Synthesis of (E)-alkenes via hydroindation of C{triple bond, long}C in InCl3-NaBH4 system

Article abstract of DOI:10.1016/j.tet.2006.05.066

In InCl₃-NaBH₄-MeCN system, terminal aryl alkynes could couple with aryl iodides and bromides to give disubstituted alkenes via hydroindation of C{triple bond, long}C. In the similar way, (E)-alkenylsilanes were synthesized via reduction of alkynylsilanes in tetrahydrofuran (THF) in high yields. The processes showed high regio- and stereoselectivity.

Full text of DOI:10.1016/j.tet.2006.05.066

Tetrahedron 62 (2006) 7712–7717
Synthesis of (E)-alkenes via hydroindation
of C^C in InCl3–NaBH4 system
Chunyan Wang,a Lei Yan,a Zhiguo Zheng,b Deyu Yanga and Yuanjiang Pana,
*
aDepartment of Chemistry, Zhejiang University, Hangzhou 310027, PR China
bZhejiang Hisoar Pharm.Co., Ltd, Taizhou 318000, PR China
Received 9 April 2006; revised 25 May 2006; accepted 26 May 2006
Available online 16 June 2006
Abstract—In InCl3–NaBH4–MeCN system, terminal aryl alkynes could couple with aryl iodides and bromides to give disubstituted alkenes
via hydroindation of C^C. In the similar way, (E)-alkenylsilanes were synthesized via reduction of alkynylsilanes in tetrahydrofuran (THF)
in high yields. The processes showed high regio- and stereoselectivity.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
synthesis.13–15 But little attention was paid to hydroindation
of alkynylsilanes. Recently, we have reported some facile
hydroindations of alkyne in InCl3–NaBH4–MeCN sys-
tem.6,7 The good results stimulated us to further explore
the reactivity of this system. We herein present a convenient
and efficient method to synthesize (E)-alkenes under mild
conditions via reduction of alkynylsilanes or coupling reac-
tions of terminal alkynes with aryl iodides or bromides in an
InCl3–NaBH4 system.
During the past few decades, hydroboration and hydroalumi-
nation have proven reliable ways to reduce carbon–carbon
multiple bonds to give alkenylmetals formed via hydrometa-
lations of C^C, which can then be utilized in various re-
actions.1 But in these reactions expensive catalyst or high
temperature is usually required. Although in the same group
with boron and aluminum in the periodic table, the study of
hydroindation is still limited compared to hydroboration and
hydroalumination. However, since dichloroindium hydride
(Cl2InH) was generated firstly via combination of Indium(III)
chloride and tributyltin hydride system,2 Cl2InH generated
in situ has been applied to many organic reactions.3–13 Espe-
cially, because of excellent properties, such as easy hand-
ling, mild conditions, and low toxicity, InCl3–NaBH4
system has attracted much interest.3–7
2. Results and discussion
2.1. Synthesis of (E)-alkenes via coupling reaction of
terminal alkynes with aryl halides in InCl3–NaBH4–
MeCN system
Initially, anhydrous InCl3, NaBH4, MeCN, phenyl acetylene,
and iodobenzene were added in sequence in one-pot at
ꢀ15 ꢁC under nitrogen atmosphere. After the reaction was
stirred overnight at room temperature, it was found that
the only coupling product (E)-stilbene (2a) was obtained
smoothly in 78% yield without 3a and 4a as byproducts
(2a:3a:4a¼100:0:0), showing high regio- and stereo-
selectivity. The product was determined by 1H NMR
(500 MHz) and MS. The E or Z stereochemistry in the dou-
ble bond C]C of products was assigned on the basis of the
In the presence of a catalytic amount of triethylborane as
a radical initiator, hydroindation of terminal alkynes with
Cl2InH generated in situ by a combination of InCl3 and
DIBAL-H gives (Z)-alkenylindiums, which couple with
aryl iodides to form (Z)-alkenes smoothly.12 However, up to
now, coupling reaction of alkenylindium with halides to give
(E)-alkenes in InCl3–NaBH4 system has not been reported.
In addition, it has been reported that alkynylsilanes were
reduced via hydroboration and hydroalumination to form
alkenylsilanes, which are important intermediates in organic
1
value of H NMR coupling constants between the olefinic
protons and comparison of their 1H NMR spectra with those
previously reported.
Under the above reaction conditions, a variety of cross-
coupling reactions of the different terminal alkynes and
halides were examined. It was found that all the terminal
aryl alkynes could couple with aryl iodides or bromides
Keywords: Hydroindation; Alkynes; (E)-Alkenes; (E)-Alkenylsilanes;
InCl3–NaBH4 system.
* Corresponding author. Tel./fax: +86 571 8795 1264; e-mail:
panyuanjiang@zju.edu.cn
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.05.066

C. Wang et al. / Tetrahedron 62 (2006) 7712–7717
7713
Table 1. The coupling reactions of terminal alkynes and halides in InCl3–NaBH4–MeCN system
R
H
H
H
R
H
InCl3-NaBH4-MeCN
+
+
R
H
-15 °C- r.t.
Arx, 15 h
Ar
H
R
Ar
H
Ar
1
(E)-2
(Z)-3
4
Entry
R
Halides
2:3:4a
Yield (2+3%)b
Ref.
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
q
r
s
Ph
p-CH3Ph
p-BrPh
p-CH3OPh
n-Bu
C6H13OCH2
Ph
p-CH3Ph
p-BrPh
p-CH3OPh
n-Bu
C6H13OCH2
Ph
p-CH3Ph
p-BrPh
p-CH3OPh
n-Bu
C6H13OCH2
Ph
1-Cyclohexen-1-yl
PhI
PhI
PhI
PhI
PhI
PhI
p-ClC6H4I
p-ClC6H4I
p-ClC6H4I
p-ClC6H4I
p-ClC6H4I
p-ClC6H4I
PhBr
PhBr
PhBr
PhBr
PhBr
100:0:0
91:9:0
95:5:0
94:6:0
—
78
80
72
73
—
—
77
74
56
61
—
—
72
67
58
62
—
—
—
68
16
16
16
16
—
—
17
18
18
17
—
95:5:0
91:9:0
88:12:0
90:10:0
—
—
92:8:0
92:8:0
93:7:0
88:12:0
—
—
—
92:8:0
16
16
16
16
—
—
—
6
PhBr
PhCl
PhI
t
Determined by 1H NMR.
Isolated by terminal alkynes.
a
b
well. The regio-, stereoselectivity, and the yield were satis-
factory. In all the reactions, no compound 4 was detected.
On the other hand, aryl chloride failed to couple with aryl
alkenylindium in InCl3–NaBH4–MeCN system (Table 1,
entry s). We also attempted to apply this reaction to aliphatic
alkynes and aliphatic bromides. But the results showed that
no coupling product was obtained (Table 1, entries e–f, k–l,
and q–r). It was interesting that the desired coupling product
was obtained when (1-cyclohexen-1-yl) ethyne was used as
the aliphatic terminal alkyne. It was considered that the
conjugated structure in the compound (1-cyclohexen-1-yl)-
ethyne stabilized the intermediate. All the results were sum-
marized in Table 1.
could be consumed completely after carrying out this reac-
tion for 3 h. But a large amount of over-reduced product
(8a) was obtained (Table 2, entry 6). Thus, the ratio of
NaBH4 and phenylalkynylsilane was decreased to 1:1, and
that led to high selectivity (6a/7a/8a¼90:5:5) with 92% total
yield (Table 2, entry 7). The products were determined by
1H NMR and GC–MS. All the results were summarized in
Table 2.
A variety of alkynylsilanes were examined (1 mmol) under
the optimized reaction conditions [InCl3 (1 mmol), NaBH4
(1 mmol), THF (5 mL), ꢀ15 ꢁC to rt, 2–4 h] (Table 2, entry
7; Scheme 2). It was found that all the arylalkynylsilanes
could react well. Both stereoselectivities and yields were sat-
isfactory. When the hydrogen atom, at the para position of
benzene ring, was substituted by a methyl group (Table 3,
entry b), the stereoselectivity decreased slightly, and more
starting material was over-reduced than phenylalkynylsilane
(Table 3, entry a). In contrast, the stereoselectivity and the
yield were rather high when this hydrogen atom was
substituted by an electron-withdrawing group such as F,
Cl, and Br (Table 3, entries c–e). When R was C4H9 or
C5H11, a mixture was obtained. About half of the starting
material was over-reduced to 8f or 8g by GC–MS (Table
3, entries f–g). Diastereoisomeric excess (E/Z) determined
by 1H NMR was 3:1. Reduction of aliphatic alkynylsilanes
to vinylsilanes is mostly difficult to control. All the results
are summarized in Table 3.
2.2. Synthesis of (E)-alkenylsilanes via hydroindation of
alkynylsilanes in InCl3–NaBH4–THF system
Considerable efforts have been devoted to develop the novel
methods of forming vinylsilanes due to their versatile role as
intermediates in organic synthesis.19 Presently, there are no
reports of vinylsilane synthesis via hydroindation of alkynyl-
silanes. So under the same conditions, the coupling of 2-phe-
nylalkynylsilane and iodobenzene was also tested in the
InCl3–NaBH4–MeCN system. Unexpectedly, the reaction
worked poorly in this system, which was proved to be more
excellent than others in many reactions. Only 2-phenylvinyl-
silanes (6a+7a) were obtained in only 2.5% conversion by
GC–MS analysis even after stirring for 20 h. In order to
improve the conversion, the reaction conditions were opti-
mized involving solvent and the amount of NaBH4. At first,
various solvents instead of MeCN were tested under the
conditions of entry 1 (Table 2). Attempts using solvents such
as ethyl ether, CH2Cl2, toluene, and acetone all failed in
which only a trace amount of the expected products were
obtained (Table 2, entries 2–5). Interestingly, THF was
found to be the best solvent in which the starting material
2.3. Synthesis of (E)-2-phenylvinylphosphonate via
hydroindation of phenylalkynylphosphonate in
InCl3–NaBH4–MeCN system
Vinylphosphonates have been widely used as synthetic inter-
mediates in organic chemistry20 and investigated as biolog-
ically active compounds.21 Recently, our group has afforded

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C. Wang et al. / Tetrahedron 62 (2006) 7712–7717
Table 2. Study of the system in reduction of phenylalkynylsilanea
SiMe3
H
Ph
H
H
brine
Ph
H
InCl3-NaBH4-MeCN
Ph
SiMe3
Ph
+
+
-15 C - r. t.
°
SiMe3
SiMe3
PhI, 15 h
(E)-6a
(Z)-7a
5a
8a
Entry
Solvent
6a:7a:8ab,c
Yield (6a+7a%)b
1
2
3
4
MeCN
Ethyl ether
CH2Cl2
Toluene
Acetone
THF
—
—
—
—
—
56:7:37
90:5:5
2.5
—
Trace
Trace
Trace
61
5
6d
7e
THF
92
a
InCl3/NaBH4/phenylalkynylsilane ¼ 1:2:1 and 20 h except that pointed out especially.
b
c
d
e
Determined by GC–MS.
Determined by 1H NMR.
3 h.
InCl3/NaBH4/phenylalkynylsilane ¼ 1:1:1, 4 h.
Table 3. Synthesis of vinylsilanes in InCl3–NaBH4–THF systema
SiMe3
R
R
H
H
R
+
brine
InCl3-NaBH4-MeCN
+
SiMe3
R
SiMe3
-15 oC - r. t.
15 h
H
H
SiMe3
5
(E)-6
(Z)-7
8
Entry
R
6:7:8b,c
Total Yield (6+7%)b
a
b
c
d
e
f
Ph
p-CH3Ph
p-FPh
p-BrPh
p-ClPh
n-C4H9
n-C5H11
90:5:5
70:5:25
88:5:7
95:2:3
90:4:6
3:1:4
92
70
88
94
93
Mixture
Mixture
g
3:1:3
a
Reaction conditions: InCl3 (1 mmol), NaBH4 (1 mmol), THF (5 mL), ꢀ15 ꢁC to rt, 2w4 h.
b
c
Determined by GC–MS.
Determined by 1H NMR.
a convenient and efficient method to synthesize vinyl-
alkynes gave (E)-alkenylindiums as major intermediate
and (Z)-alkenylindiums as minor intermediate. At last, alke-
nylindiums (C and D) coupled with aryl iodides or bromides
afforded alkenes. It was supposed that chlorobenzene was
not active enough to couple with alkenylindiums. For alky-
nylphosphonates and alkynylsilanes, the intermediate G or
H could not couple with iodobenzene to give the correspond-
ing products because of steric hindrance. In our experiments,
alkynylsilanes could be over-reduced. But in similar reaction
system only alkenylphosphonates were obtained without any
over-reduced products detected by GC–MS. The reasons
have not been clarified. It may be explained that aliphatic
alkenyl intermediates are not stable enough to couple with
aryl halides in our experiments.
phosphonates via hydroindation of 2-arylalkynylphospho-
nates in InCl3–NaBH4–MeCN system.7 In the same way,
the coupling reaction of phenylalkynylphosphonate and
iodobenzene was tested. Only 2-phenylvinylphosphonates
(10:11¼95:5) were obtained in 94% yield without any
desired coupling products (Scheme 1).
2.4. Proposed mechanism
In accordance with previous reports, a similar mechanism is
presently proposed (Schemes 2 and 3).17,18 During the reac-
tion E-configuration of the alkenyl radical (A or E) is more
stable than Z-configuration (B or F). So hydroindation of
PO(OEt)2
H
Ph
H
H
Ph
H
InCl3-NaBH4-MeCN
brine
PO(OEt)2
Ph
+
-15 C - r. t.
°
PO(OEt)2
PhI, 15 h
9
(E)-10
95
(Z)-11
:
5
total yield 94% by GC-MS
Scheme 1. Synthesis of (E)-2-phenylvinylphosphonate via hydroindation of phenylalkynylphosphonate in InCl3–NaBH4–MeCN system.

C. Wang et al. / Tetrahedron 62 (2006) 7712–7717
7715
InCl3-NaBH4 system
R
H
R
.
.
InCl2
CH
H
+
InCl2
.
RC
H
+
H
H
A
B
R
H
H
R
H
R
H
H
H
R
H
ArX
+
InCl2
Ar
InCl2
Ar
H
C
D
Scheme 2. Proposed mechanism of the coupling reaction of terminal alkynes with aryl halides in InCl3–NaBH4 system.
R
R
H
SiMe3
.
.
CSiMe3
RC
SiMe3
or
H
R
.
or
InCl2
or
+
CPO(OEt)2
InCl2
R
RC
H
PO(OEt)2
.
.
H
PO(OEt)2
H
E
F
SiMe3
H
R
R
InCl2
R
H
H
R
H
SiMe3
SiMe3
H
InCl2
or
H
SiMe3
InCl2
H2O
+
or
or
or
R
+
PO(OEt)2
R
R
H
R
H
PO(OEt)2
InCl2
PO(OEt)2
H
H
H
H
PO(OEt)2
H
G
Scheme 3. Proposed mechanism of reduction of alkynylsilanes in InCl3–NaBH4 system.
3. Conclusion
GC/MS (USA TRACE Company). Precoated thin-layer
plates of silica gel 60 GF254 (Qingdao Haiyang Chemical
Co. Ltd, Qingdao, China) were used for analytical purposed.
The reactions were carried out in pre-dried (150 ꢁC, 4 h)
glassware under the atmosphere of dry nitrogen. All solvents
and reagent were dried, deoxygenated, and redistilled before
use.
InCl3–NaBH4 system was applied to the coupling reactions
of the terminal aryl alkynes with aryl iodides or bromides in
one-pot successfully. But in this system alkynylphospho-
nates and alkynylsilanes were only reduced to alkenyl-
indiums, which could not couple with iodobenzene to
synthesize trisubstituted alkenes. Moreover, it was found
that InCl3–NaBH4–THF system is more active than InCl3–
NaBH4–MeCN system in reduction of alkynylsilanes.
Finally, many alkenylphosphonates and alkenylsilanes
were obtained by hydrolyzation of alkenylindiums. An
effective and simple approach to (E)-alkenes from alkynes
was developed in the absence of expensive catalyst under
the mild conditions.
4.2. General procedure for the cross-coupling reactions
of terminal alkynes and halides in InCl3–NaBH4–MeCN
system
After a mixture of InCl3 (2.0 mmol) in dry MeCN (10 mL)
and NaBH4 (10 mmol) was stirred for 30 min at ꢀ15 ꢁC
under nitrogen, terminal alkyne (2 mmol) and aryl iodide
or bromide (2 mmol) were added in sequence by syringe.
The cooling bath was removed. The reaction mixture was
warmed to room temperature, and stirred overnight. Satu-
rated NH4Cl solution (5 mL) was added to the reaction
mixture to destroy excessive NaBH4. The reaction mixture
was stirred for another 10 min and then filtered. The filtrate
was extracted with ether (20 mLꢂ3). The combined organic
layer was dried over anhydrous MgSO4 and concentrated in
vacuum. Purification by silica gel column (petroleum ether
bp 60–90 ꢁC as eluent) could afford the corresponding
products, which were identified by 1H NMR and MS.
4. Experimental
4.1. General
All the terminal alkynes and halides were purchased from
J&K chemical Co., or other commercial suppliers and
were used after appropriate purification (distillation). Alky-
nylsilane22 and alkynylphosphonates23 were prepared
according to the literature. Acetonitrile was freshly distilled
from phosphorus pentoxide before use. The 1H NMR spectra
were obtained with a Bruker AVANCE DRX-500 NMR
spectrometer with TMS as an internal standard and CDCl3
as solvent. GC–MS data were recorded by TRACE 2000
4.2.1. 1-[(E)-2-Phenylethenyl]benzene (2a). 1H NMR
(CDCl3, 500 MHz) d 7.12 (2H, s), 7.25–7.28 (2H, m), 7.36
(4H, t, J¼7.5 Hz), 7.52 (4H, d, J¼7.5 Hz); MS, m/z 180 (M+).

7716
C. Wang et al. / Tetrahedron 62 (2006) 7712–7717
1
4.2.2. 1-Methyl-4-[(E)-2-phenylethenyl]benzene (2b). H
NMR (CDCl3, 500 MHz) d 2.29 (3H, s), 7.00–7.45 (11H,
m); MS, m/z 194 (M+).
4.3.3. (E)-2-(4-Fluorophenylethenyl)trimethylsilane (6c).
1H NMR (CDCl3, 500 MHz) d 7.00–7.39 (4H, m), 6.80 (1H,
d, J¼19.5 Hz), 6.35 (1H, d, J¼19.5 Hz), 0.14 (9H, s); MS,
m/z 194 (M+).
1
4.2.3. 1-Bromo-4-[(E)-2-phenylethenyl]benzene (2c). H
NMR (CDCl3, 500 MHz) d 7.07–7.52 (11H, m); MS, m/z
4.3.4. (E)-2-(4-Chlorophenylethenyl)trimethylsilane
(6d). H NMR (CDCl3, 500 MHz) d 7.10–7.34 (4H, m),
1
258 (M+).
6.78 (1H, d, J¼19.7 Hz), 6.42 (1H, d, J¼19.7 Hz), 0.14
(9H, s); MS, m/z 210 (M+).
4.2.4. 1-Methoxyl-4-[(E)-2-phenylethenyl]benzene (2d).
1H NMR (CDCl3, 500 MHz) d 6.90–7.48 (11H, m), 3.86
(3H, s); MS, m/z 210 (M+).
4.3.5. (E)-2-(4-Bromophenylethenyl)trimethylsilane (6e).
1H NMR (CDCl3, 500 MHz) d 7.05–7.46 (4H, m), 6.78 (1H,
d, J¼19.3 Hz), 6.42 (1H, d, J¼19.3 Hz), 0.14 (9H, s); MS,
m/z 254 (M+).
1
4.2.5. 1-Chloro-4-[(E)-2-phenylethenyl]benzene (2g). H
NMR (CDCl3, 500 MHz) d 7.06–7.51 (11H, m); MS, m/z
214 (M+).
Acknowledgements
4.2.6. 1-[(E)-2-(4-Chlorophenyl)ethenyl]-4-methylbenz-
ene (2h). H NMR (CDCl3, 500 MHz) d 6.96–7.48 (10H,
1
The generous financial support of Natural Science Founda-
tion of China (20375036) is gratefully acknowledged.
m), 2.36 (3H, s); MS, m/z 228 (M+).
4.2.7. 1-[(E)-2-(4-Chlorophenyl)ethenyl]-4-bromobenz-
ene (2i). H NMR (CDCl3, 500 MHz) d 7.00–7.51 (10H,
References and notes
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silanes in InCl3–NaBH4 system
InCl3 (1 mmol), dry solvent (5 mL), and NaBH4 (1 mmol)
were mixed at ꢀ15 ꢁC under nitrogen. After the mixture
was stirred for 30 min, alkynylsilanes (1 mmol) were
added by syringe. The cooling bath was removed. The
reaction mixture was warmed to room temperature, stirred
for 2–4 h. Completion of the reaction was monitored by
TLC. In order to destroy the excessive NaBH4 saturated
NH4Cl solution (5 mL) was added to the reaction mixture.
The reaction mixture was stirred for another 10 min and
filtered. The filtrate was extracted with ethyl ether
(10 mLꢂ3). The combined organic layer was dried over
MgSO4 and concentrated in vacuum. Purification by silica
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4.3.1. (E)-2-(Phenylethenyl)trimethylsilane (6a). 1H NMR
(CDCl3, 500 MHz) d 7.18–7.43 (5H, m), 6.87 (1H, d,
J¼19.5 Hz), 6.48 (1H, d, J¼19.5 Hz), 0.15 (9H, s); MS,
m/z 176 (M+).
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