Oxidative coupling of carbonyl compounds by using pentavalent biphenyl-2,2′-ylenebismuth reagents

Article abstract of DOI:10.1246/cl.2007.718

p-Tolylbiphenyl-2,2′-ylenebismuth bis(trifluoromethanesulfonate) reacted with lithium enolates derived from ketones, carboxylic esters, and thioesters to afford the corresponding oxidative coupling products in good to high yields. Copyright

Full text of DOI:10.1246/cl.2007.718

718
Chemistry Letters Vol.36, No.6 (2007)
Oxidative Coupling of Carbonyl Compounds
by Using Pentavalent Biphenyl-2,20-ylenebismuth Reagents
Shohei Imachi1 and Teruaki Mukaiyamaꢀ1;2
1Center for Basic Research, The Kitasato Institute, 6-15-5 (TCI) Toshima, Kita-ku, Tokyo 114-0003
2Kitasato Institute for Life Sciences, Kitasato University, 5-9-1 Shirokane, Minato-ku, Tokyo 108-8641
(Received March 20, 2007; CL-070295; E-mail: mukaiyam@abeam.ocn.ne.jp)
p-Tolylbiphenyl-2,20-ylenebismuth bis(trifluoromethanesul-
fonate) reacted with lithium enolates derived from ketones,
carboxylic esters, and thioesters to afford the corresponding
oxidative coupling products in good to high yields.
prepared from triarylbismuth dichloride and lithium enolates
occupied two axial positions, an approach of the two oxoalkyl
groups would be prevented. The bismuth(V) reagent having a
trigonal bipyramidal structure such as triphenylbismuth dichlo-
ride would not show high reactivity toward oxidative coupling
because the phenyl groups located in equatrial positions and
the chlorine occupied axial positions.8 In the case of the cyclic
biphenylyl derivative, on the other hands, Fedorov and Finet
suggested that one phenyl group of the biphenylyl group formed
an equatorial bond to bismuth and another one lay in the
apical position.9 Therefore, it was assumed that one of the
oxoalkyl groups was blocked at the unfavorable axial positions
when the cyclic biphenylyl bismuth derivatives were used, and
the coupling reaction of oxioalkyl groups would proceed
smoothly.
The reactivity of triarylbismuth dichloride was examined by
taking the coupling reactions of lithium enolates as a model. The
reaction of triphenylbismuth dichloride with lithium enolates
generated from acetophenone and LHMDS afforded the cou-
pling products in only 12% yield in THF at 0 ꢁC. By comparison,
the coupling products were obtained in 33% yield in the same
solvent at ꢂ45 ꢁC when the cyclic bismuth 5b was used.
Next, a series of new cyclic biphenylyl derivatives was pre-
pared, and the preparation of p-tolylbiphenyl-2,20-ylenebismuth
bis(trifluoromethanesulfonate) (5f) was shown in Scheme 1 as a
typical example. According to the same procedure, other cyclic
bismuth compounds were similarly synthesized just by changing
the substituents on the Grignard reagents. Then, effects of coun-
ter ions on the bismuth reagents as well as those of electron-do-
nating and electron-withdrawing groups on the aryl derivatives
were investigated (Table 1).
Because of the unique chemical behaviors of organobismuth
compounds, a number of interesting reports on oxidation, phen-
ylation, and other organic reactions are being published.1 Gener-
ally, bismuth compounds are known to exist exclusively in two
oxidation states, namely, (III) and (V), and most of their reac-
tions were carried out by utilizing the oxidation properties of bis-
muth(V) compounds that were converted to organobismuth(III).
It was reported from our laboratory that both functionalized and
simple tertialy alcohols smoothly underwent O-phenylation
to afford sterically hindered tertialy-alkyl aryl ether by using
powerful oxidation properties of bismuth(V) compounds such
as fluorotetraphenylbismuth.2 In the present communication,
the synthesis of p-tolylbiphenyl-2,20-ylenebismuth bis(trifluoro-
methanesulfonate) and its application to the oxidative coupling
of carbonyl compounds are described.
The synthesis of 1,4-dicarbonyl compounds has attracted
interests of many chemists because these compounds are utilized
as useful intermediates for the synthesis of thienyl, pyrrol, or
furyl heterocycles.3 The method for the synthesis of 1,4-dicar-
bonyl compounds is generally classified into the following three
categories: 1) addition reaction of lithium, magnesium and
stannyl enolates to the ꢀ-halogenated carbonyl compounds,4 2)
direct oxidation of silyl enolates,5 3) oxidative coupling of
copper, silver, iron, and manganese(III) enolates.6 The oxidative
coupling reactions are conveniently used because no preparation
of silyl enolates or ꢀ-halogenated carbonyl compounds in ad-
vance is needed. Although there are synthesis of (2-oxoalkyl)-
triarylbismuthonium salts and their addition reactions with
alcohols and amines reported by Suzuki et al., there have been
no reports on the oxidative coupling of carbonyl compounds that
promoted by bismuth reagents.7
The coupling reactions were promoted more effectively
when the cyclic bismuth reagents having more labile counter
ions were used (Entries 1–4). In contrast, aryl substituents
having trifluoromethyl, methoxy and 2-methyl groups were not
effective (Entries 5, 7, and 8). It is noted then, that the desired
The coupling product and organobismuth(III) are thought to
be obtained by a reductive elimination of bismuth(V) after li-
gand exchanges of the bismuth(V) reagents with lithium enolates
(Figure 1). If two oxoalkyl groups of intermediate 1 that is
a
b
c
Bi
Bi
Bi
X
X
Ar
Bi
Ar
O
Ar
O
O
Ar3BiX2
OLi
R
2
3
R
4f X = OAc
5f X = OTf
d
R
O
R
R
Scheme 1. (a) I2, THF, rt, 1 h; (b) p-CH3C6H4-MgBr, THF, rt,
12 h, 68%; (c) CH3CO3H, CH3CO2H, THF, CH2Cl2, 0 ꢁC, 0.5 h,
89%; (d) TfOH, CH2Cl2, ꢂ78 ꢁC, 3 h, 84%.
1
+ Ar3Bi
+ 2LiX
Figure 1. A plausible reaction pathways.
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.6 (2007)
719
Table 1. Dimerization of lithium enolates of acetophenone by
using bismuth(V) compounds
1,4-diketone was obtained in high yield when biphenylyl bis-
muth containing 4-methyl group on the aryl substituent was used
(Entry 6).
1.0 equiv.
The results of a dimerization of lithium enolates derived
from ketones and carboxylic esters with cyclic biphenylyl bis-
muth 5f were summarized in Table 2. The oxidative coupling
of thioesters also proceeded smoothly under the same conditions,
and the corresponding coupling product was obtained in high
yield (Entry 7). Thus, a direct oxidative coupling of lithium
enolates generated from thioester was accomplished.
Bi
X
X
R1
2.1 equiv.
LHMDS
5
O
R2
O
OLi
Ph
Ph
Ph
THF Ph
−45 °C, 3 h
This study was supported in part by the Grant of the 21st
Century COE Program from Ministry of Education, Culture,
Sports, Science and Technology (MEXT), Japan. S. I. was grant-
ed a research Fellowship of the Japan Society for the Promotion
of Science for Young Scientists. The authors wish to thank
Ms. Tomoko Shinohara (Otsuka Pharmaceutical Co., Ltd.) for
her support in elemental analysis.
O
6a
−78 °C
7a
2.2 equiv.
0.5 h
Entry
R1
R2
X
Yield/%a
1
2
3
4
5
6
7
8
5a
5b
5c
5d
5e
5f
H
H
H
H
H
H
H
H
H
OAc
Cl
21
33
55
78
63
88
73
68
OTs
OTf
OTf
OTf
OTf
OTf
H
This paper is dedicated to the memory of the Emeritus
professor Yoshihiko Ito of Kyoto University.
CF3
CH3
H
5g
5h
CH3
H
References
OMe
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aBased on the bismuth(V) compounds.
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THF R1
−78 °C
0.5 h
R2
O
6
3 h
2.2 equiv.
7
Entry
R1
R2
Product
7b
Yield/%a
4-MeO−C6H4
4-Cl−C6H4
1
2
6b
H
H
84
81
6c
7c
3
6d
H
7d
90
O
Ph
4
6e
6f
H
Me
H
7e
7f
88
67
68
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6b
Ph
7
8
9
6g
OBn
7g
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7
6h
SPh
H
7h
87
aBased on the cyclic bismuth 5f. b3.0 equivalent of starting
carbonyl compounds and LHMDS was used.
Published on the web (Advance View) April 28, 2007; doi:10.1246/cl.2007.718