Copper(II)-catalyzed highly regio- and stereoselective hydrosilylation of unactivated internal alkynes with silylborate in water

Article abstract of DOI:10.1039/c5ob00694e

The highly regio- and stereoselective hydrosilylation of internal alkynes with silylborate catalyzed by Cu(OTf)₂ with 1,10-phenanthroline as the ligand in the presence of Cs₂CO₃ in water is developed. This protocol was applied efficiently in the aqueous synthesis of multi-substituted vinylsilanes.

Full text of DOI:10.1039/c5ob00694e

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Organic &
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DOI: 10.1039/C5OB00694E
Biomolecular Chemistry
COMMUNICATION
Copper(II)-Catalyzed Highly Regio- and Stereo-selective
Hydrosilylation of Unactivated Internal Alkynes with Silylborate in
Water
Received 00th January 20xx,
Accepted 00th January 20xx
a,b
a,b
a
a
c
DOI: 10.1039/x0xx00000x
Qing-Qing Xuan, Chuan-Li Ren, Li Liu,* Dong Wang and Chao-Jun Li *
www.rsc.org/
The highly regio- and stereoselective hydrosilylation of internal hydrosilylation of internal alkynes in water is still a challenge task. It
is necessary to search for novel catalytic system, efficient catalyst
and silylation reagent, for aqueous hydrosilylation of unactivated
internal alkynes.
alkynes with silylborate catalyzed by Cu(OTf)2 with 1,10-
phenanthroline as ligand in the presence of Cs CO in water was
developed. This protocol was applied efficiently in the aqueous
synthesis of multi-substituted vinylsilanes.
2
3
Commercially available Me PhSiBpin is a powerful and efficient
2
9
silylation reagent. The electronegativity difference between boron
and silicon allows the chemoselective activation of the Si-B bond. It
Vinylmetal species are extremely important building blocks in
organic synthesis. Among them, vinylsilanes are becoming more
and more attractive due to their stability, low toxicity, and easy of
10
has been applied broadly in the formation of C-Si bond. Loh and
Oestreich reported copper(I)-catalyzed hydrosilylation of alkynes
11
1
and allenes with silylborate. Tsuji and co-workers reported the
copper-catalyzed silylcarboxylation of internal alkynes by employing
carbon dioxide and silylborates to give the silylactone in high yield
handling. Hydrosilylation of alkynes is the most straightforward
2
3
4
5
method to access vinylsilanes. Although Fe-, Co- and Ni-
complexes have been used as catalysts for the hydrosilylation of
alkynes with silanes, the more widely employed were high-cost
noble metal catalysts. Moreover, when unactivated internal alkynes
underwent the hydrosilylations with silanes, it usually gave inferior
12
with excellent selectivity. As for the aqueous C-Si bond formation
13
14
by employing silylborate, most recently, Santos and Lipshutz,
respectively developed copper-catalyzed silylation of conjugated
alkynes with carbonyl or carboxyl moiety in water through a
conjugated addition process. Until now, there was no report on the
hydrosilylation of unactivated internal alkynes with silylborate in
water.
6
regio- and stereosectivity. Due to the difficulty 0f control in the
regio- and stereoselectivity, few examples of efficient and highly
stereo- and regioselective hydrosilylation of unactivated internal
alkynes were reported.
Herein, we like to report the Cu(OTf) -catalyzed highly stereo-
Recently, on the basis of the environmental concern, aqueous
organic reactions catalyzed by transition-metals have made great
2
and regioselective synthesis of multi-substituted vinylsilanes in
7
water with unactivated internalkynes by employing Me PhSiBpin as
progresses. However, there were few examples of the
2
8
an efficient silylation reagent.
hydrosilylation of alkynes with silanes in water. Li, de Jesús, E. and
Initially, we examined the reaction of 1-phenyl-propyne (1a)
Terennas. respectively reported the synthesis of vinylsilanes by
hydrosilylations of terminal alkynes with silanes in water.
Unfortunately, when internal alkyne, 1-phenyl-propyne, was
with Me PhSiBpin (2) in the presence of Cu(OTf) (2.5 mol%) and
2
2
Cs CO (10 mol%) in water under N atmosphere. When no ligand
2
3
2
was used, a trace amount of product of 3a (- + -isomer) could be
detected (Table 1, entry 1).
Then, various ligands (Fig. 1) were screened. In the presence of
L1 and L2, only trace product was detected (Table 1, entries 2 and
treated with triethylsilane in the presence of catalyst RuCl(PPh) (5
3
8a
mol%) in water, we only got complicated mixtures. Thus, the
development of highly regio- and stereoselective method for the
3
). However, when pyridine L3 was added as the ligand in the
a.
absence of additional base, product 3a was obtained in 22% yield.
Beijing National Laboratory for Molecular Sciences (BNLMS),. CAS Key Laboratory
of Molecular Recognition and Function, Institute of Chemistry, Chinese Academy
of Sciences, Beijing 100190, China. E-mail: lliu@iccas.ac.cn; Fax: +86-10-
b.62554449; Tel: +86-10-62554614
To our delight, the addition of Cs CO (10 mol%) as additional base
2
3
15
increased the yield to 66%, and the regioisomeric ratio (/) was
raised from 83 : 17 to 92 : 8 (Table1, entries 4 and 5). Thus, both
additional base and ligand are crucial for copper catalyzed reaction
of internal alkyne with silylboronate in water. When pyridine
derived ligands L4-L6 were employed in the aqueous reaction of 1a
University of Chinese Academy of Sciences, Beijing 100049, China
c.
Department of Chemistry, McGill University, Montreal, Quebec, Canada H3A0B8
†
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/c000000x/
with 2 catalyzed by Cu(OTf) , excellent yields of 3a were obtained,
2
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COMMUNICATION
Journal Name
but with slightly less regioselectivities (entries 6~8). On the contrary,
Additional base affected the yield and rVeigewioAsretilcelecOtinvliintye
4
-hydroxymethyl-pyridine L7 gave relatively lower yield (80%) with significantly. Among the screened bases, CDs OCIO: 10c.1o0u3l9d/Cp5rOovBi0d0e69th4Ee
2
3
high regioselectivity (/ = 92 : 8) (entry 9). 2,2’-bipyridine ligand L8 best result (entry 11 vs 12~15). It is noteworthy that the use of KOH
gave moderate yield and regioselectivity (Table 1, entry 10). To our as a base gave poor yield and regioselectivity (entry 15). Therefore,
delight, 1,10-phenanthroline L9, which has more rigid structure for aqueous reaction of 1a with 2, t-BuOK is not an optimal base as
than L8, afforded both high yield (97%) and excellent it is prone to hydrolysis to KOH in water. In summary, the best
regioselectivety (/ = 94 : 6) (Table 1, entry 11). However, when conditions for the reaction of internal alkynes with silylboronate in
the loading of L9 was increased from 3 mol% to 6 mol%, the water included that: Cu(OTf)
reaction of 1a with 2 did not proceed at all (entry 16). It could be L9 (3 mol%) as the ligand and additional base Cs
assumed that the inactivated intermediate of Cu(II) with 1,10- under N atmosphere at room temperature. The protocol showed
phenanthroline was formed (Fig. 2), which could not go through preference syn-addition leading to excellent -selectivity with the
(2.5 mol%) with 1,10-phenanthroline
2
CO (10 mol%)
2
3
2
16
the subsequent transmetalation process.
absolute (E)-configuration of formed double bond.
Figure 2. Unactive complex of Cu(II) with 1,10-phenanthroline
Methanol has been employed widely as proton donor and
1
1
solvent in the reactions of Me PhSiBpin. However, in the reaction
2
Figure 1. Ligands for Cu(II)-catalyzed reaction of 1a with
water.
2 in
of 1a with 2 under the catalysis of Cu(OTf) /1,10-phenanthroline,
2
the amount of water in co-solvent (CH OH/H O) affected the yields
3
2
of products 3a (- + -) strongly (Table 2). When the ratio of
methanol to water in co-solvent was 1 : 4, the reaction system
became heterogeneous, the yield of 3a increased to 76% with high
a
Table 1. Cu(II)-Catalyzed Hydrosilylation of 1a with 2 in Water.

-selectivity (Table 2, entry 3). Interestingly, it was found that for
the reaction of 1a with 2, the results obtained in heterogeneous
system were better than that in homogeneous system (Table 2,
entries 3, 4 vs 1, 2)
b
Entry
Ligand (mol%)
Base
Yield of 3a (%)
c
(
/)
a
d
e
Table 2. Cu(II)-Catalyzed Hydrosilylation of 1a with 2 in Water.
1
---
Cs CO
2
trace
trace
trace
3
3
3
e
e
2
3
L1 (6)
L2 (6)
L3 (6)
L3 (6)
L4 (6)
L5 (6)
L6 (6)
L7 (6)
L8 (3)
L9 (3)
L9 (3)
L9 (3)
L9 (3)
L9 (3)
L9 (6)
Cs CO
2
b
Entry
Solvent
Yield (%)
Cs CO
2
c
(/)
f
4
---
22 (83/17)
66 (92/8)
99 (87/13)
99 (88/12)
91 (87/13)
80 (92/8)
70 (84/16)
97 (94/6)
26 (81/19)
34 (81/19)
5
6
7
8
9
Cs CO
2
3
3
3
3
3
3
3
1
2
3
4
CH OH (homogenous)
28 (90/10)
56 (90/10)
76 (92/8)
97 (92/8)
3
Cs CO
2
Cs CO
2
CH OH/H O(1:1) (homogeneous)
3 2
Cs CO
2
CH OH/H O(1:4) (heterogeneous)
3
2
Cs CO
2
1
0
1
Cs CO
2
H O (heterogeneous)
2
1
Cs CO
2
a
1
1
1
1
1
2
3
4
5
6
K CO
2
General procedure: A mixture of 1a (1.0 equiv), 2 (1.5 equiv),
L9 (3 mol%), Cs CO (10 mol%) and Cu(OTf) (2.5 mol%) in
3
Na CO
2
3
2
2
3
b
e
water (1 mL) was stirred under N at room temperature. The
2
KOAc
KOH
trace
1
c
yields of 3a(+) were determined by H NMR analysis. The
ratio of (/) was determined by H NMR analysis.
16 (81/19)
NR
1
Cs CO
2
3
a
General procedure: A mixture of 1a (1.0 equiv), 2 (1.5 equiv),
ligand (3~6 mol%), base (10 mol%) and Cu(OTf) (2.5 mol%)
2
With optimized conditions in hand, the substrate-scope of the
reaction of internal alkynes with Me PhSiBpin in water was
2
b
in water (1 mL) was stirred under N at room temperature. The
2
1
c
yields of 3a were determined by H NMR analysis. The ratio
/) was determined by H NMR analysis. Without the use of
ligand. the ratio was not detected. Without the use of base.
1
d
investigated (Table 3). Substrates possessing both electron-
donating and electron-withdrawing groups on the phenyl ring could
react efficiently, giving highly regio- and stereospecific vinylsilanes.
A wide range of functional groups in the phenyl ring, including
halide, cyano, ester groups, were tolerant to the reaction conditions.
(
e
f
2
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Even the ester group was not hydrolyzed in such aqueous basic of the pyridine ring might coordinate with vinyl coppVeierwcAormticpleleOxnlitnoe
media. The substrate with long alkyl chain (1j) gave lower yield of enhance the -regioselectivity. When trimDeOthI:y1l0s.i1ly0l3a9c/eCt5yOleBn0e06(914pE)
(
3
E)-3j (65%), but with the high regioselectivity (/ = 90 : 10) (Table was used in the reaction with 2 in water, the -regioselective
, entry 10). It was noteworthy that the internal alkynes bearing product 3p was obtained with exclusively E- configuration in 95%
pyridinyl (1n) and thiophenyl (1o) gave high yields and excellent yield (entry 16), while under the Pt(DVDS)-P catalysis, the
regioselectivities (Table 3, entries 14 and 15). Especially, for 1n, it stereoselectivity of the hydrosilylation of 1o with triethylsilane was
8a
gave -3n as a sole isomer. It was suggested that the nitrogen atom poor (E/Z = 35 : 65).
We also conducted the reaction of internal alkynes 1a with 2 in
D2O (Scheme 1). The reaction could afford deuterated (E)- vinyl-
silanes stereospcifically in high yield (-4a+-4a) (96%) with
excellent regioselectivity (/ = 94 : 6).
a
Table 3. The aqueous synthesis of multisubstituted vinylsilanes
Scheme 1. The reaction was carried out in D O.
2
Santos proposed that Cu(II)-catalyst could activate Si-B bond
1
7
through transmetalation to form Cu-Si intermediate, which
undergoes Michael addition with ,-unsaturated carbonyl
substrates. Thus we assume that the silylcupration of internal
alkynes also proceeds through transmetalation process. LCu(OTf)2
can be converted to more activated LCu(OH) assisted by Cs CO in
2
2
3
water. Then LCu(OH)2 undergoes σ-bound metathesis with
Me PhSiBpin, generating intermediate I. The intermediate I can be
2
coordinated with alkyne, forming intermediate II. Subsequently, the
C-C triple bond inserts into the Cu-Si bond, giving intermediate III.
Finally the vinyl copper species undergoes hydrolysis to afford
vinylsilanes and LCu(OH) is regenerated.
2
Figure 3. The proposed mechanism of the silylcupration of internal
alkynes
a
General procedure: A mixture of alkynes (1.0 equiv), 2 (1.5 equiv),
L9 (3 mol%), Cs CO (10 mol%) and Cu(OTf) (2.5 mol%) in 1 ml
2
3
2
b
water was stirred under N at room temperature, determined by
NOESY, isolated yield, the ratio (/) was determined by H NMR
2
Conclusions
c
d
1
In conclusion, we have developed an efficient system for the
synthesis of multisubstituted vinylsilanes via silylcupration of
unactivated internal alkynes. The reactions were conducted in
water which was environmentally friendly. The aqueous
analysis.
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hydrosilylation exhibited high efficiency and excellent regio- and
Journal Name
Petrignet, J.; Díaz Álvarez, J.; Robeyns, K.; Riant, O. Angew.
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stereoselectivity. This method, using low cost Cu(OTf) as catalyst
DOI: 10.1039/C5OB00694E
2
Chem., Int. Ed. 2013, 52, 1785; c) O’Brien, J. M.; Hoveyda, A.
with 1,10-phenanthroline (L9) as ligand, and water as solvent and
proton source, provided a convenient and practical route to
multisubstituted vinylsilanes.
H. J. Am. Chem. Soc., 2011, 133, 7712; d)Walter, C.; Auer, G.;
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Acknowledgements
We thank the National Natural Science Foundation of China (Nos.
Wang, D.; Chen, Y. J.; Li, C. J. J. Org. Chem., 2013, 78, 11076.
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