co(acac) /dpephos or Co(acac) /Xantphos can selectively
Received: August 27, 2018
2
2
produce β-(E) dihydrovinylsilanes for the hydrosilylation of
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02746
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
facilitate the general, highly efficient, and Markovnikov
hydrosilylation of alkynes to α-vinylsilanes is still elusive and
remains a challenge.
Driven by the well-defined ligand-promoted base metals for
efficient and selective hydrosilylation of alkynes, we herein
report a new type of NNN (amine-pyridine-imine, API) pincer
(E))-vinylsilanes (3a/4a = 84/16) with α-vinylsilane 3a as the
major isomer, while only 66% NMR yield to α-vinylsilane was
achieved (see Table 1, entry 1). Based on this finding, a variety
of API·CoX (1a−1f) with different substitutes at either amine
2
fragment or imines fragment in the API skeleton were
subjected to the reaction conditions. We were pleased to
find that the 1,3,5-trimethyl imine-derived ligands cobalt
chloride complex (1d) affords maximum regioselectivity (up to
93:7) while the NMR yield was improved to 90% (entry 4).
The use of more sterically hindered 2,6-diisopropyl imine (1b
ligand and its corresponding cobalt complexes (API·CoX ) for
2
extremely efficient and Markovnikov-selective hydrosilylation
of terminal alkynes with primary silanes. A broad range of
alkynes could be effectively hydrosilylated with silanes in the
presence of API·CoX , giving α-vinylsilanes in good to high
and 1c) or diethyl-substituted-NH (1f) did not give any
2
2
yields with excellent regioselectivity.
improvement in the yield and regioselectivity to the desired
vinylsilane. Comparatively, the cobalt bromide complexes were
inferior to their chloride counterpart (entries 3 and 5 in Table
1) under identical conditions. Subsequently, various solvents
were screened to improve the reaction efficiency (Table 1,
entries 7−11). It turns out that 1,4-dioxane showed roughly
equal reactivity and regioselectivity to that in THF, while
toluene, dichloromethane, ether, and hexane all gave relatively
poorer outcomes.
NNN pincer ligands could be readily synthesized from
commercially available 2-acetyl-6-methylpyridine (see details
in the Supporting Information). Reaction of NNN pincer
ligands with CoX (X = Cl or Br) in THF gave the respective
2
API·CoX complexes, which were further structurally verified
2
by the representative crystal structure of 1d (see Table 1). The
as-synthesized complexes are air- and moisture-stable, and are
operationally simple and easy to handle on the laboratory
bench.
Further studies show that the catalyst loading (1d) can be
reduced to 0.5 mol % (see Table 2). In this case, the reaction
a
Table 1. Optimization for Alkyne Hydrosilylation
Table 2. Catalytic Performance with Lower Catalyst
Loading under Optimized Conditions
x
time, t
[s]
conversion GC yield
selectivity,
3a/4a [%]
TOF
−1
[
mol %]
[%]
[%]
[h ]
0
0
0
0
.5
5
5
30
60
>99
17
65
88
15
62
86
90/10
92/8
88/12
84/16
126720
127059
87529
60706
.085
.085
.085
98
could complete instantaneously in 5 s to produce the silylated
products in 88% yield with excellent regioselectivity (3a/4a =
b
c
time, t conversion
yield
[%]
selectivity,
b
entry catalyst solvent
[min]
[%]
3a/4a [%]
9
0/10), in which the turnover frequency is as high as 126 720
1
2
3
4
5
6
7
8
9
0
1
1a
1b
1c
1d
1e
1f
1d
1d
1d
1d
1d
THF
THF
THF
THF
THF
THF
dioxane
hexane
ether
toluene
DCM
5
5
5
5
5
5
5
60
5
5
>99
>99
>99
>99
>99
>99
>99
30
66
78
73
90
80
80
91
N.D.
75
85
84/16
90/10
91/9
93/7
92/8
93/7
92/8
85/15
88/12
92/8
92/8
−
1
h . To the best of our knowledge, this is the most efficient
cobalt catalysis system for alkyne hydrosilylation so far.
further decrease of the catalyst loading results in relatively
lower regioselectivity (3a/4a = 84/16) (see Table 2). In
addition, the hydrosilylation did not proceed at all in the
13b
A
absence of either 1d or NaBHEt , albeit with prolonging
3
reaction times (see entries 4 and 5 in Table S1 in the
Supporting Information), indicating the essential role of 1d
and activator for the reaction.
To explore the general applicability of this protocol, a variety
of alkynes were subjected to the optimized conditions for
selective hydrosilylation, and the results are summarized in
Scheme 2. Overall, a wide range of alkynes bearing electron-
donating or electron-withdrawing substituents on the phenyl
95
98
98
1
1
5
78
a
Reaction conditions: phenylacetylene (0.5 mmol), PhSiH3 (0.5
mmol), catalyst (2 mol %), NaHBEt (6 mol %), solvent (1 mL),
3
b
room temperature. Determined by GC-FID using mesitylene as an
c
1
internal standard. Determined by H NMR using mesitylene as an
internal standard.
ring reacts with PhSiH smoothly to afford the corresponding
3
α-vinylsilanes (3a−3x) in moderate to good yields and
excellent regioselectivities. The substituents in the para, meta,
and ortho of the phenyl ring are compatible with the reaction
conditions. The substrates with the ortho position appeared to
be less reactive than their para- or meta-substituted analogues;
therefore, prolonging the reaction time or increasing the
catalyst loading is demanded for full conversion (2l and 2n).
Moreover, the ortho-substituted alkynes gave relatively lower
α-selectivity, compared with para- or meta-substituted
We started our investigations by testing the hydrosilylation
of phenylacetylene (2a) with PhSiH as a benchmark reaction.
3
The reaction was initially performed in the presence of 2
mol % of cobalt precatalyst 1a and 6 mol % of NaBHEt as an
3
activator in tetrahydrofuran (THF) at room temperature. We
found that 1a demonstrates high activity and the reaction is
rapidly completed within 5 min to afford a mixture of (α + β-
B
DOI: 10.1021/acs.orglett.8b02746
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 2. Scope of Alkynes
no reaction occurs for Ph SiH with more steric hindrance,
3
albeit with a prolonging reaction time.
To highlight the practical utility of this procedure for the
synthesis of α-vinylsilane, we conducted the reaction on gram-
scale under the standard conditions. The hydrosilylation of 2a
(
6 mmol) with PhSiH in the presence of 1 mol % 1d afforded
3
1.05 g of 3a in 83% isolated yield with 92:8 α/β selectivity
(
Scheme 3).
Scheme 3. Gram-Scale Synthesis of 3a
16
Deuterium-labeling experiments (Scheme 4) were con-
ducted to gain insight into the Co-catalyzed Markovnikov
Scheme 4. Deuterium-Labelling Experiments
a
Reaction conditions: alkyne (0.5 mmol), PhSiH (0.5 mmol), 1d (2
3
b
mol %), NaHBEt (6 mol %), THF (1 mL), room temperature. 1 h.
3
c
d
e
1
d (5 mol %), 2.5 h. 1 mL 1,4-dioxane instead of THF. The ratio of
t/4t determined by H NMR. Parentheses represents the ratio of 3/
1
3
4
determined by GC-FID. Yields of isolated product are given.
analogues (2h, 2i, and 2q). These outcomes indicate the ortho-
substituted groups have a negative influence on the reaction
efficiency, in terms of activity and regioselectivity, most likely
due to the steric effect. A broad range of functional groups are
tolerated to give high activity and excellent regioselectivity,
showcasing good functionalities of this catalysis system.
Halogen-substituted phenyl alkynes gave the Markovnikov
hydrosilylation product smoothly in high regioselectivity and
no observation of dehalogenation products (2k, 2l, 2m, and
hydrosilylation of alkynes. Both the reaction of deuterated 2a/
PhSiH (eq 1 in Scheme 4) and 2a/PhSiD (eq 2 in Scheme 4)
afforded deuterated 3a with high yield and α-selectivity (92:8).
Furthermore, the deuterium ratio in 3a showed that both syn-
3
3
2
n). Other functional groups, such as −NH (2x), −N(CH )
2
3 2
(2u), −CN (2v), −OH (2t) are compatible with the present
conditions to afford their corresponding α-vinylsilanes in high
yields. In addition, heteroaromatic alkynes, such as 3-
ethynylpyridine (2w) and 2-ethynylthiophene (2y), proceed
smoothly to deliver the corresponding α-vinylsilanes products
in satisfactory yield with high regioselectivity. The 1,4-
and anti-addition pathways of PhSiH to alkyne occur in the
3
reaction process, indicating that more than one intermediate
exists during the alkyne addition, which is consistent with the
17
previous report. However, a direct D/H exchange pathway
18
could not be excluded at this point.
diethynylbenzene (2z) can also react with PhSiH smoothly
In addition, two competing reactions (Scheme 4, eqs 3 and
4) were conducted to investigate kinetic isotope effect (KIE)
of Co-catalyzed hydrosilylation of alkynes. A typical secondary
KIE value (k /k = 1.63) was detected for the reaction of
3
to form the divinylsilane compound in moderate yield with
excellent regioselectivity. Note that the internal alkynes (2ab
and 2ac) efficiently underwent the hydrosilylation reaction
under the optimized conditions too, affording the syn-addition
products in good yield. However, when the terminal alkyl
alkyne (2aa) was subjected to the standard conditions, a
mixture of Markovnikov and anti- Markovnikov vinylsilanes
H
D
phenylacetylene (1.0 equiv) with PhSiH (1.0 equiv) and
3
PhSiD (1.0 equiv) (eq 4 in Scheme 4), suggesting that the
3
oxidative addition of hydrosilane is not involved in the rate-
determining step. Alternatively, for the reaction of phenyl-
acetylene (1.0 equiv) and D-phenylacetylene (1.0 equiv) with
PhSiH (1.0 equiv) (eq 3), an equal KIE value (k /k = 1.63)
(
α:β ratio = 45:55) was obtained, albeit in high yield.
Primary, secondary, and tertiary hydrosilanes are also
3
H
D
suitable for the hydrosilylation reaction with phenylacetylene
to afford their corresponding α-vinylsilanes in appreciable
yields (Table S1, entries 13−16). However, the reaction
efficiency and regioselectivity are highly hydrosilane substrate-
dependent. Longer reaction times, in particular, are necessary
for the tertiary hydrosilane to achieve good results. Note that
was determined. The deuterated products in two reactions
showed that the syn-addition of the silane to alkyne is a more
favorable pathway than the anti-addition.
Based on the results of the deuterium labeling experiments
and the precedent of cobalt-catalyzed hydrosilylation of
2d,19
unsaturated compounds,
a mechanism accounting for the
C
DOI: 10.1021/acs.orglett.8b02746
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
formation of α-vinlysilane is presented in Scheme 5. We
propose that a low-valent cobalt(I) silyl intermediate (A)
Accession Codes
2d,20
CCDC 1860603−1860604 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033.
Scheme 5. Proposed Mechanism
AUTHOR INFORMATION
■
Corresponding Author
*
E-mail: yangyong@qibebt.ac.cn.
ORCID
Yong Yang: 0000-0002-6118-5200
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors would like to acknowledge the financial support
from QIBEBT and the 13th-Five Key Project of the Chinese
Academy of Sciences (Grant No. Y7720519KL).
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