8
20
Chemistry Letters Vol.35, No.7 (2006)
Beneficial Effect of TMSCl in the Lewis Acid-mediated Carboxylation
of Aromatic Compounds with Carbon Dioxide
ꢀ
Koji Nemoto, Hiroki Yoshida, Yutaka Suzuki, Naoya Morohashi, and Tetsutaro Hattori
Department of Environmental Studies, Graduate School of Environmental Studies, Tohoku University,
-6-11 Aramaki-Aoba, Aoba-ku, Sendai 980-8579
6
(Received May 8, 2006; CL-060538; E-mail: hattori@orgsynth.che.tohoku.ac.jp)
The Lewis acid-mediated carboxylation of aromatic com-
conditions by addition of 1.2 mol equiv. of TMSCl, while the
yield dropped to 10% (o-:p- = 3:97) in the absence of TMSCl.
Electrophilic silylation of aromatic hydrocarbons with halosi-
lanes in the presence of a Friedel–Crafts catalyst has yet to be
realized, which is attributed to the great ease of protodesilylation
of the resultant arylsilanes under the acidic conditions. However,
Olah et al. found that a certain, albeit small amount (ꢁ1%) of
arylsilane could be obtained in the reaction of benzene, as well
pounds with CO2 is significantly promoted by the addition of a
large excess of chlorotrimethylsilane (TMSCl) to give arylcar-
boxylic acids in good to excellent yields.
Chemical fixation of CO2 has been the subject of much
1
interest in the field of organic synthesis. Although many CO2-
fixation reactions have been developed to date, they are, in most
cases, classified as ‘‘nucleophilic fixation,’’ as represented by the
Kolbe–Schmitt reaction, carbonation of Grignard reagents, and
as toluene, with TMSCl in the presence of AlCl by addition
3
of a hindered base, e.g. diisopropylethylamine (DIEA), to trap
8
HCl eliminated in the silylation. Thus, the carboxylation was
2
palladium-catalyzed cycloaddition of diene with CO2, in which
examined in the presence of hindered bases. The substrate was
used as a solvent instead of benzene to avoid formation of
trimethylphenylsilane, which should lead to contamination of
an anionic species, as well as a catalytically-activated species,
nucleophilically attacks to unactivated CO2. On the other hand,
few examples have been known for ‘‘electrophilic fixation,’’ in
which CO2 is activated by coordination to a Lewis acid and
electrophilically adds to a C=C bond, as is often the case with
6
the desired carboxylic acids with benzoic acid. Although intro-
duction of several mol % of DIEA, as well as 2,6-di-tert-butyl-4-
methylpyridine, against the Lewis acid actually promoted the
reaction, addition exceeding a poor upper limit lowered the yield
(Table 1, Entries 1–6), which would be attributed to the com-
3
,4
other carbonyl compounds. It has been known that aromatic
compounds are directly carboxylated with CO2 with the aid of
aluminum-based Lewis acids to give arylcarboxylic acids,
plexation of the amine with AlBr to reduce the Lewis acidity.
3
5
though generally in poor yields. The reaction is believed to
However, the yield could be improved by increasing the amount
9
involve the electrophilic attack of the Lewis acid-activated
CO2 to the aromatic ring to form an arenium intermediate, which
eliminates a proton to give a carboxylic acid after aqueous
workup (the SEAr mechanism). On the other hand, the SEAr re-
action of arylsilanes occurs at the ipso position bearing the silyl
moiety, owing to the stabilizing effect in the transition state by
the (p–ꢀ)ꢁ conjugation between the Si–C bond and the develop-
of TMSCl without the use of the amines (Entries 7–11). The
more TMSCl was added, the more the yield increased to reach
96% based on the amount of the AlBr . This suggests that
3
3
1 mol equiv. of AlBr is consumed through the overall reaction.
Xylenes and mesitylene could also be effectively carboxyl-
ated by employing 5.0 mol equiv. of TMSCl (Table 2). In the
case of o- and p-xylene, 2,4-dimethylbenzoic acid was obtained
4
ing positive charge (the ꢂ-effect). We have recently reported
that aryltrimethylsilanes are efficiently carboxylated with CO2
Table 1. Carboxylation of toluene in the presence of TMSC1
and amine
6
in the presence of AlBr3. In this reaction, trimethyl(o-tolyl)-
silane gave not only o-methylbenzoic acid (the ipso-substitution
product) but also p-methylbenzoic acid. Formation of the latter
was rationalized by the sequential reactions: initial protodesilyl-
CH3
CO H
2
CO (3.0 MPa), TMSCl
2
AlBr (1.0 mmol), Amine
3
H C
7
3
ation of the o-tolylsilane with incidental HBr in the reaction
r.t., 3 h
system, subsequent silylation of the resultant toluene at the para
position to give p-tolylsilane, followed by the carboxylation at
the ipso carbon of the silyl moiety. The silyl-transfer mechanism
prompted us to examine the carboxylation of aromatic hydrocar-
bons in the presence of TMSCl in expectation of in-situ genera-
tion of arylsilanes under the Lewis acidic conditions. Herein, we
wish to report that TMSCl significantly promotes the Friedel–
Crafts-type carboxylation of aromatic hydrocarbons.
b
TMSCl/
AlBr3a
Amine/
AlBr3a
Yield /%
(o-:p- ratio)
Entry
Amine
1
2
3
4
5
6
7
8
9
1.0
1.0
1.0
1.0
1.0
1.0
2.0
3.0
4.0
5.0
10.0
b
—
DIEA
—
0.010
0.050
0.10
0.50
1.0
21 (1:3)
29 (1:3)
32 (1:3)
26 (1:3)
19 (1:3)
9 (1:3)
6
In our previous paper, we reported that trimethyl(p-tolyl)-
silane (1.5 mmol), on treatment with 2.0 mol equiv. of AlBr3
—
—
—
—
—
—
—
45 (1:4)
63 (1:4)
73 (1:5)
80 (1:5)
96 (1:6)
3
in benzene (1.0 cm ) under CO2 pressure (3.0 MPa) at room
—
—
temperature, gave a 1:9 mixture of o- and p-methylbenzoic acid
in 50% yield after chromatographic purification. We have found
that a comparable crop (54%) of the acid mixture (o-:p- = 1:3)
can be obtained by the reaction of toluene under the same
1
1
0
1
—
a
Molar ratio. Based on the quantity of AlBr3.
Copyright Ó 2006 The Chemical Society of Japan