Cobalt-Catalyzed Alkyne Hydrosilylation and Sequential Vinylsilane Hydroboration with Markovnikov Selectivity

Article abstract of DOI:10.1002/anie.201605615

A pyridinebis(oxazoline) cobalt complex is a very efficient precatalyst for the hydrosilylation of terminal alkynes with Ph₂SiH₂, providing α-vinylsilanes with high (Markovnikov) regioselectivity and broad functional-group tolerance. The vinylsilane products can be further converted into geminal borosilanes through Markovnikov hydroboration with pinacolborane and a bis(imino)pyridine cobalt catalyst.

Full text of DOI:10.1002/anie.201605615

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International Edition: DOI: 10.1002/anie.201605615
German Edition: DOI: 10.1002/ange.201605615
Vinylsilanes
Cobalt-Catalyzed Alkyne Hydrosilylation and Sequential Vinylsilane
Hydroboration with Markovnikov Selectivity
Ziqing Zuo, Ji Yang, and Zheng Huang*
Abstract: A pyridinebis(oxazoline) cobalt complex is a very
efficient precatalyst for the hydrosilylation of terminal alkynes
with Ph2SiH2, providing a-vinylsilanes with high (Markovni-
kov) regioselectivity and broad functional-group tolerance.
The vinylsilane products can be further converted into geminal
borosilanes through Markovnikov hydroboration with pina-
colborane and a bis(imino)pyridine cobalt catalyst.
V
inylsilanes are versatile synthetic building blocks owing to
their stability, ease of handling, non-toxicity, and propensity
to undergo a variety of transformations.[1] Alkyne hydro-
silylation is the most atom-economic method to access such
compounds.[2] One important issue in the hydrosilylation of
terminal alkynes is the control of the regio- and stereochem-
istry because the reaction can generate (E)-b-, (Z)-b-, and
a-vinylsilanes. Indeed, although many catalysts can be used
for anti-Markovnikov selective hydrosilylation to form
b-vinylsilanes,[3] reactions of alkynes without a directing
group that proceed with Markovnikov selectivity (a-selectiv-
ity) are rare.[4] In the early 2000s, the groups of Trost[4a,b] and
Yamamoto[4c] independently reported the Markovnikov
hydrosilylation of terminal alkyl alkynes to form a-vinyl-
silanes with pentamethylcyclopentadiene ruthenium catalysts.
Cossy and co-workers showed that Ru alkylidene complexes
also catalyze the hydrosilylation of alkyl alkynes to form
a-vinylsilanes with a high degree of regioselectivity.[4d] How-
ever, whereas these Ru catalysts gave high Markovnikov
regioselectivity when alkyl alkynes were used as the sub-
strates, a-selective hydrosilylations of terminal aryl alkynes
have hardly been described (Scheme 1a).[4,5]
Scheme 1. Markovnikov hydrosilylation of terminal alkynes and hydro-
boration of vinylsilanes.
a-vinylsilanes from challenging terminal aryl alkynes. Fur-
thermore, in the presence of a bis(imino)pyridine Co catalyst,
the resulting a-vinylsilanes underwent selective Markovnikov
hydroboration to produce geminal borosilanes with a tetra-
substituted carbon center (Scheme 1b).
We commenced our study by examining Co and Fe
catalysts with tridentate ligands for the hydrosilylation of
phenylacetylene 1a with Ph2SiH2 (Table 1). In the presence of
our iminopyridine-oxazoline (IPO) cobalt complex (IPO)-
CoCl2 (3a; 2 mol%)[10g] and NaBHEt3 (4 mol%) as the
catalyst activator in THF at room temperature, a-vinylsilane
2a and (E)-b-vinylsilane (E)-2a’ were formed as the major
and minor product, respectively [2a/(E)-2a’ = 81:19, 74%
combined yield]. The corresponding (Z)-b-vinylsilane (Z)-2a’
was not observed (entry 1). Switching the precatalyst to a Co
complex with a PNN ligand (4a)[10f] resulted in a slightly
improved regioselectivity for the Markovnikov product
(2a/(E)-2a’ = 87:13, 66% yield; entry 3). Whereas (IPO)Fe
complex 3b was found to be inactive (entry 2), the (PNN)Fe
complex 4b gave the three isomers 2a, (E)-2a’, and (Z)-2a’ in
a 47:20:33 ratio (60% combined yield; entry 4). The bis-
(imino)pyridine Co and Fe complexes (PDI)CoCl2 (5a) and
(PDI)FeBr2 (5b)[12] (2 mol%) also did not effect the desired
hydrosilylation (entries 5 and 6).[13]
To further improve the catalytic efficiency and the
Markovnikov selectivity, a series of pyridinebis(oxazoline)
Co complexes, (RPyBox)CoCl2 (6a and 6c–6g), and one Fe
complex (6b) were prepared (see the Supporting Information
for details and Table 1 for the single-crystal structure of
6c).[14] Among them, the Co complexes with iPr (6a) and tBu
(6c) substituents at the oxazoline rings gave the best activity
and selectivity (entries 7 and 9). The reaction proceeded to
completion within one hour using only 0.5 mol% of 6c, and
furnished the desired product in 94% yield with a 2a/(E)-2a’
ratio of 93:7 (entry 10). The catalyst activator is not limited to
NaBHEt3 as reactions with MeLi, TMSCH2Li, MeMgBr, or
PhMgBr (2 equiv relative to Co) as the activator all gave the
The metal-catalyzed hydroboration of vinylsilanes can
generate borosilanes, which are likely to be valuable synthetic
intermediates for the construction of complex molecules.[6]
Whereas several methods are available for the synthesis of
vicinal borosilanes,[7] there are only few approaches towards
geminal borosilanes,[8] and methods that provide access to
derivatives with a tetrasubstituted carbon center are partic-
ularly rare.[9] Driven by our interest in developing base-metal
catalysts for alkene/alkyne functionalizations,[10,11] we herein
report a method for the Markovnikov hydrosilylation of
terminal alkynes with a pyridinebis(oxazoline) Co catalyst.
The process is highly selective for the formation of
[*] Z. Zuo, J. Yang, Prof. Dr. Z. Huang
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
345 Lingling Road, Shanghai 200032 (China)
E-mail: huangzh@sioc.ac.cn
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201605615.
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Table 1: Catalyst screen for the hydrosilylation of 1a with Ph2SiH2.[a]
Entry
Catalyst (mol%)
t [h]
Yield [%]
2a/(E)-2a’/(Z)-2a’
1
2
3
4
5
6
7
8
9
3a (2)
3b (2)
4a (2)
4b (2)
5a (2)
5b (2)
6a (2)
6b (2)
6c (2)
6c (0.5)
12
12
12
12
12
12
12
12
12
1
74
<5[b]
66
81:19:0
–
87:13:0
47:20:33
–
60
<5[b]
<5[b]
93
–
93:7:0
–
94:6:0
93:7:0
<5[b]
92
10
94
[a] Reaction conditions: 1a (0.5 mmol), PhSiH2 (0.5 mmol), THF (1 mL).
Yields of isolated products are given unless otherwise noted. The
product ratios were determined by GC analysis. [b] Determined by GC
analysis.
desired product in high yield and selectivity (see the
Supporting Information). The Fe analogue (iPrPyBox)FeBr
(6b), however, gave only a trace amount of the hydro-
silylation product (entry 8).
Utilizing the (PyBox)Co complex 6c as the precatalyst, we
examined the alkyne substrate scope of the Markovnikov
hydrosilylation (Scheme 2). The catalyst enabled the hydro-
silylation of a diverse array of terminal aryl alkynes with
Ph2SiH2, providing the corresponding a-vinylsilanes with high
regioselectivities [a/(E)-b ratios of 92:8 to 99:1]; the (Z)-b
isomers were never observed. Aryl alkynes bearing either
electron-donating or electron-withdrawing groups were effec-
tively hydrosilylated, and substituents in the para, meta, and
ortho positions of the aryl ring are compatible with the
reaction conditions. However, substrates with ortho substitu-
ents appeared to be less reactive than their para- or meta-
substituted analogues, and thus required longer reaction times
(2 h for 2c and 2h) or higher catalyst loadings (2 mol% for 2 f
and 2q). On the other hand, ortho-substituted alkynes
generally gave higher a-selectivities than the sterically less
demanding para- or meta-substituted analogues (e.g., 2c: 98:2
vs. 2b: 93:7; 2h: 97:3 vs. 2g: 92:8), but the reasons for this
“ortho effect” on the selectivity remain to be elucidated.
Halogen-substituted aryl alkynes, including the para-bromo-
substituted one, underwent Markovnikov hydrosilylation in
high yields, and dehalogenation products were not observed.
Tertiary amine (2i), ether (2g, 2h, 2k), ester (2m), and even
Scheme 2. Markovnikov hydrosilylation of various alkynes. Reaction
conditions: 1 (0.5 mmol), Ph2SiH2 (0.5 mmol), 6c (0.5 mol%),
NaBHEt3 (1 mol%), THF (2 mL), RT, 1 h. Yields of isolated products
are given. The product ratios [a/(E)-b] were determined by GC
analysis. [a] 2 h. [b] With 2 mol% 6c for 10 h. [c] With 1 mol% 6c.
[d] With 3 mol% 6c for 12 h. [e] With 1 mol% 6c for 5 h. Product ratio
1
determined by H NMR spectroscopy.
ketone (2l) functional groups were also tolerated. Hetero-
aromatic alkynes, such as 3-ethynylthiophene and 3-ethynyl-
pyridine, gave the corresponding products (2v and 2w) with
high site selectivity. Ferrocenyl alkyne provided the desired
product 2x in 89% yield with 97:3 selectivity. The reaction of
an internal aryl alkyne, 1-phenyl-2-methylacetylene, exclu-
sively provided the syn addition product 2y, but the regiose-
lectivity was moderate (69:31).
Terminal alkyl alkynes also underwent the hydrosilylation
reaction under the optimized conditions to afford the
products in good yields. The Markovnikov products were
obtained as the major products, although the regioselectivity
(ca. 4:1) was relatively low compared to that achieved with
2
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terminal aryl alkynes (see above). Chloride, ether, and
genative silylation of 2j in the presence of an Ir catalyst[18]
phthalimide moieties in the alkyl chains were tolerated as
demonstrated by the formation of 2aa, 2ae, and 2af. The
hydrosilylation of 1-ethynylcyclohexene provided the desired
a-vinylsilane 2ad in high yield with very high regioselectivity
(97:3), whereas 1-ethynylcyclohexane only reacted with 87:13
selectivity (2ac). The preliminary data, together with the
results obtained with aryl-substituted alkynes, suggest that the
gave sila-acenaphthene 9j in useful yield [Eq. (4); dtbpy =
4,4ꢀ-di-tert-butyl-2,2ꢀ-dipyridyl, nbe = norbornene].
=
presence of a C C bond in conjugation with the alkyne may
have an advantageous effect on the Markovnikov selectivity.
On the basis of the data of selective syn H/Si addition to
internal alkynes (2y, Scheme 2) and precedents regarding
relevant Co-catalyzed alkene and alkyne hydrosilylation
reactions,[3k,10a,15] we propose a silyl migration mechanism
involving CoI silyl intermediate A[16] (Scheme 3). Most likely
More interestingly, the a-vinylsilanes react with pinacol-
borane (HBpin) to form geminal borosilanes 10 with excel-
lent regioselectivity. The formation of the sterically crowded
tetrasubstituted carbon center through Markovnikov hydro-
boration is remarkable because Lu and co-workers[19] and our
group[10g] previously demonstrated that IPO Co precatalyst 3a
gave exclusively the anti-Markovnikov hydroboration prod-
ucts when 1,1-disubstituted aryl alkyl alkenes were used as the
substrates. In contrast, a catalyst screen revealed that the Co
and Fe complexes of IPO (3a, 3b), PDI (5a, 5b), and Pybox
(6a, 6b) were all selective for the formation of geminal
borosilane 10a in the reaction of 2a with HBpin (see the
Supporting Information). Among them, PDI Co complex 5a
was most effective; in the presence of this precatalyst
(2 mol%) and NaBHEt3 (4 mol%), 10a was formed in 90%
yield. For comparison, the reaction of a-methylstyrene with
HBpin catalyzed by 5a gave a trace amount of the anti-
Markovnikov product (< 5%), but no Markovnikov product.
These results indicate that substitution of the alkyl group in
1,1-disubstituted styrenes with a silyl moiety leads to an
inversion of the site selectivity in the hydroboration with
HBpin.
Scheme 3. Proposed mechanism for the Co-catalyzed Markovnikov
alkyne hydrosilylation.
for steric reasons, the alkyne undergoes selective 1,2-insertion
À
into the Co Si bond to form Co alkenyl species C, which then
reacts with Ph2SiH2 to form the Markovnikov product and
regenerate A.
Using the standard reaction conditions, we developed
procedures to perform the reaction on gram scale. The
hydrosilylation of 1a (15 mmol) with 1 equiv of Ph2SiH2
afforded 3.9 g of the hydrosilylation product (90% yield of
isolated product) with 95:5 a/b selectivity [Eq. (1)]. The
To delineate the scope of the method, we studied the
hydroboration of various a-vinylsilanes with 5a as the
precatalyst (Scheme 4). All reactions were selective for the
generation of the geminal borosilane, and the vicinal boro-
silanes were never detected.[20] Most reactions proceeded to
high conversion within 12 hours at room temperature using
vinylsilane products can undergo various transformations.
Tamao oxidation, for example, gave the corresponding
ketones (7) in high yields [Eq. (2)]. In the presence of a Ru
catalyst,[17] the Si H bond in 2a was successfully converted
À
À
into a Si OH bond, enabling the isolation of silanol 8a in
Scheme 4. Markovnikov hydroboration of various a-vinylsilanes. Reac-
tion conditions: 2 (0.2 mmol), HBpin (0.24 mmol), 5a (5 mol%),
NaBHEt3 (10 mol%), THF (1 mL), RT, 12 h. Yields of isolated products
are given. [a] With 2 mol% 3a. [b] At 608C. [c] With 5 mol%
=
91% yield [Eq. (3)]. Furthermore, intramolecular dehydro-
(
MesPDI)CoCl2 as the precatalyst (MesPDI=2,4,6-Me3C6H2N
CMe]2C5H3N). [d] With 10 mol% 5a.
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5 mol% of the catalyst. However, the method works more
efficiently for 1,1-disubstituted aryl silyl alkenes with elec-
tron-withdrawing groups than for those bearing electron-
donating groups. For instance, the reaction of a para-
methoxy-substituted vinylsilane required an elevated temper-
ature (608C) and gave product 10g in 52% yield over 12 h,
whereas the reactions of substrates with a fluoro or chloro
substituent in para position proceeded at 258C to form the
desired products in high yields (10p: 85%, 10s: 87%) within
the same time frame. Fluorine substitution of the meta and
ortho positions of the aromatic ring was tolerated (10n: 90%,
10o: 90%). However, the para-bromo-substituted vinylsilane
gave the desired product 10t in low yield (30%). Finally,
substrates bearing 2-naphthyl and 3-thiophenyl substituents
afforded the corresponding products in good yields (10k:
76%, 10v: 94%).
The geminal borosilanes may find application in the
synthesis of multifunctionalized molecules.[6] To our delight,
preliminary studies showed that treatment of borosilanes 10
with H2O2 and NaHCO3 at room temperature resulted in the
À
À
selective conversion of the C B bond into a C OH bond
while the silyl group remained intact during the oxidation
process [Eq. (5)]. This method provides a convenient route to
a-silyl-substituted tertiary alcohols 11 from alkynes.
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In summary, we have demonstrated that synthetically
valuable a-vinylsilanes can be selectively prepared from
terminal alkynes by Co-catalyzed Markovnikov hydrosilyla-
tion. These a-vinylsilanes were also used as substrates for
a Co-catalyzed Markovnikov hydroboration to generate
novel geminal borosilanes, which are difficult to access by
other means. Both processes use sustainable, inexpensive Co
catalysts with commercially available ligands. To the best of
our knowledge, this method represents the first approach
towards geminal borosilanes with tetrasubstituted carbon
centers from simple alkynes.
Acknowledgements
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Financial support was provided by the National Basic
Research Program of China (2015CB856600) and the
National Natural Science Foundation of China (21422209,
21432011, 21272255, and 21421091).
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Received: June 9, 2016
[13] Aside from the hydrosilylation products, only traces of the
hydrogenation products were observed.
Revised: July 12, 2016
Published online: && &&, &&&&
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Vinylsilanes
Z. Zuo, J. Yang,
Z. Huang*
&&&& — &&&&
Cobalt-Catalyzed Alkyne Hydrosilylation
and Sequential Vinylsilane Hydroboration
with Markovnikov Selectivity
Terminal alkynes can be converted into a-
vinylsilanes in a cobalt-catalyzed Mar-
kovnikov alkyne hydrosilylation reaction
with Ph2SiH2. The reaction products were
further transformed into novel geminal
borosilanes in a cobalt-catalyzed hydro-
boration process, also with Markovnikov
selectivity.
6
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
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