Angewandte
Chemie
DOI: 10.1002/anie.201203778
Synthetic Methods
Gallium Tribromide Catalyzed Coupling Reaction of Alkenyl Ethers
with Ketene Silyl Acetals**
Yoshihiro Nishimoto, Hiroki Ueda, Makoto Yasuda, and Akio Baba*
The coupling reaction of alkenyl electrophiles with organo-
metallics has been strenuously developed because it is
a powerful tool for the construction of p-conjugated carbon
frameworks, which are very valuable as functional materials.[1]
In general, alkenyl halides and pseudohalides are used as
electrophilic coupling partners in the presence of a transition-
metal catalyst. The use of these electrophiles, however, has
some drawbacks in terms of high cost, storage instability, and
the production of metal halide wastes. To overcome these
drawbacks, significant effort has gone into replacing alkenyl
halides with alkenyl alcohol derivatives such as alkenyl
ethers,[2] acetates,[3] carbonates,[4] carbamates,[5] phosphates,[6]
and silyl enol ethers.[7] Therefore, some success using alkyl-
and arylmetals have been reported. However, to the best of
our knowledge, the cross-coupling between alkenyl alcohol
derivatives and metal enolates has never before been
achieved[8] in spite of the vital importance of a-alkenyl
carbonyls.[9]
Table 1: Effect of catalysts in coupling of alkenyl ether with ketene silyl
acetal.[a]
Entry
Catalyst
(equiv)
T
[8C]
Yield
[%][b]
1
2
3
4
5
6
7
8
9
InBr3 (1)
À20
80
80
RT
80
80
80
80
80
quant
86
95
19
6
0
4
0
InBr3 (0.1)
GaBr3 (0.1)
GaBr3 (0.1)
BiBr3 (0.1)
–
BF3·OEt2 (0.1)
AlCl3 (0.1)
TiCl4 (0.1)
trace
[a] 1a (1 mmol), 2a (1.5 mmol), ClCH2CH2Cl (2 mL), 2 h. [b] Determined
by 1H NMR spectroscopy.
Recently, we developed carbometalations of alkenes and
alkynes using ketene silyl acetals and metal halides such as
InBr3,[10] GaBr3,[11] and BiBr3 to conveniently produce the
derivative. This result indicated that 1a acted as a vinylation
reagent through the removal of the butoxy group. Gratify-
ingly, the coupling reaction was promoted at 808C by
a catalytic amount of InBr3 (10 mol%) to give the product
3aa in 86% yield (Table 1, entry 2). GaBr3 was found to be
more effective, thus affording 3aa in 95% yield (Table 1,
entry 3),[11] and these conditions were determined to be
optimal. In contrast, BiBr3 showed quite a low catalytic effect
(Table 1, entry 5) although it reportedly promoted the
effective carbometalation of alkynes.[12] No coupling, of
course, took place without a catalyst (Table 1, entry 6), and
representative Lewis acids such as BF3·OEt2, AlCl3, and TiCl4
were not effective (Table 1, entries 7–9).
With the optimum reaction conditions in hand, the scope
of with respect to the ketene silyl acetals 2 was investigated as
shown in Scheme 1. Dialkylketene silyl acetals gave the
corresponding a-vinyl esters in high yields (3ab, 3ac, and
3ad). The alkylarylketene silyl acetal also showed good
reactivity, thus furnishing 3ae. In the case of monosubstituted
ketene silyl acetals, alkylketene and arylketene substrates
gave the desired products 3af and 3ag in 90 and 83% yields,
respectively. The phenoxy-substituted ketene silyl acetal
furnished the a-phenoxy a-vinyl ester 3ah in 44% yield,
and the reaction using an unsubstituted ketene silyl acetal also
took place, although the yield was only 20% (3ai). Hetero-
aryl-substituted ketene silyl acetals were found to be facile
nucleophiles (3aj and 3ak).
[12]
corresponding organometallics, wherein the independent anti
addition of metal species and nucleophiles took place without
the preliminarily generation of an organometallic intermedi-
ate. Herein, we wish to extend this carbometalation to a new
type of catalytic coupling between alkenyl ethers and ketene
silyl acetals.
At first, we focused on the carbometalation of butyl vinyl
ether (1a) using dimethylketene butyl trimethylsilyl acetal
(2a) and an equimolar amount of InBr3 (Table 1, entry 1), in
which the effective interaction between the double bond of 1a
and InBr3 was expected because the electron density of 1a is
higher than that of the alkenes employed in our reported
system.[10b] Although a vigorous reaction took place, even at
low temperature as expected, the coupling product 3aa was
obtained quantitatively instead of the expected alkylindium
[*] Dr. Y. Nishimoto, H. Ueda, Dr. M. Yasuda, Prof. Dr. A. Baba
Department of Applied Chemistry, Graduate School of Engineering
Osaka University
2-1, Yamada-oka, Suita (Japan)
E-mail: baba@chem.eng.osaka-u.ac.jp
[**] This work was supported by a Grant-in-Aid for Scientific Research on
Innovative Areas (No. 22106527, “Organic Synthesis Based on
Reaction Integration. Development of New Methods and Creation
of New Substances” and No. 23105525, “Molecular Activation
Directed toward Straightforward Synthesis”) and Challenging
Exploratory Research (No. 23655083) from the Ministry of Educa-
tion, Culture, Sports, Science and Technology (Japan).
Furthermore, the applicable alkenyl ethers are shown in
Table 2. The isopropyl vinyl ether (1b) gave the desired
product in 50% yield, which was lower than that of 1a as
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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