Nemoto et al.
JOCArticle
4
esters. It is also a recent important finding that alkali
This increase in the yield was attributed to the ability of
metallic aluminum to scavenge HCl liberated by carbox-
ylation to give AlCl . The in situ-generated AlCl initiates
5
-10
11
metals can mediate metalation,
metal insertion into carbon-halogen bonds;
finding provides an otherwise difficult access to functionalized
transmetalation, and
1
2-15
this
3
3
further carboxylation, while HCl disturbs the reaction.
Therefore, the aluminum powder acts in two distinct capa-
5
,6,11,12
7,13
6,8,14
zinc,
9
organomagnesium,
aluminum,
15
copper,
10
21
cities for promoting carboxylation. In the same paper, the
cadmium, and indium reagents, which can be used for the
nucleophilic carboxylation, either by itself or in combina-
authors proposed an activation mechanism of CO using
2
AlCl based on DFT calculation. Munshi and Beckman have
3
reported that the carboxylation of toluene proceeds effi-
ciently by incubating AlCl under CO pressure prior to
16
tion with the catalytic metal-carbon bond insertion.
The transition metal-mediated or catalyzed hydrocarbox-
1
7
18
ylation and alkylative carboxylation of unsaturated C-C
bonds are also promising approaches for obtaining carboxylic
acids.
3
2
the reaction and that this manipulation enables weaker
Lewis acids, such as SnCl , MoCl , and TiCl , to replace
4
5
4
22
On the other hand, it has been known that aromatic
compounds can be directly carboxylated with CO with the
aid of aluminum-based Lewis acids. The reaction is
believed to be an electrophilic fixation of CO , in which a
AlCl without appreciable loss of the product yield. These
3
observations indicate that the complexation between a Lewis
acid and CO is important for the present carboxylation,
which supports Olah’s mechanism.
It is known that a trialkylsilyl group on an aromatic nucleus
2
1
9,20
2
2
Lewis acid-activated CO molecule electrophilically attacks
2
an aromatic nucleus, according to the electrophilic aromatic
promotes the S Ar reaction and that it directs the reaction to
E
the ipso position. This is because the silyl moiety stabilizes the
transition state, leading to the formation of a benzenonium
2
substitution (S Ar) mechanism. Carboxylic acids are gen-
1
E
erally obtained in poor yields using this procedure because of
the low electrophilicity of CO and/or side reactions caused
intermediate by the (p-σ) conjugation between the Si-C bond
2
π
23
by the strong Lewis acidity of aluminum-based compounds.
Recently, the research group of Olah and Prakash has
reported that the yield of carboxylic acids can be improved
and developing positive charge (the β-effect). We reported
that trimethylsilyl-substituted benzene, toluene, and naphtha-
lene underwent Lewis acid-mediated carboxylation more effi-
2
1
24
by the addition of aluminum powder together with AlCl3.
ciently than the corresponding arenes. During this study, we
have found that the carboxylation of arenes is significantly
promoted by the addition of trialkyl- or triarylsilyl chlorides.
Herein, we wish to report in detail the beneficial effect of silyl
chlorides in the Lewis acid-mediated carboxylation of arenes,
(
4) (a) Ukai, K.; Aoki, M.; Takaya, J.; Iwasawa, N. J. Am. Chem. Soc.
2
2
2
006, 128, 8706. (b) Takaya, J.; Tadami, S.; Ukai, K.; Iwasawa, N. Org. Lett.
008, 10, 2697. (c) Ohishi, T.; Nishiura, M.; Hou, Z. Angew. Chem., Int. Ed.
008, 47, 5792.
25
(
5) Krasovskiy, A.; Krasovskaya, V.; Knochel, P. Angew. Chem., Int. Ed.
006, 45, 2958.
6) Conway, B.; Hevia, E.; Kennedy, A. R.; Mulvey, R. E. Chem.
Commun. 2007, 2864.
7) Naka, H.; Uchiyama, M.; Matsumoto, Y.; Wheatley, A. E. H.;
McPartlin, M.; Morey, J. V.; Kondo, Y. J. Am. Chem. Soc. 2007, 129, 1921.
8) (a) Seggio, A.; Lannou, M.-I.; Chevallier, F.; Nobuto, D.; Uchiyama,
as well as halobenzenes.
2
(
Results and Discussion
(
Carboxylation of Arenes and Halobenzenes. First, toluene
was carboxylated under CO pressure at room temperature
(
2
M.; Golhen, S.; Roisnel, T.; Mongin, F. Chem.;Eur. J. 2007, 13, 9982. (b)
Wunderlich, S. H.; Knochel, P. Angew. Chem., Int. Ed. 2007, 46, 7685. (c)
Bresser, T.; Mosrin, M.; Monzon, G.; Knochel, P. J. Org. Chem. 2010, 75,
with various silyl halides acting as promoters (Table 1). The
initial CO pressure was adjusted to 3.0 MPa and AlBr was
19g
employed as a Lewis acid, according to our previous study.
2
3
4
686.
9) Usui, S.; Hashimoto, Y.; Morey, J. V.; Wheatley, A. E. H.; Uchiyama,
M. J. Am. Chem. Soc. 2007, 129, 15102.
10) L’Helgoual’ch, J.-M.; Bentabed-Ababsa, G.; Chevallier, F.;
Yonehara, M.; Uchiyama, M.; Derdour, A.; Mongin, F. Chem. Commun.
008, 5375.
(
The reaction was carried out in the neat substrate, and the
yield of toluic acid was calculated on the basis of the amount
of AlBr , assuming that 1 mol of AlBr is consumed per mole
(
3
3
2
(
(
11) Krasovskiy, A.; Knochel, P. Angew. Chem., Int. Ed. 2004, 43, 3333.
12) Piller, F. M.; Appukkuttan, P.; Gavryushin, A.; Helm, M.; Knochel,
of toluic acid produced (vide infra). Carboxylation carried out
with 1.0 molar equiv of Me SiClto AlBr gavea 1:3 mixture of
o- and p-toluic acid in a 21% yield (entry 2), while the yield
3
3
P. Angew. Chem., Int. Ed. 2008, 47, 6802.
13) Bl u€ mke, T.; Chen, Y.-H.; Peng, Z.; Knochel, P. Nat. Chem. 2010, 2,
13.
14) Krasovskiy, A.; Malakhov, V.; Gavryushin, A.; Knochel, P. Angew.
Chem., Int. Ed. 2006, 45, 6040.
(
3
dropped to 10% (o:p = 1:33) in the absence of Me SiCl
3
(
(entry 1). Other trimethylsilyl halides were less effective than
Me SiCl (entries 3 and 4). The replacement of the methyl
(
(
(
15) Chen, Y.-H.; Knochel, P. Angew. Chem., Int. Ed. 2008, 47, 7648.
16) Kobayashi, K.; Kondo, Y. Org. Lett. 2009, 11, 2035.
17) (a) Saito, S.; Nakagawa, S.; Koizumi, T.; Hirayama, K.; Yamamoto,
3
groups of Me SiCl with long chains or branched alkyl groups
3
favorably affected the yield of toluic acid; the introduction of
one such substituent increased the yield to 30% (entries 10
and 11), whereas the introduction of three such substituents
increased the yield to 40% (entries 14 and 15). The phenyl
group was more effective, giving yields of 51%, 75%, and 94%
by replacing one through three of the methyl groups, respec-
tively (entries 19, 21, and 24). The difference in magnitude of
the promoting effect of the silyl chlorides seems to originate
from the difference in magnitude of the electron-donating effect
Y. J. Org. Chem. 1999, 64, 3975. (b) Aoki, M.; Kaneko, M.; Izumi, S.; Ukai,
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Shimizu, K.; Takimoto, M.; Sato, Y.; Mori, M. Org. Lett. 2005, 7, 195. (c)
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1
7
2
976, 421589). (f) Huesler, R.; Orban, I.; Holer, M. Eur. Pat. Appl. 1996, EP
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(22) Munshi, P.; Beckman, E. J. Ind. Eng. Chem. Res. 2009, 48, 1059.
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(
20) See also: Nemoto, K.; Onozawa, S.; Egusa, N.; Morohashi, N.;
Hattori, T. Tetrahedron Lett. 2009, 50, 4512.
21) Olah, G. A.; T o€ r o€ k, B.; Joschek, J. P.; Bucsi, I.; Esteves, P. M.; Rasul,
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(
7
856 J. Org. Chem. Vol. 75, No. 22, 2010