Rhodium-catalyzed homocoupling of (1-Acyloxyvinyl)silanes: Synthesis of 1,3-diene-2,3-diyl diesters and their derivatives

Article abstract of DOI:10.1055/s-0029-1217968

1,3-Diene-2,3-diyl diesters are synthesized by rhodium-catalyzed homocoupling of (1-acyloxyvinyl)silanes. Double transmetalation between vinylsilane and rhodium intermediate in a single catalytic cycle is the key in this reaction. From the homocoupling pr

Full text of DOI:10.1055/s-0029-1217968

LETTER
2831
Rhodium-Catalyzed Homocoupling of (1-Acyloxyvinyl)silanes: Synthesis of
1,3-Diene-2,3-diyl Diesters and Their Derivatives
R
hodium-Catalyze
a
d
H
omocou
n
pling of Vin
n
ylsilanes i Yue, Hiroki Yamamoto,1 Motoki Yamane*
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
21 Nanynag Link, Singapore 637371, Singapore
Fax +6567911961; E-mail: yamane@ntu.edu.sg
Received 29 June 2009
of 1,3-diene-2,3-diyl diesters because trimethyl(1-acyloxy-
Abstract: 1,3-Diene-2,3-diyl diesters are synthesized by rhodium-
vinyl)silane had been known to be a reactive substrate for
transmetalation with rhodium(I) complex. Notably, (1-
acyloxyvinyl)silanes are easily prepared in a single step
catalyzed homocoupling of (1-acyloxyvinyl)silanes. Double trans-
metalation between vinylsilane and rhodium intermediate in a sin-
gle catalytic cycle is the key in this reaction. From the
homocoupling products, symmetrical a-diketone and its monoace- from the corresponding acylsilanes via O-acylation of
tal are easily synthesized.
their lithium enolates, which would allow us to prepare a
wide variety of symmetrical 1,3-diene-2,3-diyl diesters in
stereoselective manner.
Key words: homocoupling, rhodium catalyst, organosilicon com-
pounds, vinyl ester, conjugated diene
Similar oxidative homocoupling of organometallic re-
agents, such as organoboron compounds, were reported to
afford dienes and biaryls.13–15 However, none of these
methods are applicable for the synthesis of 1,3-diene-2,3-
diyl diesters because the corresponding (1-acyloxyvi-
nyl)metals are not readily available. Among (1-acyloxyvi-
nyl)metals, triorgano(1-acyloxyvinyl)silane is the only
main-group organometallic compound that is stable
enough to handle in the air and can be prepared easily.
Herein we present a simple method to prepare 1,3-diene-
2,3-diyl diesters via rhodium-catalyzed homocoupling of
vinylsilane.
1,3-Dienes are an important class of compounds that oc-
cur in many natural products.2,3 They are also used as im-
portant ligands in transition-metal-catalyzed reactions,4
useful precursors in organic synthesis,5 as well as in poly-
mer chemistry.6 Among them, acyloxy- or alkoxy-substi-
tuted dienes are attractive tools especially for the
synthesis of cyclic compounds via Diels–Alder reaction.7
Therefore 1,3-diene-2,3-diyl diester, which possesses two
acyloxy groups, is also expected to be a useful synthetic
intermediate. However, there is no report for practical
synthesis of 1,3-diene-2,3-diyl diesters.8
Z
We have reported [RhCl(CO)2]2-catalyzed cross-coupling
reaction of triorgano(vinyl)silane and acid anhydride.9 In
this reaction, vinylrhodium(I) complex is the key interme-
diate generated via simple transmetalation between trior-
gano(vinyl)silane and rhodium(I) complex. The
conventional transmetalation of organosilicon com-
pounds in the palladium-catalyzed coupling reactions re-
quires activating reagents such as fluoride ion10 and/or
substitution with hydroxy or alkoxy groups on the silicon
atom of the vinylsilanes.11,12 In contrast, there is no re-
quirement of such activation and even trimethylsilyl(vi-
nyl)silanes are also utilizable in the present
transmetalation process. We envisioned that this trans-
metalation would be extended to homocoupling of vinyl-
silane as depicted in Scheme 1. Transmetalation between
XSiMe3
R
RhIX
2
3
Z
R
Z
Z
RhIIIX
Z
R
R
RhI
C
SiMe3
1
A
R
Z
R
RhIIIX
X
Y
YSiMe3
B
Y
Scheme 1 Hypothetical mechanism for homocoupling of vinylsila-
nes
rhodium(I) complex and vinylsilane 1 would form vinyl- (1-Benzoyloxyvinyl)silane 1a was prepared via O-ben-
rhodium(I) complex A. After oxidation, second transmet- zoylation of the corresponding acylsilane lithium enolate
alation between vinylrhodium(III) intermediate B and selectively in E-configuration.9 Because a-halocarbonyl
vinylsilane 1 would occur to generate divinylrhodium(III) compounds are known to act as the oxidants for palladi-
complex C. The successive reductive elimination would um-catalyzed oxidative homocoupling of organometallic
furnish 1,3-diene 3, the homocouping product. This meth- compounds, we used a-bromoacetophenone. Vinylsilane
od was expected to be a synthetic method for preparation 1a was mixed with a-bromoacetophenone in the presence
of a catalytic amount of [RhCl(CO)2]2 and heated in tolu-
ene at 80 °C (Scheme 2). After heating for 1.5 hours, ho-
mocoupling product 3a was obtained in about 50% yield.
The cross-coupling product 2 was not formed at all, and
SYNLETT 2009, No. 17, pp 2831–2835
Advanced online publication: 09.09.2009
DOI: 10.1055/s-0029-1217968; Art ID: D17109ST
© Georg Thieme Verlag Stuttgart · New York
x
x
.x
x
.2
0
0
9

2832
Y. Yue et al.
LETTER
Table 1 Investigation of Additives for Rhodium-Catalyzed Homo-
coupling of Vinylsilane 1a
acetophenone was obtained as the coproduct. The Z,Z-di-
ene structure of 3a was confirmed by X-ray crystallo-
graphic analysis as shown in Figure 1.16 It is noteworthy
that triorgano(vinyl)silane having trimethylsilyl group,
the simplest silyl group, could be used for this homocou-
pling reaction without adding activating reagents such as
fluoride.
5 mol% [RhCl(CO)2]2
OCOPh
additive
Ph(CH2)2
SiMe3
toluene, 80 °C
1a
OCOPh
OCOPh
Ph(CH2)2
+
Ph(CH2)2
(CH2)2Ph
3a OCOPh
H
4
OCOPh
SiMe3
O
5 mol% [RhCl(CO)2]2
toluene, 80 °C
+
Ph(CH2)2
Ph(CH2)2
Br
Entry Additivea
Time (h) Product (%)b
Ph
1a
3a
–
4
OCOPh
PhCO2
O
1
2
3
Cu(OAc)2
TEMPOc
chloranild
(3)
(3)
(3)
5
5
5
–
–
4
Ph(CH2)2
(CH2)2Ph
OCOPh
Ph
5
2 0%
3a < 50%
48
Scheme 2 Rhodium-catalyzed homocoupling of vinylsilane 1a
O
4e
5
(3)
(3)
1.5
ca. 50
21
9
Br
Ph
Ph
Ph
O
10
22
26
Cl
O
6
7
(3)
5
28
Cl
Ph
O
(3)
(3)
12
trace
43
ca. 50
24
Br
OMe
O
8
9
10
11
1.5
Cl3C
Cl3C
OMe
(3)
(2)
(1.1)
1.5
7.5
7.5
92
67
64
<8
11
12
O
CCl3
a Number in parentheses indicates the molar amounts of the additive.
b Isolated yield.
c 2,2,6,6-Tetramethylpiperidine-1-oxyl.
d 2,3,5,6-Tetrachloro-1,4-benzoquinone.
e Benzophenone was obtained as coproduct.
Figure 1 ORTEP structure of 1,3-diene-2,3-diyl diester 3a16
Thus, we found that the homocoupling of (1-benzoyl-
oxyvinyl)silane 1a proceeded and a-bromoacetophenone
acted as an oxidant for change rhodium(I) to rhodium(III).
Then we investigated the additives that would give homo-
coupling product in better yield. The results are summa-
rized in Table 1. An inorganic oxidant, such as copper
diacetate, was not effective and did not give the product at
all (entry 1). 2,2,6,6-Tetramethylpiperidine-N-oxyl
(TEMPO) that is known as an efficient oxidant14a for ho-
mocoupling reaction of boronic acids was not suitable
(entry 2). When we used 2,3,5,6-tetrachloro-1,4-benzo-
quinone (chloranil), diene 3a was obtained in 48% yield
(entry 3). a-Chloro ketones also gave the diene 3a, how-
ever, the yields were deduced in 20–30% (entries 5 and 6).
Finally, we found that hexachloroacetone is sufficiently
effective, and the homocoupling product 3a was obtained
in 92% yield (entry 9). When we reduced the amount of
hexachloroacetone, the yield was decreased (entries 10
and 11).
Under the optimized reaction conditions, various substi-
tuted vinylsilanes were subjected to the homocoupling re-
action. The results are summarized in Table 2. Not only
trimethylsilyl derivative, but dimethyl(phenyl)vinylsilane
also provided the homocoupling product 3a in 72% yield
(entry 2). The structure of acyloxy group on 1-position of
vinyl group did not show significant difference and (1-
acetylvinyl)silane 1c gave 1,3-diene-2,3-diyl diacetate 3c
in 74% yield. Vinylsilanes 1d and 1e that have a carbonate
part are applied to synthesize 1,3-diene-2,3-diyl dicarbon-
ate 3d and 3e in moderate yield (entries 4 and 5). Carbon
substituents on 2-position of vinylsilane did not affect the
yield of homocoupling product and (2-phenylvinyl)silane
1f and 1g gave the corresponding desired products 3f and
3g in moderate to good yields (entries 6 and 7). Mean-
while, the reaction of 2-nonsubstituted vinylsilane 1h was
not successful due to the thermal instability of the product
under the reaction conditions (entry 8). Acyloxy function-
Synlett 2009, No. 17, 2831–2835 © Thieme Stuttgart · New York

LETTER
Rhodium-Catalyzed Homocoupling of Vinylsilanes
2833
Table 2 Rhodium-Catalyzed Homocoupling of Vinylsilane 1a
R1
5 mol% [RhCl(CO)2]2
Cl3CCOCCl3
R1
R2
R2
R2
toluene, 80 °C
SiR3
R1
1
3
Entry
1
Vinylsilane
1
Time (h)
1.5
Diene
3
Yield (%)b
92
OCOPh
SiMe3
OCOPh
SiMe2Ph
PhCOO
Ph(CH2)2
1a
1b
1c
1d
1e
1f
3a
Ph(CH2)2
2
2
PhCOO
Ph(CH2)2
2
3
4
5
6
7
5
5
3a
3c
3d
3e
3f
72
74
55
51c
90
45
Ph(CH2)2
Ph(CH2)2
Ph(CH2)2
Ph(CH2)2
OCOMe
MeCOO
Ph(CH2)2
SiMe3
2
OCO2Et
EtOCOO
Ph(CH2)2
5
SiMe3
2
2
OCO2Ph
PhOCOO
Ph(CH2)2
16
9
SiMe3
OCOPh
SiMe3
PhCOO
Ph
Ph
Ph
2
2
OCOMe
SiMe3
MeCOO
Ph
1g
20
3g
OCOPh
SiMe3
Ph(CH2)2
PhCOO
8
9
1h
1i
24
5
3h
3i
0d
0
2
Ph(CH2)2
SiMe2Ph
2
10
11
1j
24
24
3j
18
21
SiMe3
O
O
2
Ph
N
N
Ph
N
N
1k
3k
SiMe3
N
N
2
a Molar ratio: vinylsilane 1/[RhCl(CO)2]2/Cl3CCOCCl3 = 1:0.05:3.
b Isolated yield.
c Vinylsilane was recovered in 20%.
d Vinylsilane was recovered in 14%.
ality is essential for this homocoupling reaction. In fact, considered as a doubly masked form of a-diketone, which
simple (2-alkylvinyl)silane 1i did not give homocoupling is a useful synthetic intermediate to prepare heterocyclic
product at all (entry 9). The ester and carbonate part may compounds. So we tried the conversion of the product,
coordinate to the rhodium center to enhance the rate of 1,3-diene-2,3-diyl diester to a-diketone (Scheme 3).9
transmetalation. Heterocyclic vinylsilanes 1j and 1k were When dienediyl diester 3a was treated with potassium car-
also subjected to the reaction to yield 2,2¢-bibenzofuran 3j bonate in methanol at room temperature, a,a-dimethoxy
and 4,4¢-bitriazole 3k although the yields are not satisfac- ketone 5 was obtained in 87% yield. This was converted
tory (entries 10 and 11).
into symmetrical a-diketone 6 by acidic hydrolysis with
trifluoroacetic acid in 70% yield. Thus, using the rhodi-
um-catalyzed homocoupling as an aid we provided the
Since vinyl ester is a masked form of carbonyl compound,
the homocoupling product, 1,3-diene-2,3-diyl diester, is
OCOPh
O
MeO
Ph(CH2)2
OMe
CF3CO2H
K2CO3
Ph(CH2)2
Ph(CH2)2
CH2Cl2
r.t., 2 h
MeOH
r.t., 24 h
(CH2)2Ph
OCOPh
(CH2)2Ph
87%
(CH2)2Ph
70%
O
O
3a
5
6
Scheme 3 Conversion of diene 3a to a,a-dimethoxy ketone and a-diketone
Synlett 2009, No. 17, 2831–2835 © Thieme Stuttgart · New York

2834
Y. Yue et al.
LETTER
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General Procedure for Rh-Catalyzed Homocoupling of Vinylsi-
lane 1
To a solution of the corresponding vinylsilane 1 (0.2 mmol) and
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(3Z,5Z)-1,8-Diphenyl-3,5-diene-4,5-diyl Dibenzoate (3a)
White solid; mp 123–124 °C (EtOAc). 1H NMR (400 MHz,
CDCl3): d = 2.34 (4 H, dt, J = 7.2, 7.6 Hz), 2.67 (4 H, t, J = 7.6 Hz),
5.61 (2 H, t, J = 7.2 Hz), 7.07–7.11 (5 H, m), 7.13–7.19 (5 H, m),
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Acknowledgment
We thank Nanyang Technological University for the generous
financial support. We also thank Dr. Yongxin Li and Ms. Shu Qi
Cheong for their technical assistance with the X-ray crystallo-
graphic analysis.
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Synlett 2009, No. 17, 2831–2835 © Thieme Stuttgart · New York

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(15) For metal-mediated homocoupling of vinylsilanes, see:
(a) Weber, W. P.; Felix, R. A.; Willard, A. K.; Koenig, K. E.
Tetrahedron Lett. 1971, 4701. (b) Itami, K.; Ushiogi, Y.;
Nokami, T.; Ohashi, Y.; Yoshida, T. Org. Lett. 2004, 6,
3695.
(16) CCDC 737226 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Synlett 2009, No. 17, 2831–2835 © Thieme Stuttgart · New York

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