Lewis acid-mediated carboxylation of aryl- and allylsilanes with carbon dioxide

Article abstract of DOI:10.1246/cl.2003.454

Aryl- and allylsilanes are carboxylated with CO₂ with the aid of aluminum-based Lewis acids, to give aromatic and β, γ-unsaturated carboxylic acids in fair to good yields, respectively. Formation of the former is rationalized by an aromatic electrophilic substitution by CO₂ activated by the Lewis acid, while the latter a nucleophilic addition of in situ-generated allylaluminum species to CO₂.

Full text of DOI:10.1246/cl.2003.454

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
454
Chemistry Letters Vol.32, No.5 (2003)
Lewis Acid-Mediated Carboxylation of Aryl- and Allylsilanes with Carbon Dioxide
Tetsutaro Hattori,ꢀ Yutaka Suzuki, and Sotaro Miyanoꢀ
Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University,
Aramaki-Aoba 07, Aoba-ku, Sendai 980-8579
(Received January 30, 2003; CL-030092)
Aryl- and allylsilanes are carboxylated with CO2 with the
aid of aluminum-based Lewis acids, to give aromatic and
b; g-unsaturated carboxylic acids in fair to good yields, respec-
tively. Formation of the former is rationalized by an aromatic
electrophilic substitution by CO2 activated by the Lewis acid,
while the latter a nucleophilic addition of in situ-generated al-
lylaluminum species to CO2.
by using AlBr3 as the Lewis acid (Table 1). Benzene was em-
ployed as the solvent because it remained almost intact when
treated with CO2 under the conditions. Although the carboxyla-
tion of phenylsilane and (p-fluorophenyl)silane was sluggish,
tolylsilanes gave regioisomers of methylbenzoic acid in moder-
ate yields (entries 1–5). The product distribution is rationalized
based on the SEAr mechanism as follows: In o-tolylsilane, the
ipso carbon of the silyl moiety, which was activated by the o-
methyl group, underwent the carboxylation to give o-methyl-
benzoic acid. However, steric hindrance between the silyl and
the methyl group of the substrate induced protodesilylation5
with incidental HBr in the reaction system and subsequent sily-
lation of the resulting toluene at the para-position to give p-
tolylsilane,6 carboxylation of which gave p-methylbenzoic acid.
Preferential formation of the p-substituted product indicates
that the latter reaction path was predominant compared with
the former (entry 3), which coincides with the fact that p-tolyl-
silane afforded p-methylbenzoic acid in high selectivity (entry
5). It is of interest to note that m-tolylsilane afforded o-methyl-
benzoic acid in higher yield (up to 30%) than o-tolylsilane (8%)
(entries 3 and 4). In m-tolylsilane, both the methyl and the silyl
group activated the ortho- and para-positions to the silyl
moiety,7 during which the most sterically favorable para-posi-
tion seemed to undergo the carboxylation to give, after protode-
silylation of the resulting silane, o-methylbenzoic acid.
Naphthylsilanes also afforded naphthoic acids in good yields
(entries 6 and 7). It should be recalled that direct carboxylation
Chemical fixation of CO2 is a topic of growing interest in
the field of organic synthesis.1 Although nucleophilic alkylation
or arylation of CO2 with carbanions, such as organometallic
species and phenolic enolates, provides a convenient access to
carboxylic acids,1 there is no established protocol for electro-
philic one, in which CO2 is activated by coordination to a Lewis
acid and then attacks to a C=C bond, as is often the case with
other carbonyl compounds.2;3 Several papers have dealt with the
direct carboxylation of aromatic hydrocarbons with CO2 with
the aid of Lewis acids.4 We ourselves have recently found that
fused aromatic compounds were carboxylated regioselectively
by CO2 in the presence of aluminum-based Lewis acids.4e
The reaction mechanism is not clear but may involve 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).
In this respect, it should be noted that Olah et al. have recently
proposed an activation mechanism of CO2 by AlCl3 based on ab
initio molecular orbital calculations of the conformations of
possible complexes between the two.4f On the other hand, it
is well known that arylsilanes readily undergo electrophilic dis-
placement of the silyl moiety at the ipso position due to the sta-
bilizing effect in the transition state by the (p-s)p conjugation
between the Si–C bond and the developing positive charge
(the b-effect), while the same effect induces allylsilanes to un-
dergo an SE20-type displacement at the g-position to the silyl
moiety.3 Herein, we report that aryl- and allylsilanes are readily
carboxylated with CO2 in the presence of aluminum-based
Lewis acids. Mechanistic differences between the reactions
are discussed.
Table 1. Carboxylation of arylsilanesa
Product
(distribution)
Entry
Substrate
Yield / %
1
2
3
Ph-SiMe3
p-F-C6H4-SiMe3
o-Me-C6H4-SiMe3 Me-C6H4-CO2H
(o-:p-=3:10)
Ph-CO2H
p-F-C6H4-CO2H
20
5
35
4
5
6
7
m-Me-C6H4-SiMe3 Me-C6H4-CO2H
(o-:m-:p-=2:tracel:1)
p-Me-C6H4-SiMe3 Me-C6H4-CO2H
(o-:p-=1:9)
47
50
58
73
The general procedure for the carboxylation is as follows:
In a 50 cm3 autoclave equipped with a glass inner tube and a
magnetic stirring bar were charged
a substrate (1.00–
1.50 mmol), 2.0 mol equiv. of a Lewis acid, and a solvent
(1.0–2.0 cm3) under N2 and the apparatus was purged with
CO2 by repeated pressurization and subsequent expansion, the
final pressure being adjusted to 3.0 MPa. The mixture was stir-
red at room temperature for 3 h. After aqueous workup, the pro-
duct was purified by Kugelrohr distillation or column chromato-
graphy on silica gel with diethyl ether–hexane (1:1) as the
eluent.
1-Naphthyl-SiMe3
Naphthyl-CO2H
(1-:2-=1:trace)
Naphthyl-CO2H
(1-:2-=5:1)
2-Naphthyl-SiMe3
8
9
Ph-SiCl3
p-Me-C6H4-SiCl3
Ph-CO2H
Me-C6H4-CO2H
(o-:p-=1:2)
15
54
aReaction conditions: Substrate, 1.5 mmol; Solvent, 1.0 cm3.
First, carboxylation of aryltrimethylsilanes was examined
Copyright Ó 2003 The Chemical Society of Japan

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Chemistry Letters Vol.32, No.5 (2003)
455
of naphthalene gave 1-naphthoic acid regioselectively but the
yield was moderate (38%) even at elevated temperature
(40 ꢁC) because of the high tendency of the substrate toward de-
hydrocoupling to form polyaryls (the Scholl reaction).4e One
might suspect that the carboxylation proceeds via arylaluminum
intermediates generated in situ from arylsilanes and AlBr3. In
this respect, however, Austin et al. reported that trichlorophen-
ylsilane on treatment with AlCl3 did not afford arylaluminum
species.5 Trichlorophenyl- and trichloro-p-tolylsilanes were
carboxylated under the standard conditions to give the corre-
sponding carboxylic acids (entries 8 and 9), though the regio-
selectivity in the latter reaction was reduced compared to that
in the reaction of trimethyl-p-tolylsilane (entry 5) possibly ow-
ing to weaker b-effect of the trichlorosilyl moiety. These obser-
vations coincide well with the SEAr mechanism, though the
transmetalation cannot completely be ruled out.
Vinylsilanes were also expected to undergo the carboxyla-
tion at the a-carbon owing to the b-effect.3;8 However, tri-
methylvinylsilane polymerized under the conditions, giving no
acid product. This led us to examine a weaker Lewis acid,
MeAlCl2: By using 2.0 mol equiv. of MeAlCl2 (1.0 M solution
in hexane), trimethyl(penta-1,3-dienyl)silane and 1,3-bis(tri-
methylsilanyl)propene were carboxylated to give hexa-2,4-die-
noic acid and but-3-enoic acid in 34 and 63% yields, respec-
tively, while other vinylsilanes, including trimethylvinylsilane
and bicyclo[2.2.1]hept-2-en-2-yltrimethylsilane, gave only a
trace amount of the corresponding carboxylic acids.
Scheme 1.
state (Scheme 1). However, it seems that allylsilanes are not al-
ways converted into allylaluminums under the reaction condi-
tions. For example, trimethyl(penta-2,4-dienyl)silane, when
treated with CO2 in the presence of Me2AlCl, gave hexa-3,5-
dienoic acid (entry 4), while the same treatment of trimethyl
(penta-2,4-dienyl)stannum gave 2-vinylbut-3-enoic acid in
67% yield. Although the latter reaction is more likely to proceed
via the transmetalation between the allylstannum and
Me2AlCl,10 it is difficult to explain the formation of the differ-
ent products from the same aluminum species.
In conclusion, we have shown here that aryl- and allylsi-
lanes are readily carboxylated with CO2 in the presence of alu-
minum-based Lewis acids. Further studies on the reaction me-
chanisms are in progress.
This work was supported in part by a grant from the Min-
istry of Education, Culture, Sports, Science, and Technology,
Japan (Grant-in-Aid for Scientific Research No. 13650908).
Next, carboxylation of allyltrimethylsilanes were examined
by using MeAlCl2 as the Lewis acid (Table 2). Several simple
allylsilanes were carboxylated to give allylic acids in moderate
yields (entries 1–3). It should be noted that both but-2-enyltri-
methylsilane and trimethyl(1-methylallyl)silane gave 2-methyl-
but-3-enoic acid (entries 2 and 3), formation of which from the
latter allylsilane contradicts the SE20 mechanism. In addition,
allyltrimethylsilane was reported to undergo transmetalation
with GaCl3 to give an allylgallium species.9 Therefore, the car-
boxylation of allylsilanes is likely to proceed via allylaluminum
species. Formation of 2-methylbut-3-enoic acid from both the
terminal and internal silanes can be explained by the equili-
brium between in situ-generated terminal and internal allylalu-
minum species and nucleophilic addition of the more stable
terminal allylaluminum to CO2 via a six-membered transition
References
1
Reviews: a) M. Aresta, E. Quaranta, and I. Tommasi, New. J.
Chem., 18, 133 (1994). b) H. Arakawa, M. Aresta, J. N. Armor,
M. A. Barteau, E. J. Beckman, A. T. Bell, J. E. Bercaw, C. Creutz,
E. Dinjus, D. A. Dixon, K. Domen, D. L. DuBois, J.
Eckert, E. Fujita, D. H. Gibson, W. A. Goddard, D. W.
Goodman, J. Keller, G. J. Kubas, H. H. Kung, J. E. Lyons, L. E.
Manzer, T. J. Marks, K. Morokuma, K. M. Nicholas, R.
Periana, L. Que, J. Rostrup-Nielson, W. M. H. Sachtler, L. D.
Schmidt, A. Sen, G. A. Somorjai, P. C. Stair, B. R. Stults, and
W. Tumas, Chem. Rev., 101, 953 (2001).
2
3
4
For example, see: a) R. W. Hoffmann, Angew. Chem., Int. Ed.
Engl., 21, 555 (1982). b) T. Ooi, M. Takahashi, and K. Maruoka,
J. Am. Chem. Soc., 118, 11307 (1996).
a) K. D. C. Eaborn and D. R. M. Walton, Organomet. Chem.
Synth., 1, 151 (1970/1971). b) A. Hosomi and H. Sakurai, Tetrahe-
dron Lett., 1976, 1295.
a) J. F. Norris and J. E. Wood, III, J. Am. Soc. Chem., 62, 1428
(1940). b) M. G. Gonikberg and A. E. Gavrilova, Zh. Obshch.
Khim., 22, 1388 (1952). c) S. Fumasoni and M. Collepardi, Ann.
Chim., 54, 1122 (1964). d) R. Huesler, I. Orban, and M. Holer,
European Patent 706987 (1996). e) Y. Suzuki, T. Hattori, T.
Okuzawa, and S. Miyano, Chem. Lett., 2002, 102. f) G. A. Olah,
Table 2. Carboxylation of allylsilanesa
B. Torok, J. P. Joschek, I. Bucsi, P. M. Esteves, G. Rasul, and
¨ ¨
G. K. S. Prakash, J. Am. Chem. Soc., 124, 11379(2002) and refer-
ences cited therein.
5
6
J. D. Austin, C. Eaborn, and J. D. Smith, J. Chem. Soc., 1963,
4744.
G. A. Olah, T. Bach, and G. K. S. Prakash, J. Org. Chem., 54, 3770
(1989).
7
8
J. L. Speier, J. Am. Chem. Soc., 75, 2930 (1953).
Recent examples of carboxylation of unsaturated hydrocarbons: a)
H. Senboku, H. Kanaya, and M. Tokuda, Synlett, 2002, 140. b) M.
Takimoto, K. Shimizu, and M. Mori, Org. Lett., 3, 3345 (2001).
9M. Yamaguchi, T. Sotokawa, and M. Hirama,
1997, 743.
10 J. J. Eisch, K. Mackenzie, H. Windisch, and C. Kruger, Eur. J. In-
¨
org. Chem., 1999, 153.
Chem. Commun.,
aReaction conditions: Substrate, 1.00 mmol; Solvent,
b
2.0 cm3. Me2AlCl was employed instead of MeAlCl2.
Published on the web (Advance View) April 23, 2003; DOI 10.1246/cl.2003.454