Synthesis of multi-substituted vinylsilanes via copper(i)-catalyzed hydrosilylation reactions of allenes and propiolate derivatives with silylboronates

Article abstract of DOI:10.1039/c4cc01722f

An efficient and general copper(i)-catalyzed method for the synthesis of multi-substituted vinylsilanes is reported. Multi-substituted allenes with electron-withdrawing groups and propiolate derivatives reacted well with (dimethylphenylsilyl)boronic acid

Full text of DOI:10.1039/c4cc01722f

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Synthesis of multi-substituted vinylsilanes via
copper(I)-catalyzed hydrosilylation reactions
of allenes and propiolate derivatives with
silylboronates†
Cite this: Chem. Commun., 2014,
50, 7195
Received 7th March 2014,
Accepted 15th May 2014
Yun-He Xu,*a Liu-Hai Wu,a Jun Wanga and Teck-Peng Loh*ab
DOI: 10.1039/c4cc01722f
www.rsc.org/chemcomm
An efficient and general copper(I)-catalyzed method for the syn- high selectivities.17 Subsequently, Tsuji and co-workers elegantly
thesis of multi-substituted vinylsilanes is reported. Multi-substituted described a Cu(I)-catalyzed silacarboxylation reaction of internal
allenes with electron-withdrawing groups and propiolate derivatives alkynes with carbon dioxide and silylboranes.18 In 2013, Hoveyda’s
reacted well with (dimethylphenylsilyl)boronic acid pinacol ester to group reported an efficient NHC–Cu-catalyzed hydrosilylation of
afford silyl-substituted butenoate derivatives and b-silyl-substituted terminal alkynes also using 1 to generate linear (E)-b-vinylsilanes.19
acrylate derivatives, respectively. The corresponding products could So far, there has been no report on the copper-catalyzed hydrosilyla-
be obtained in moderate to high yields and with good to excellent tion of electron-deficient allenes or propiolate derivatives.20 The need
stereoselectivities.
to use a strong base NaOtBu to initiate the copper(I) catalytic cycle in
the previously reported works by using silylboronate reagent 1 limits
The development of new and efficient methods for the synthesis of the use of this method for base sensitive substrates.17–19 To circum-
versatile vinylsilanes has attracted the interest of many organic vent this issue in order to generate multi-substituted vinylsilanes, we
chemists.1 Accordingly, various noble transition-metal catalyzed report a copper(I)-catalyzed protosilylation reaction of allenes and
hydrosilylation of alkynes using silanes has been reported to propargyl ester derivatives to produce multi-substituted vinylsilanes
furnish this class of compounds in good yields.2 Recently, silyl- with very high stereoselectivities under very mild reaction conditions.
boronate reagents developed by Suginome’s group have been widely
Initially, 1 and 2a were chosen as the model substrates to
used for the silylation reactions to forge new C–Si bonds.3 The optimize this reaction. In the presence of a strong base KOtBu, only
research groups of Ito,4 Tanaka,5 Suginome with Ohmura,6 32% yield of the desired product was obtained (Table 1, entry 1). To
Oestreich,7 Moberg,8 Sato,9 Cheng,10 Hayashi,11 Hoveyda,12 Riant,13 clarify what leads to the relatively low yield of the product, a few
and Sawamura14 et al. have contributed greatly in this field. Among control experiments were carried out. We found that when 2a, CuCl
the metals, copper salts have been recognized to be a good choice as (10 mol%) and KOtBu (11 mol%) in THF were stirred for 24 hours in
catalysts because of their low cost and environmental friendly the absence of 1, the starting material 2a was decomposed. Without
merits.15 However, not much attention has been focused on the the use of KOtBu, only a trace amount of the desired product was
copper-catalyzed Si–B activation and transformations. The pioneer- obtained but most of 2a could be recovered (Table 1, entry 2). On this
ing work on silylcupration reaction of alkynes with Me2PhSi–BEt3Li basis, we attempted to use less basic Et3N as the additive. To our
has been reported by Oshima and co-workers.16 In this work, for delight, 14% product yield could be obtained with the recovery of
most of the cases, a mixture of vinylsilane regioisomers was obtained. starting material 2a (Table 1, entry 3). Diverse solvents were then
It was not until about 25 years later that Loh and co-workers screened and it was found that the yield of the desired product could
developed a CuCl/Johnphos system catalyzed silylcupration reaction be improved to 69% by using a protic solvent such as methanol
of terminal alkynes using Suginome’s reagent ((dimethylphenylsilyl)- (Table 1, entry 6). The use of tert-butanol gave the best results in
boronic acid pinacol ester 1) in the presence of MeOH to afford terms of yield (Table 1, entries 7–12 and 16). We observed that other
the corresponding branched vinylsilanes in good yields and phosphine ligands were efficient but not necessary to afford the
desired product 3a in this reaction (Table 1, entries 10–12 and 15).
a Hefei National Laboratory for Physical Sciences at the Microscale and Department
of Chemistry, University of Science and Technology of China, Hefei, 230026,
China. E-mail: xyh0709@ustc.edu.cn
b Division of Chemistry and Biological Chemistry, School of Physical and
Mathematical Sciences, Nanyang Technological University, Singapore 637371.
E-mail: teckpeng@ntu.edu.sg
Further control experiments were carried out and it was shown that
both the copper catalyst and Et3N were crucial in this catalytic
reaction (Table 1, entries 13 and 14). Ultimately, the optimized
reaction conditions were established as follows: 10 mol% CuBr as
a catalyst and 11 mol% dppe as a ligand using 11 mol% Et3N as an
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cc01722f additive in tert-butanol, heated at 40 1C.
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Table 1 Optimization of the reaction conditionsa
b-silylation products were obtained (Table 2, entries 3d–3g). 1,3-
Disubstituted allenes were also treated as substrates in our reaction
and the corresponding trisubstituted vinylsilanes could be generated
in reasonable yields (Table 2, entries 3h, 3i and 3j). Furthermore,
we noticed that the multi-substituted allenes (Table 2, entries 3k,
3l and 3m) are good substrates for this reaction. In all cases, only the
b-substituted products were obtained in high yields.
After the successful synthesis of dimethylphenylsilyl-substituted
butenoate derivatives, next we turn our attention to study the
hydrosilylation of propiolate derivatives which would provide very
useful Si-substituted acrylate derivatives.21 We chose methyl non-2-
ynoate and (dimethylphenylsilyl)boronic acid pinacol ester 1 to
optimize the reaction conditions (see the details in the ESI†). To
our surprise, we found that in the absence of any phosphine ligand,
the exclusive (E)-isomer 5a could be generated in a very high yield
(92%) when the reaction was carried out using 10 mol% CuBr as a
catalyst combined with 11 mol% Et3N in MeOH.22
Entry Catalyst Ligand
Additive Alcohol Solvent T (1C) Yieldb (%)
1
2
3
4
5
6
7
8
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuBr
CuI
CuBr
CuBr
CuBr
CuBr
—
dppe
dppe
dppe
dppe
dppe
dppe
dppe
dppe
dppe
dppe
Johnphos NEt3
Xantphos NEt3
dppe
dppe
—
KOtBu MeOH THF
30
30
30
32
Trace
14
No reaction
38
69
75
75
74
70
68
—
MeOH THF
MeOH THF
MeOH Toluene 30
MeOH DMSO 30
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
—
—
—
—
—
—
—
—
—
—
—
MeOH 30
tBuOH 30
tBuOH 30
tBuOH 30
tBuOH 30
tBuOH 30
tBuOH 30
tBuOH 30
tBuOH 30
tBuOH 30
tBuOH 40
9
10
11
12
13
14
15
16
67
—
No reaction
No reaction
46
81
NEt3
NEt3
NEt3
CuBr
CuBr
Next, we examined the scope of the reaction using various
propiolate derivatives. It was found that all the substrates posses-
sing aliphatic side-chains could provide the desired products in
very high yields (Table 3, entries 5a–5g). Functional groups
dppe
a
All reactions unless otherwise stated were carried out with 2a (0.3 mmol),
1 (0.33 mmol), 10 mol% of the copper catalyst (0.03 mmol), 11 mol%
ligand, 11 mol% additive and 2 equiv. of alcohol in the given solvent
b
(1.0 mL). Isolated yield.
Table 3 Synthesis of multi-substituted vinylsilanes using internal alkynesa
With the optimized conditions in hand, we next tested the scope
of allenes for this Cu(I)-catalyzed hydrosilylation reaction using
(dimethylphenylsilyl)boronic acid pinacol ester 1. The results are
shown in Table 2. Firstly, we extended this reaction to the mono-
substituted allenic ester and a sulfone. The desired products were
obtained in good yields, respectively (Table 2, entries 3b and 3c). In
all the cases using 1,1-disubstituted allenes, very high yields of the
Table 2 Synthesis of multi-substituted vinylsilanes using internal alkynesa
a
a
Unless noted otherwise, the reaction conditions are as follows: allene
Unless noted otherwise, the reaction conditions are as follows: alkyne
(0.3 mmol), 1 (0.33 mmol), CuBr (10 mol%), dppe (11 mol%) and Et3N (0.3 mmol), 1 (0.6 mmol), CuBr (10 mol%) and Et3N (11 mol%) in
b
b
(11 mol%) in tBuOH (1.0 mL) heated at 40 1C for 24 h. Isolated yields. MeOH (1.0 mL) stirred at 28 1C for 24 h. Isolated yields.
7196 | Chem. Commun., 2014, 50, 7195--7197
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We also thank the Singapore Ministry of Education Academic
Research Fund (MOE2011-T2-1-013 and NEA1002111) for the
funding of this research.
Notes and references
´
1 B. Marciniec, H. Maciejewski, C. Pietraszuk and P. Pawluc, in
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Scheme 1 Proposed mechanism for the synthesis of vinylsilanes from allenes.
5 S.-Y. Onozawa, Y. Hatanaka and M. Tanaka, Chem. Commun., 1997, 1229.
6 (a) T. Ohmura and M. Suginome, Org. Lett., 2006, 8, 2503;
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including chloride (Table 3, entry 5c), protected hydroxy
(Table 3, 5e) and bulky cyclopropane groups (Table 3, entry
5f) are well tolerated in this reaction. b-Aromatic ring substi-
tuted propiolates also worked well in this reaction (Table 3,
entries 5h–5m). The electronic properties of the substituents on
the aromatic ring have no apparent effect on the yield of the
products. Heterocycles such as a thiophene group containing
substrate also furnished the product 5p in 87% yield (Table 3,
entry 5n). When the substrates were changed to butynone
derivatives, the desired products could also be obtained in 10 K.-J. Chang, D. K. Rayaburapu, F.-Y. Yang and C.-H. Cheng, J. Am.
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moderate to good yields (Table 3, entries 5o and 5p).
Based on the observed results, a possible mechanism was
110, 5579.
proposed as shown in Scheme 1. Firstly, the Et3N could help 12 K.-S. Lee and A. H. Hoveyda, J. Am. Chem. Soc., 2010, 132, 2898.
13 (a) A. Welle, J. Petrignet, B. Tinant, J. Wouters and O. Riant,
to activate the B–Si bond via possible coordination between
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nitrogen and boron atoms.23 In the presence of a copper catalyst,
´
J. Petrignet, J. D. Alvarez, K. Robeyns and O. Riant, Angew. Chem., Int.
species A could be generated. Due to the electron density bias of
allenes with an electron-withdrawing group, an addition of Si–Cu
into the double bond will afford an allylic copper species B which
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15 S. E. Allen, R. R. Walvoord, R. Padilla-Salinas and M. C. Kozlowski,
Chem. Rev., 2013, 113, 6234 and references therein.
could then undergo protonolysis in the presence of tBuOH to give 16 K. Nozaki, K. Wakamatsu, T. Nonaka, W. Tu¨ckmantel, K. Oshima
the desired product 3.24 The reactive L–Cu–OtBu complex is then
and K. Utimoto, Tetrahedron Lett., 1986, 27, 2007.
regenerated and will be involved in the next catalytic cycle. As for
17 P. Wang, X.-L. Yeo and T.-P. Loh, J. Am. Chem. Soc., 2011, 133, 1254.
18 T. Fujihara, Y. Tani, K. Semba, J. Terao and Y. Tsuji, Angew. Chem.,
the hydrosilylation of propiolate derivatives, a similar mechanism
as depicted in Scheme 1 could also be proposed. The higher
reactivity of propiolate derivatives permits an easy transformation
of the silyl group into the triple bond.
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19 F. Meng, H. Jang and A. H. Hoveyda, Chem. – Eur. J., 2013, 19, 3204.
20 An example of copper(I)-catalyzed conjugate silyl addition to a,b-
unsaturated aldehydes, see: (a) I. Ibrahem, S. Santoro, F. Himo and
´
A. Cordova, Adv. Synth. Catal., 2011, 353, 245. Example of Lewis acid
catalyzed hydrosilylation of allenes and alkynes using silanes see:
(b) T. Sudo, N. Asao, V. Gevorgyan and Y. Yamamoto, J. Org. Chem.,
1999, 64, 2494. Examples on Cu-catalyzed borylation of allenes;
(c) W. Yuan, X. Zhang, Y. Yu and S. Ma, Chem. – Eur. J., 2013,
19, 7193; (d) F. Meng, H. Jang, B. Jung and A. H. Hoveyda, Angew.
Chem., Int. Ed., 2013, 52, 5046.
In conclusion, we have found that copper salts can catalyze
the silylation reactions of allenes and propiolate derivatives
with silylboronates under mild reaction conditions. These
methods provide facile access to versatile silyl-substituted
homoallylic esters and b-silyl-substituted acrylates in moderate
to excellent E/Z selectivities. This study also reveals that weak
bases such as triethylamine could activate the Si–B bond to
initiate the catalytic cycle for the transformation of a silyl group
into unsaturated bonds. The use of this strategy to carry out
other organic transformations is in progress.
21 M. A. Brook, Silicon in Organic, Organometallic and Polymer Chemistry,
Wiley, New York, 2000.
22 (a) The E-configuration of product 5a was determined from the 1D
NOE study, see the details in the ESI†. Two accounts of copper(II)-
catalyzed silylation of activated alkynes in water appeared during
our submission of this work. See: (b) J. A. Calderone and W. L.
Santos, Angew. Chem., Int. Ed., 2014, 53, 4154; (c) R. T. H. Linstadt,
C. A. Peterson, D. J. Lippincott, C. I. Jette and B. H. Lipshutz, Angew.
Chem., Int. Ed., 2014, 53, 4159.
We thank Prof. S. Ma of SIOC for his generous gift of the
chiral allene compound for the mechanism study. We are 23 Pyridine assisted Si–B bond activation, see: (a) Q.-Q. Xuan, N.-J. Zhong,
C.-L. Ren, L. Liu, D. Wang, Y.-J. Chen and C.-J. Li, J. Org. Chem., 2013,
78, 11076; (b) J. A. Calderone and W. L. Santos, Org. Lett., 2012, 14, 2090.
24 Please see a brief proof for the proposed mechanism in the ESI†
grateful for financial support by the National Science Foundation
for Young Scientists of China (No. 21202156) and the Natural
Science Foundation of Anhui Province (No. 1308085QB37).
section by using a chiral allene 2n to generate a racemic product 3n.
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Chem. Commun., 2014, 50, 7195--7197 | 7197