Published on the web August 25, 2012
913
Electrophilic Aromatic Substitution of Arenes with CO Mediated by R SiB(C F )
2
3
6
5 4
Megumi Konno, Masafumi Chiba, Koji Nemoto, and Tetsutaro Hattori*
Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University,
-6-11 Aramaki-Aoba, Aoba-ku, Sendai, Miyagi 980-8579
6
(
Received May 16, 2012; CL-120419; E-mail: hattori@orgsynth.che.tohoku.ac.jp)
5
The FriedelCrafts-type carboxylation of arenes has been
achieved by activating CO2 with silylium borates. The reaction
exhibits broader substrate applicability than does our previously
reported AlX3/R3SiX-mediated carboxylation.
triaryl- or trialkylsilane to give a silylium ion. Taking advantage
of this property, Kawashima and co-workers have recently
reported the successful intramolecular sila-FriedelCrafts reac-
tion of biphenyl-2-yldiphenylsilanes using Ph CB(C F ) in the
3
6 5 4
6
presence of 2,6-lutidine. We envisaged that the electrophilic
carboxylation of aromatics may be achieved using silylium
7
A growing sense of crisis with regard to the exhaustion
of fossil resources and global climate change has prompted
chemists to develop efficient methods for the utilization of CO2
borates to activate CO2.
First, the carboxylation of mesitylene was tested in the neat
substrate in the presence of equimolar amounts of Ph SiH and
Ph CB(C F ) under CO pressure at room temperature (Table 1,
3
1
as a renewable chemical feedstock. The reaction of CO2 with
3
6
5 4
2
carbon nucleophiles has been highlighted recently, and remark-
able progress has been made with the development of the
transition-metal-catalyzed carboxylation of organometallics bear-
Entry 1). The reaction actually gave mesitoic acid accompanied
by dimesityl ketone. The formation of the ketone is rationalized
by the initial carboxylation of mesitylene, followed by the
FriedelCrafts acylation of mesitylene with the resulting triphen-
ylsilyl mesitoate with the aid of HB(C F ) , which is generated in
2
ing weakly polarized metalcarbon bonds, as well as with direct
carboxylation via the transition-metal-catalyzed cleavage of an
6
5 4
3
aromatic CH bond. On the other hand, it has long been known
situ by the carboxylation (vide infra). This could be avoided by
the addition of 2,6-di-tert-butylpyridine (DTBP), though the
yield of mesitoic acid was reduced (Entry 3); the use of the
sterically less demanding 2,6-lutidine instead of DTBP disturbed
the carboxylation (Entry 2), presumably owing to the formation
of an adduct(s) with the silylium ion and/or siloxycarbonylium
that strong Lewis acids such as aluminum halides mediate the
4
direct carboxylation of aromatic compounds with CO2. The
reaction is believed to proceed via the electrophilic aromatic
substitution (SEAr) mechanism, in which the Lewis acid activated
4
b
CO2 serves as an electrophile. However, the applicability of the
electrophilic carboxylation is severely limited because of the low
electrophilicity of CO2 and/or side reactions caused by the strong
Lewis acids employed. Recently, we reported that the combined
use of AlBr3 and a trialkyl- or triarylsilyl chloride efficiently
promotes the electrophilic carboxylation of arenes and haloben-
zenes.4d Mechanistic studies strongly suggested that a silyl halide
reacts with CO2 in cooperation with AlX3 to give a silyl-
haloformate-like active species, although its precise structure is
not clear at present. The formation of the active CO2 species
can be interpreted by two different mechanistic pathways
+
Ph SiOC¸O (vide infra). The replacement of Ph SiH with
3
3
Et SiH, in expectation of a higher reactivity of the silylium ion of
3
the latter toward CO , lowered the product yields (Entry 4); this
2
was attributed to the slow formation of Et SiB(C F ) under the
3
6 5 4
reaction conditions employed. In relation to this, Lambert et al.
reported that Et SiB(C F ) was prepared quantitatively by
3
6 5 4
immersing Ph CB(C F ) in an excess of Et SiH for two days,
3
6
5 4
3
5
while Ph SiH gave a complex mixture by the same treatment.
3
+
Table 1. Carboxylation of mesitylene with the aid of R3Si in the
a
neat substrate
(
Scheme 1). In the first, CO2 is activated by AlX3 in such a
4
b
CO2
manner as proposed by Olah et al. to form an aluminum-
haloformate-like species, which then reacts with a silyl halide to
give a silyl haloformate (path a). Alternatively, AlX3 may react
with a silyl halide to give a silylium ion, which activates CO2 to
form a silyl haloformate (path b). We are particularly interested
in the possibility of the latter path because it may lead to the
development of a method for the activation of CO2 using reagents
compatible with acid-sensitive substances. It is well known that
the hydrogen atom binds more strongly to carbon than to silicon,
which allows the tritylium ion to abstract a hydrogen atom from a
O
R SiB(C F )
3
6
5 4
CO2H
base
+
r.t.
Acidb Ketoneb
Entry R3Si+ source
Base
/
%
/%
1
2
3
4
5
6
7
8
Ph3SiH + Ph3CB(C6F5)4 none
Ph3SiH + Ph3CB(C6F5)4 2,6-lutidine trace
Ph3SiH + Ph3CB(C6F5)4 DTBP
Et3SiH + Ph3CB(C6F5)4 none
52
40
®
®
31
®
®
14
®
36
35
quant
83
86
66
Et3SiB(C6F5)4
Et3SiB(C6F5)4
Et3SiB(C6F5)4
Et3SiB(C6F5)4
none
none
none
DTBP
c
O
CO2
R SiX
d
3
path a
path b
AlX2
X
O
O
AlX3
SiR3
a
X
O
3
Reaction conditions: mesitylene (5.0 mmol), ether Ph SiH and
R SiX
CO2
3
R Si
AlX4
3 6 5 4 3 6 5 4
Ph CB(C F ) (0.20 mmol each) or Et SiB(C F ) prepared from
3
Et3SiH and Ph3CB(C6F5)4 (0.20 mmol each), base (0.20 mmol),
b
CO2 (3.0 MPa), room temp, 18 h. Isolated yield based on the
Scheme 1. Possible mechanisms for the formation of a silyl
haloformate-like active species from CO2 and R3SiX with the aid of
AlX3. The structures of species are simplified.
+
c
quantity of R3Si . The reaction was carried out under CO2
(0.1 MPa) atmosphere. Reaction time was extended to 36 h.
d
Chem. Lett. 2012, 41, 913914
© 2012 The Chemical Society of Japan