Copper-catalyzed highly regioselective silylcupration of terminal alkynes to form α-vinylsilanes

Article abstract of DOI:10.1021/ja109464u

A highly regioselective synthesis of branched vinylsilanes through silicon-copper additions to terminal alkynes catalyzed by copper(I) was developed using methanol as additive. The corresponding vinylsilanes were obtained with excellent branched to linear selectivity of up to 99/1 in good yields.

Full text of DOI:10.1021/ja109464u

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pubs.acs.org/JACS
Copper-Catalyzed Highly Regioselective Silylcupration of Terminal
Alkynes to Form r-Vinylsilanes
Peng Wang, Xue-Liang Yeo, and Teck-Peng Loh*
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
Singapore 637371
S Supporting Information
b
Scheme 1. Isomers in the Silylcupration of Terminal Alkynes
Based on Si-B Activation Catalyzed by Copper
ABSTRACT: A highly regioselective synthesis of branched
vinylsilanes through silicon-copper additions to terminal
alkynes catalyzed by copper(I) was developed using metha-
nol as additive. The corresponding vinylsilanes were ob-
tained with excellent branched to linear selectivity of up to
99/1 in good yields.
Table 1. Copper-Catalyzed Silylcupration of 1- Octynea
rganosilanes are versatile building blocks in organic synth-
O
esis and material chemistry.1 Among them, vinylsilanes have
been known to be attractive intermediates since they can serve as
stable vinylic anion or cation synthetic equivalents.2 However,
the widespread utilization of these compounds has been impe-
ded by the lack of practical methods for accessing stereo- and
regiodefined vinylsilanes. The most straightforward and atom-
economical method to access vinylsilanes is via the regioselective
hydrosilylation of alkynes.3 However, only few catalytic systems,
such as Ru-based catalysts, have been established for the gen-
erally selective synthesis of R-vinylsilanes.4 Apart from this, the
silylmetalation of alkynes offers an alternative and attractive stra-
tegy for the efficient synthesis of stereodefined vinylsilanes.5 For
instance, Uchiyama et al. employed a bulky zincate (SiBNOL-
Zn-ate) to promote the chemo- and regioselective silylzincation
of terminal alkynes, affording R-vinylsilanes without any catalyst.6
However, it is still a challenge to gain highly branched selectivity
with general terminal alkynes through silylcupration.7 Recently,
many research groups reported Cu-B additions to multiple
bonds through activation of B2(pin)2 with copper alkoxide,8 and
the similar strategy was applied to form a C-Si bond.9,10 Herein
we report the low-cost copper-catalyzed highly chemo- and
regioselective silylcupration of unfunctionalized and functiona-
lized terminal alkynes based on Si-B activation,11 resulting in a
high degree of Markovnikov selectivity (Scheme 1).
entry
phosphine
T (°C)
yield (%)b
ratio (B:L)c
1
2
3
4
5
6
7
8
dppe
-20
-20
-20
-20
-20
-20
0
47
14
12
62
70
73
80
93
50:50
18:82
5:95
dppf
Xantphos
PEt3
86:14
93:7
PtBu3
Johnphos
Johnphos
Johnphos
99:1
99:1
95:5
rt
a Reaction conditions unless otherwise specified: 1-octyne (0.3 mmol,
1 equiv), PhMe2SiB(pin) (1.1 equiv), CuCl (0.1 equiv), phosphine (0.1
equiv), NaOtBu (0.11 equiv) and MeOH (2.0 equiv) in THF (1.0 mL)
for 24 h. b Isolated yields of the mixture of isomers. c Determined by 1H
NMR analysis of the crude mixture.
and the desired R-vinylsilane was obtained in moderate yield
(entry 4). On the basis of these results, we proposed that the
utilization of electron-rich and bulky monophosphine ligands
might increase reactivity and improve selectivity. Better yield and
regioselectivity were indeed observed when using PtBu3 (entry
5). It was gratifying to find that the reaction gave the best results
with good yield of 80% and excellent regioselectivity (B:L 99:1)
at 0 °C with the stable Johnphos ligand developed by Buchwald
(entry 7).12 When the reaction was performed at ambient tempe-
rature, better yield was achieved, albeit with slight deterioration
of selectivity (entry 8).
Table 1 summarizes the representative results for the silylcu-
pration reaction of 1-octyne. The reaction of 1-octyne with
PhMe2SiB(pin) was carried out in the presence of CuCl (10 mol %)
and NaOtBu (11 mol %) in THF under different reaction
conditions. Various phosphine ligands were screened to improve
the selectivity and yield. In the presence of the bidentate ligand
dppe, the product was obtained in 47% isolated yield without regio-
selectivity (B:L 50:50; Table 1, entry 1). Later dppf and Xant-
phos ligands were tried; vinylsilanes were obtained in poor yields,
but with good linear to branched selectivities (entries 2 and 3).
Interestingly, the regioselectivity was reversed when using PEt3
Received: October 20, 2010
Published: January 5, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ja109464u J. Am. Chem. Soc. 2011, 133, 1254–1256
|

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Table 2. Highly Branch-Selective Synthesis of Vinylsilanes of
Various Terminal Alkynes Catalyzed by Coppera
Scheme 2. Proposed Catalytic Cycle for Silicon-Copper
Additions to Terminal Alkynes
branched regioselectivities (entries 1, 8, and 11). It was note-
worthy that double bond moiety in the substrate remained intact
(entry 10). Another important feature of the process was that
coordinating groups did not affect the regioselectivities (entries 6
and 12), though only moderate yields were obtained. When phe-
nylacetylene was employed as a substrate the regioselectivity dra-
matically decreased presumably due to the electron deficient nature
of the terminal alkyne carbon (entry 9).
Until now the mechanism of the silylcupration of acetylenes
has not been well-studied in literature. To the best of our know-
ledge, there is only one proposed mechanism that is similar to the
carbocupration reaction.7d It was previously proposed that the
carbocupration of alkynes proceeds via direct 1,2-addition of
R-Cu to the alkyne, leading to the four-centered addition inter-
mediate, and the Cu(I) oxidation state remains unchanged during
the reaction.13 Later Nakamura et al. deduced through systema-
tical calculations that the carbocupration reaction proceeds via an
oxidative addition to give a Cu(III) species which undergoes
reductive elimination to give the vinyl copper species.14 On the
basis of our experimental results, a plausible catalytic cycle is
shown in Scheme 2. We believe that a LCu(I)Cl complex could
easily react with NaOtBu to form a LCuOtBu complex, which
undergoes formal σ-bond metathesis with PhMe2SiB(pin) to
generate the Cu-Si key intermediate I. In order to prove initial
Si-B activation by a LCuOtBu complex, tBuOH instead of MeOH
was used as additive with model reaction conditions (Table 1,
entry 7), and similar results were gained (B:L 96:4, 79% yield).
This supports our proposed initial activation step, although a
LCuOMe complex is catalytically active when using NaOtBu
(11 mol %) in excess MeOH (2.0 equiv). Then, Cu of the Cu-Si
key intermediate I coordinates with the alkyne substrate to give
complex II. Subsequently the C-C triple bond inserts into the
Cu-Si bond, forming vinyl copper species IV, through a possible
four-membered transition state III. Finally the vinyl copper
species undergo protonolysis in the presence of methanol to
afford the product V and regenerate the catalyst VI LCuOMe
complex. It should be noted that another mechanistic pathway
which involves an oxidative addition and reductive elimination
process is also possible and cannot be ruled out.7d
a Conditions: 1 (0.3 mmol), Ph(Me)2SiB(pin) (0.33 mmol), CuCl (0.03
mmol), NaOtBu (0.033 mmol), Johnphos (0.03 mmol) and MeOH (2.0
equiv) in THF (1.0 mL), nitrogen, 0 °C, 24 h. b Isolated yields of the
mixture of isomers. c Determined by 1H NMR analysis of the crude mixture.
With the optimized conditions in hand, we next tested the
scope of the reactions using various terminal alkynes. The results
are summarized in Table 2. Substrates possessing both electron-
donating (entries 4, 6, and 12) and electron-withdrawing groups
(entries 2, 3, and 5) all can react efficiently. A wide range of
functional groups, including halide, cyano, ester, hydroxyl, and
protected amino groups, are tolerant, and the substrate bearing
an ester group gave the best yield (entry 5). Terminal alkynes
containing alkyl, benzyl, cycloalkyl groups also showed highly
In conclusion, we have demonstrated an efficient way for the
highly Markovnikov-selective silylcupration of various terminal
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dx.doi.org/10.1021/ja109464u |J. Am. Chem. Soc. 2011, 133, 1254–1256

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COMMUNICATION
alkynes. The reaction is tolerant to a wide range of functional groups,
including chloride, hydroxyl, ester, cyano, amino groups and
gives R-vinylsilanes in moderate to good yields with excellent
regioselectivities. This method, using the low-cost transition metal
copper, provides a new entry for the construction of the C-Si bond
and opens up many possibilities of installing Si into organic
molecules via Cu-Si intermediates. Efforts to delineate the
mechanism and to apply this method to other addition
reactions are in progress.
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’ ASSOCIATED CONTENT
S
Supporting Information. Additional experimental pro-
b
cedure and spectral data. This material is available free of charge
via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
teckpeng@ntu.edu.sg
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’ ACKNOWLEDGMENT
This research was supported financially by the Nanyang Tech-
nological University and the Singapore Ministry of Education
Academic Research Fund Tier 2 (T206B1221; T207B1220RS).
Prof. S. H. Hong is acknowledged for his kind help.
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