Efficient chemoselective carboxylation of aromatics to arylcarboxylic acids with a superelectrophilically activated carbon dioxide-Al2Cl6/Al system

Article abstract of DOI:10.1021/ja020787o

Aromatic carboxylic acids are obtained in good to excellent yield essentially free of diaryl ketones by carboxylation of aromatics with a carbon dioxide-Al₂Cl₆/Al system at moderate temperatures (20-80°C). To optimize reaction condit

Full text of DOI:10.1021/ja020787o

Published on Web 08/29/2002
Efficient Chemoselective Carboxylation of Aromatics to
Arylcarboxylic Acids with a Superelectrophilically Activated
Carbon Dioxide-Al
George A. Olah,* B e´ la T o¨ r o¨ k, Jens P. Joschek, Imre Bucsi, Pierre M. Esteves,
2
Cl
6
/Al System
†
Golam Rasul, and G. K. Surya Prakash*
Contribution from the Loker Hydrocarbon Research Institute and Department of Chemistry,
UniVersity of Southern California, UniVersity Park, Los Angeles, California 90089-1661
Received June 3, 2002
Abstract: Aromatic carboxylic acids are obtained in good to excellent yield essentially free of diaryl ketones
by carboxylation of aromatics with a carbon dioxide-Al
C). To optimize reaction conditions and study the reaction mechanism, experimental variables including
temperature, amount of Al Cl /Al, various Lewis acids, role of metal additive, carbon dioxide pressure, etc.
were studied. The carboxylation reaction was found to be stoichiometric rather than catalytic, with aluminum
chloride forming a dichloroaluminate of carboxylic acids. Although the carboxylation takes place using AlCl
2 6
Cl /Al system at moderate temperatures (20-80
°
2
6
3
itself, the presence of metal additives, especially Al, increased the yield and selectivity of carboxylic acids.
Because it was not possible to distinguish between two possible mechanistic pathways of the reaction on
the basis of the experimental results, theoretical calculations using density functional theory (DFT) were
also carried out. One possible pathway involves an initial complex between benzene and Al
subsequent formation of organoaluminum intermediates (PhAlCl and PhAl Cl ). The other proceeds through
the formation of various complexes of CO with aluminum chloride (AlCl , n ) 1-4. The calculations
have shown that the organometallic pathway, leading eventually through the formation of phenylaluminum
2 6
Cl , with
2
2
5
2
3 n
)
dichloride, is endothermic by 33 kcal/mol. In contrast, the preferred CO
2
3
-AlCl complex forms in an
+
exothermic reaction (-6.0 kcal/mol) as does CO
2
AlCl
2
. On the basis of both experimental and calculational
findings, the most feasible reaction mechanism proposed involves superelectrophilic aluminum chloride
activated carbon dioxide reacting with the aromatics in a typical electrophilic substitution.
Introduction
One of the most versatile processes for carbon-carbon bond
formation is Friedel-Crafts chemistry such as alkylation and
6
Carbon dioxide as a greenhouse gas contributes significantly
to global warming. The Kyoto Treaty of December 1997
intends to restrict the emission of greenhouse gases. Although
carbon dioxide is one of these gases, its concentration in the
1
acylation. Friedel-Crafts acylation is the preferred method for
6
7
2
the preparation of aromatic ketones. Arylcarboxylic acids are
involved in important pharmaceuticals belonging to the family
of nonsteroidal antiinflammatory drugs (NSAIDs), which are
air is only 0.0034%. However, its contribution to the overall
8
-10
greenhouse effect is close to 50%.3
the most widely prescribed medications.
The chemical fixation of carbon dioxide is a viable approach
to mitigate excessive carbon dioxide preferably by transforma-
tion to valuable products. Although several methods are known
for the chemical conversion of carbon dioxide, they are generally
(
5) Arakawa, H.; Aresta, M.; Armor, J. N.; Barteau, M. A.; Beckman, E. J.;
Bell, A. T.; Bercaw, J. E.; Creutz, C.; Dinjus, E.; Dixon, D. A.; Domen,
K.; DuBois, D.; Eckert, J.; Fujita, E.; Gibson, D. H.; Goddard, W. A.;
Goodman, D. W.; Keller, J.; Kubas, G. J.; Kung, H. H.; Lyons, J. E.;
Manzer, L. E.; Marks, T. J.; Morokuma, K.; Nicholas, K. M.; Periana, R.;
Que, L.; Rostrup-Nielson, J.; Sachtler, W. M. H.; Schmidt, L. D.; Sen, A.;
Somorjai, G. A.; Stair, P. C.; Stults, B. R.; Tumas, W. Chem. ReV. 2001,
101, 953.
4
very energy demanding, limiting the practicality of their use.
Developing new methods for CO2 fixation is thus highly
(6) Olah, G. A., Ed. Friedel-Crafts and Related Reactions; Wiley: New York,
5
desirable.
1964.
(7) (a) Olah, G. A.; Olah, J. A. In Friedel-Crafts and Related Reactions; Olah,
*
To whom correspondence should be addressed. E-mail: olah@usc.edu.
Present address: Department of Chemistry, Michigan Technological
G. A., Ed.; Wiley: New York, 1963; Vol. III, Chapter XXXIX, p 1257.
†
(
b) Simonetta, M.; Carr a´ , S. In The Chemistry of Functional Groups: The
University, 1400 Townsend Drive, Houghton, MI 49931.
Chemistry of Carboxylic Acids and Esters; Patai, S., Ed.; Wiley: Chichester,
New York, 1969; Chapter 1, p 1. Ogliaruso, M.; Wolfe, J. F. In The
Chemistry of Functional Groups: Synthesis of Carboxylic Acids and Their
DeriVatiVes; Patai, S., Rappoport, Z., Eds.; Wiley: Chichester, 1991; p 1.
(c) Ito, T.; Sugahara, N.; Kindaichi, Y.; Takami, Y. Nippon Kagaku Kaishi
1976, 353. (d) Suzuki, Y.; Hattori, T.; Okuzawa, T.; Miyano, S. Chem.
Lett. 2002, 102.
(
(
(
1) Bakker, D.; Watson, A. Nature 2001, 410, 765. Abelson, P. H. Science
2
000, 289, 1293.
2) Kyoto Protocol to the United Nations Framework Convention on Climate
Change, 1997.
3) Mintzer, I. M. Energy, Greenhouse Gases Climate Change. In Annual
ReViews of Energy; Hollander, J. M., Socolow, R. H., Sternlich, D., Eds.;
1
990; Vol. 15, p 513.
(8) Geis, G. S. J. Pharmacol. 1999, 56, 31.
(
4) Stocchi, E. Industrial Chemistry; Ellis Horwood: New York, 1990. Inoue,
S., Yamazaki, N., Eds. Organic and Bioorganic Chemistry of Carbon
Dioxide; Kodansha: Tokyo, Wiley: New York, 1982. Halman, M. M.
Chemical Fixation of Carbon Dioxide; CRC Press: Boca Raton, 1993.
(9) (a) Fryers, G. Aspirin-Towards 2000, Proc. Int. Meeting Sponsored by Eur.
Aspirin Foundation; Royal Society of Medicine: London, 1990. (b)
Weissmann, G. Sci. Am. 1991, 264, 84. Patrono, C. N. Eng. J. Med. 1994,
330, 1287.
10.1021/ja020787o CCC: $22.00 © 2002 American Chemical Society
J. AM. CHEM. SOC. 2002, 124, 11379-11391
9
11379

A R T I C L E S
Olah et al.
The most desirable route for the synthesis of arylcarboxylic
acids would be the direct use of carbon dioxide. Friedel and
Crafts themselves observed that a minor amount of benzoic acid
was formed when carbon dioxide was bubbled through a mixture
of aluminum chloride and benzene heated to the boiling point
Table 1. Carboxylation of Benzene with CO2 in the Presence of
AlCl3/Al (57 atm CO2, 18 h)
product yield (%)
benzene
mL
AlCl
(g/g)
3
/Al
T
°C
benzoic
acid
di-phenyl
methane
benzophenone
otherb
1
1
of the latter. A small amount of hydrogen chloride evolved
simultaneously. They suggested the following path, involving
20
10/1.2
10/1.2
2.5/0.6
2.5/0.6
2.5/0.6
2.5/0.6
2.5/0.6a
120
120
110
90
80
70
41
15
35
29
9
1
1
1
1
0
0
0
0
18
25
1
1
the initial formation of phenylaluminum dichloride.
9
27
88
62
C H + Al Cl f C H Al Cl + HCl
40
6
6
2
6
6
5
2
5
20
70
6
13
C H Al Cl + CO f C H COOAl Cl
a
Reaction time: 68 h. b Other - unidentified higher Mw products.
6
5
2
5
2
6
5
2
5
trisubstituted derivatives with the carbon dioxide-AlCl3/Al
reagent system.
C H COOAl Cl + H O f C H COOH + Al Cl OH
6
5
2
5
2
6
5
2
5
Many previous attempts to make the reaction practical have
failed. Pressures of 50-60 bar at 80-150 °C were used in
obtaining benzoic acid, p-chlorobenzoic acid, p-toluic acid, or
Results and Discussion
The impetus for the use of the AlCl3/Al system came by the
early work of Hall and Nash in the late 1930s, who found that
ethylene when reacted with AlCl3/Al gave ethylaluminum
2
,4-dimethylbenzoic acid from the corresponding aromatics in
low yields. In the reactions, however, secondary reaction
products such as benzophenones and diphenylmethanes were
formed in major amounts.7
1
8
sesquichloride. In our studies of such organoaluminum
systems, reaction with carbon dioxide gave carboxylic acids,
a
1
2
including methyl malonic acid.
Carbon dioxide in the presence of aluminum chloride reacts
more readily with the electron-rich oxygenated aromatics.
However, complex mixtures are formed. Thus, phenol and
carbon dioxide in the presence of aluminum chloride give 2,4-
or 4,4′-dihydroxybenzophenone, salicylic acid, aurin, or diphenyl
ether by variation of the temperature and pressure. Cresols yield
products of similar type. With the more reactive phenols, zinc
or ferric chloride can replace aluminum chloride as the catalyst.
Dimethylaniline gives tetramethyl-p,p′-diaminodiphenylmethane
and some p-dimethylaminobenzoic acid.7 The only relatively
After screening many carbon dioxide-Lewis acid halide
systems, we pursued the carboxylation of benzene by the
aluminum trichloride-aluminum system in detail.
a
7
c
successful results were achieved with ferrocene derivatives.
Naphthalenes, anthracenes, and phenanthrenes have given the
corresponding carboxylic acids in poor yields.7d
In our previous work using superacidic conditions, aromatics
such as benzene with carbon dioxide gave mainly benzophenone,
indicating that any intermediate benzoic acid formed under the
reaction conditions reacts further, thus benzoylating excess
We systematically investigated a number of experimental
variables such as reaction temperature, time, and catalyst-
reactant ratio. Representative results are tabulated in Table 1.
It was found that, at more elevated temperatures (studied up
to 120 °C), no benzoic acid could be observed, and benzophe-
none and diphenylmethane were the only isolable products.
Benzoic acid initially formed clearly is further transformed by
its Friedel-Crafts benzoylation of excess benzene to benzophe-
none and then by an ionic hydrogenation pathway to diphenyl-
methane (Table 1). However, lower temperatures resulted in a
gradual increase in yield of the benzoic acid. Benzoic acid
became significant at 90 °C and was formed exclusively at 70
benzene.1
2a
Despite all attempts,1
3-17
no satisfactory conditions were,
however, found for the direct carboxylation of aromatics with
carbon dioxide to arylcarboxylic acids, and this remained a long-
standing challenge.
We now report the first efficient and chemoselective prepara-
tion of aromatic carboxylic acids by direct superelectrophilic
carboxylation of benzene, and some of its mono-, di-, and
(
10) (a) Ito, Y.; Kato, H.; Yasuda, S.; Kato, N.; Yoshida, T.; Yamamoto, Y. Y.
°C. According to these results, several benzene derivatives were
Japanese Patent, JP 92-279165 19920925; Chem. Abstr. 1994, 121, 255669.
(b) Ito, Y.; Kato, H.; Yasuda, S.; Kato, N.; Yoshida, T.; Ueshima, M.
studied in detail in their carboxylation reaction. The optimized
results are summarized in Table 2.
Japanese Patent, JP 92-216666 19920723; Chem. Abstr. 1994, 121, 133985.
11) (a) Friedel, C.; Crafts, J. M. Compt. Rend. 1878, 86, 1368. (b) Friedel, C.;
Crafts, J. M. Ann. Chim. Phys. 1883, 14, 433.
(
(
The results show that the direct carboxylation could be carried
out at moderate temperatures in good to excellent yields, without
the formation of a significant amount of diaryl ketones, which
are readily formed at higher temperatures. It is also important
to note that with more active substrates such as mesitylene,
carboxylation to the corresponding acid (80% yield) takes place
even at 20 °C.
12) (a) Olah, G. A.; Farooq, O.; Farnia, S. M. F.; Bruce, M. R.; Clouet, F. L.;
Morton, P. R.; Prakash, G. K. S.; Stevens, R. S.; Bau, R.; Lammertsma,
K.; Suzer, S.; Andrews, L. J. Am. Chem. Soc. 1988, 110, 3231. (b) Olah,
G. A.; Prakash, G. K. S.; Rasul, G. ArkiVoc 2002, 7.
(
(
(
13) Morgan, G. T.; Pratt, D. D. Br. Pat. 1930, 353, 464.
14) Fumasoni, S.; Collepardi, M. Ann. Chim. 1964, 54, 1122.
15) Lebedev, B. L.; Pastuhova, I. V.; Eiduc, Y. T. IzV. Akad. Nauk, Ser. Khim.
1
972, 4, 967.
(
16) Bottacio, G.; Chiusoli, G. P.; Felicioli, M. G. Gazz. Chim. Ital. 1973, 103,
05.
1
(
17) Kouwenhoven, H. W.; van Bekkum, H. In Handbook of Heterogeneous
Catalysis; Ertl, G., Kn o¨ zinger, H., Weitkamp, J., Eds.; Wiley-VCH: New
York-Weinheim, 1997; Vol. 5, p 2358.
(18) (a) Hall, F. C.; Nash, A. W. J. Inst. Pet. Technol. 1937, 23, 679. (b) Hall,
F. C.; Nash, A. W. J. Inst. Pet. Technol. 1938, 24, 471.
11380 J. AM. CHEM. SOC.
9
VOL. 124, NO. 38, 2002

Chemoselective Carboxylation of Aromatics
A R T I C L E S
Table 2. Carboxylation of Benzene and Substituted Aromatics
Scheme 1
with a CO2/AlCl3/Al System (57 atm CO2 and 18 h Reaction Time)
Table 3. Effect of Different Lewis and Brønsted Acids on the
Carboxylation of Benzene with CO2 (57 atm CO2)
T
(°C)
yielda
(%)
product selectivity
acid ketone
other
products
acid
a
Isolated yields. b 68 h reaction time. c 2,3-Dimethylbenzene carboxylic
AlCl3
AlBr3
SbF5
80
80
40
50
57
100
85
0
15
acid/3,4-dimethylbenzene carboxylic acid.
phenol (5%)
a
The amount of the acid formed in the carboxylation reaction
was found to be dependent on the amount of Al2Cl6/Al used.
Although carboxylic acid formation was also observed without
the added Al metal, addition of Al powder to the system
increased the yield of the carboxylic acids significantly.
Deactivated aromatics such as halobenzenes gave lower yields,
whereas nitrobenzene gave only some byproducts but no
carboxylic acid.
For acid, based on 1:1 AlCl3:product ratio.
Despite the wide range of solid and liquid Lewis and Brønsted
acids tested, it was found that except for aluminum halides none
of the other acids (TiCl4, FeCl3, FeBr3, GaCl3, Ga(OTf)3, CF3-
SO3H, K-10, Nafion-H) were able to promote the effective
carboxylation reaction. A small amount of phenol was isolated
in the reaction with SbF5 most likely as a result of oxidation.
Using TiCl4 at higher temperature (150 °C), we obtained a small
amount of diphenylmethane derivatives, while the other acids
were inactive. Except for the isomerization and the cracking
reactions, no carboxylic acid formation was observed. Because
aluminum chloride itself is also inactive in its hydrated forms
Alkylbenzenes tend to isomerize (disproportionate) when
treated with anhydrous aluminum chloride, decreasing the yield
of the expected carboxylic acids. In some cases, when the
starting aromatic compound underwent isomerization or dis-
proportionation, the formation of derived substituted aromatic
carboxylic acids was observed (Scheme 1). In the case of
(
AlCl3‚H2O to AlCl3‚6H2O), it became clear that the optimum
range of acid catalysts for the reaction is very narrow.
The effects of reaction time and the amount of Al2Cl6/Al were
also studied to learn more about the nature of the reaction. The
carboxylation of toluene, which can be carried out at lower
temperature due to the more active nature of toluene, was
selected to test the reaction. The results are shown in Figures 1
and 2.
1
9
bromobenzene, only exclusive disproportionation took place.
The product distribution in the reaction of tert-butylbenzene
indicates that disproportionation and the formation of di-tert-
butylbenzene are faster than carboxylation. Di-tert-butylbenzene
is more reactive than the monosubstituted compound. The
reactivity difference between the mono- and di-tert-butylbenzene
is the determining factor for the product distribution (Scheme
The yield of p-toluic acid continuously increases with the
reaction time; after 12 h, however, the increase becomes very
slow, and after 18 h the curve turns to saturation. This indicates
that the reaction reached its maximum yield, and further
extension in reaction time does not result in further enhancement
of the yield. On the basis of this, the effect of the amount of
AlCl3/Al was studied in 18 h long reactions. As Figure 2
demonstrates, the amount of carboxylic acid produced ap-
proximately linearly increases with the amount of aluminum
chloride. This observation strongly suggests that the reaction
1
).
To learn more about the carboxylation reaction and to find
optimized conditions, we extended the study to several other
variables such as the nature and amount of the acid, role of
metal additives, etc. First, the effects of several Lewis and
Brønsted acids were tested. The results are tabulated in Table
3
.
(19) Olah, G. A.; Tolgyesi, W. S.; Dear, R. E. A. J. Org. Chem. 1962, 27,
3
441.
J. AM. CHEM. SOC.
9
VOL. 124, NO. 38, 2002 11381

A R T I C L E S
Olah et al.
Table 4. Effect of CO2 Pressure on the Yield and Selectivity of
Carboxylation of Benzene with CO2 in the Presence of a AlCl3/Al
System
m
(
AlCl
m
(g)
Al
T
(°C)
benzene
(mL)
CO
(atm)
2
yield
(g)
yielda
(%)
3
g)
product
1
1
.25
.25
0.3
0.3
80
80
20
20
7
14
benzophenone
acid/ketone
0.9
0.72
53
∼60
6
5/35
acid/ketone
5/15
benzoic acid
1
1
.25
0.3
0.3
80
80
20
20
29
57
0.75
1.0
∼65
8
.25
88
Figure 1. Effect of reaction time on carboxylation of toluene to p-toluic
acid with CO2 in the presence of AlCl3/Al (56 atm CO2, 20 mL of toluene,
T ) 60 °C).
a
Based on 1:1 AlCl3:product ratio.
CO2 pressure (57 atm), pure benzoic acid could be isolated in
good yield. In contrast, at the lowest CO2 pressure (7 atm)
studied, benzophenone was isolated in only a moderate yield.
The Al-halides/Al system appears to be very important in
initiating the reaction. As mentioned earlier (Table 3), aluminum
halides (AlCl3, AlBr3) themselves are able to initiate the
carboxylation of aromatics, albeit in lower yields and selectivi-
ties. In view of our earlier study on superelectrophilic “:AlCl2”
1
2
species generated from AlCl3 and Al, it was decided to study
the role of aluminum and other added metals in the case of the
carboxylation of m-xylene. The results are summarized in Table
5
.
As shown, AlCl3 itself produced the 2,4-dimethylbenzoic acid
Figure 2. Effect of amount of AlCl3/Al on carboxylation of toluene to
p-toluic acid with CO2 (57 atm CO2, 20 mL of toluene, T ) 60 °C, AlCl3/
Al molar ratio ) 1).
in only moderate yield. Addition of metallic Mg in a 1:1 molar
ratio slightly increased the yield. However, in contrast with the
metal-free reaction, wherein the product was obtained as a brown
oil and needed further purification, Mg addition into the system
resulted in a pure product. In the case of addition of Zn to AlCl3,
similar observations were made; however, the yields are
considerably higher. The most significant effect was observed
using metallic aluminum as an additive. Adding aluminum in a
1:1 molar ratio greatly increased the systems’ efficiency; the
product was formed in a very high yield and chemoselectivity.
3
similar to most Friedel-Crafts acylation reactions is of sto-
ichiometric nature. Another observation, that the yields calcu-
lated on the basis of the reactant:AlCl3 ) 1:1 ratio never exceed
the theoretical 100% (the yield is usually ∼80%), also supports
the above suggestion. Therefore, the AlCl3/Al system is
considered to be a reagent in this system rather than a catalyst.
Consequently, the yields of carboxylic acids were calculated
on the basis of the 1:1 AlCl3:product ratio.
Gradually increasing the Al/AlCl ratio to 2 and eventually 3,
we obtained yields exceeding 100% (based on AlCl ). Thus,
3
Screening various solvents for the reaction, we found that
the best solvents are an excess of the aromatic compounds
themselves. Efforts to increase the yields and the selectivity
toward carboxylic acid formation using other solvents, however,
failed. Benzophenone is formed selectively but in low yield (9%)
in supercritical CO2. Common organic hydrocarbons underwent
transformations such as isomerization or disproportionation and
were not found to be inert under the reaction conditions. Because
it is known that AlCl3 even at relatively modest temperatures
initiates isomerization and cracking of common solvents, these
observations were expected.19
The effect of the pressure of carbon dioxide on the reaction
was, however, significant. The reactions were carried out under
the usual experimental conditions with only the CO2 pressure
changed. Significant changes were observed both in yield and
in selectivity. The data are summarized in Table 4.
added metallic aluminum has a significant role including
leaching HCl formed in the system to form an excess of
aluminum chloride in situ.
The overall reaction can be described as follows:
The reaction goes through several equilibria, however, and
involves HCl formation. The role of Mg and Zn is to shift these
equilibria into the product formation by binding the HCl formed.
However, unlike AlCl3, Mg or Zn halides are not able to initiate
the carboxylation reaction. In contrast, with the added aluminum,
the metal reacts with the HCl to form aluminum chloride in
situ that is able to initiate further carboxylation. One cannot
distinguish between the yield-enhancing effect and HCl binding.
However, yields higher than 100% strongly suggest that
formation of aluminum chloride from metallic aluminum takes
place, giving unexpectedly high yields of carboxylic acids.
The results unambiguously show that a gradual increase in
initial carbon dioxide pressure (measured at room temperature,
before reaction) resulted in significant enhancement both in
overall yield and in selectivity of benzoic acid. At an initial
11382 J. AM. CHEM. SOC.
9
VOL. 124, NO. 38, 2002

Chemoselective Carboxylation of Aromatics
A R T I C L E S
Table 5. Effect of Metal Additives on the Yield of Carboxylation of
m-Xylene with CO2 in the Presence of a AlCl3/Metal System (57
atm CO2)
Table 6. Reaction of Phenylaluminum Compounds with Carbon
Dioxide
products (%)a
T
reactant
solvent
°C
benzoic acid
biphenyl
phenol
Ph3Al
Ph3Al
benzene
hexane
benzene
ether
hexane
hexane
benzene
hexane
hexane
80
80
80
80
80
RT
80
80
RT
25
65
35
traces
92
57
41
93
59
23
1
PhAlCl2
PhAlCl2
PhAlCl2
PhAlCl2
PhAlBr2
PhAlBr2
PhAlBr2
AlCl
mole ratio
3
/metal
T
(°C)
m-xylene
(mL)
time
(h)
yielda
(%)
5
2
metal
N/A
1:1
1:1
1:3
1:1
1:2
1:3
40
40
40
40
40
40
40
20
20
20
20
20
20
20
20
20
20
20
20
20
20
42
46
69
59
92
Mg
Zn
Zn
Al
Al
Al
2
a
Isolated yields.
104
111
studied the behavior of arylaluminum species such as triphen-
yaluminum and its derivatives, phenylaluminum dichloride and
phenylaluminum dibromide, as possible intermediates under the
reaction conditions. The results are summarized in Table 6.
As shown, Ph3Al readily reacts with CO2 to form benzoic
acid in both benzene and hexane solution, although the yields
are only moderate. As a major byproduct, biphenyl was obtained
in benzene. It is, most likely, the product of a side reaction
between the reactant and solvent. In hexane solvent, no biphenyl
was formed. These data are in agreement with the literature
a
Based on 1:1 AlCl3:product ratio.
Scheme 2
Furthermore, the effect of HCl itself on the system was
determined by adding up to 21 atm of HCl gas. No carboxylation
was observed. When air or oxygen (21 atm) was added, no
benzoic acid was formed; however, a small amount of phenol
was isolated. In experiments under 6 and 2 atm of air pressure,
benzoic acid was obtained in 28 and 32% yields, respectively.
This indicates that, although the added gas (air or O2) decreased
the yields, they were not able to suppress the carboxylation
reaction. The AlBr3/Al system shows similar features to the
AlCl3/Al system.
The mechanism of the AlCl3-initiated direct carboxylation
reaction of aromatics is open for discussion since the first
original explanation offered by Friedel and Crafts 120 years
ago. There are alternate pathways to be considered to account
for the observed reaction of aromatics with carbon dioxide.
A possible reaction path would involve protonation of the
aromatic hydrocarbon by the HCl-AlCl3 (formed from H2O
impurities and AlCl3) superacidic system and subsequent
2
1
reports. We prepared the proposed intermediate PhAlCl2
2
2
(obtained as a dimer) and treated it with CO2 in different
solvents. Using benzene as a solvent, we found that the question
arises that PhAlCl2 still may act as a Lewis acid and react with
excess benzene under the experimental conditions. This was
avoided by carrying out the reaction in hexane. In hexane, an
excellent yield of benzoic acid was obtained (92%). In this case,
the solvent cannot react with the Lewis acid; the product acid
is formed as a result of CO2 insertion into the C-Al bond.
According to the above-mentioned results, we cannot, how-
ever, entirely rule out the intermediacy of PhAlCl2 in the
carboxylation reaction. Therefore, experiments were carried out
to detect the presence of PhAlCl2 in the reaction mixture. First,
AlCl3 and then AlCl3/Al were heated in benzene at 80 °C for
20 h under argon pressure. After cooling, the remaining solid
was filtered off under an inert atmosphere, and the liquid sample
was studied. After removal of benzene, a small amount of solid
-
-
quenching of the arenium ion by AlCl3OH anion as an AlCl2
equivalent. This is similar to Kovacic’s amination reaction using
ClN3-AlCl320 (Scheme 2).
1
was obtained that was dissolved in C6D6, and its H NMR
According to this reaction path, m-toluic acid should be
formed from toluene. However, this is not the case. Our
experimental data clearly show (Table 2) that p-toluic acid is
formed with high selectivity. In addition, the reaction does not
occur in the presence of excess HCl gas. As a result, the
protolytic benzene activation by the superacidic HCl-AlCl3
system as a pathway can be clearly ruled out.
spectrum was recorded. None of these samples, however,
showed any evidence for the presence of PhAlCl2 in the reaction
mixtures. Unlike the formation of ethylaluminum-sesquichloride
reported by Hall and Nash, benzene does not react with AlCl3/
Al mixtures to produce phenylaluminum derivatives.
18
We were not able to detect PhAlCl2 or PhAlBr2 as an
intermediate in the reaction by NMR; however, its concentration
could be very low in equilibria. The sensitivity of NMR is
generally low, and therefore it may not be possible to observe
species in very low concentrations. Experimental results obtained
with PhAlCl and PhAlBr show that the reaction of CO with
The other alternative routes are (i) the formation of an
organoaluminum compound (PhAlCl2 and its AlCl3 complexes)
and its subsequent reaction with CO2 and (ii) activation of
carbon dioxide by aluminum chloride and its electrophilic attack
on the aromatic ring.
2
2
2
these organoaluminum compounds is a fast reaction. Thus, the
failure to observe the intermediate by NMR in the reaction
mixture is not sufficient to rule out this path.
In their original paper,11 Friedel and Crafts already proposed
the possibility of an organometallic pathway, the formation of
phenylaluminum dichloride as an intermediate.1 We carefully
1
(
20) (a) Kovacic, P.; Levisky, J. A. J. Am. Chem. Soc. 1966, 88, 100. (b) Kovacic,
P.; Levisky, J. A. J. Am. Chem. Soc. 1966, 88, 1000. (c) Kovacic, P.;
Lowery, M.; Field, K. W. Chem. ReV. 1970, 70, 639. (d) Strand, J. W.;
Kovacic, P. J. Am. Chem. Soc. 1973, 95, 2977.
J. AM. CHEM. SOC.
9
VOL. 124, NO. 38, 2002 11383

A R T I C L E S
Olah et al.
Some of the experimental data seem to support this pathway
as the reaction does not take place in the presence of excess
HCl, and air (or oxygen) has some detrimental effect on it
with a single benzene molecule, and this may not be a realistic
model for processes in solution.
To represent a more probable situation, the complexes of
benzene with the monomeric as well as dimeric aluminum
chloride were calculated at the DFT B3LYP/6-311+G*//
B3LYP/6-31+G* level. Further solvation by a second benzene
molecule was also taken into account in the cases where
solvation effects could be significant (Figure 3).
2
1
(organoaluminum compounds are known to be air sensitive ).
The second pathway would involve the activation of carbon
dioxide by complexation with the acid (either by a Lewis or a
Brønsted acid). The linear OdCdO molecule, however, has to
bend significantly to develop an empty p-orbital on the carbon.
A weak, Lewis acid complexation would leave CO2 still mostly
Figure 3 illustrates the optimized geometries of the structures
found as minima on the PES of benzene and AlCl3 and Al2Cl6.
Species 1 is the oriented π-complex, similar to the one predicted
+
linear. As compared to NO2 , which is also linear, but more
+
polarizable, CO2A would not be an efficient carboxylating
agent in its own right.23 However, the formation of a rearranged
CO2-Al2Cl6 complex or the superelectrophilic activation of
carbon dioxide by Al2Cl6 could result in a reasonably active
carbon dioxide-aluminum chloride species to react with aromatic
hydrocarbons. To study these possibilities, density functional
theory (DFT) studies on the activation process were carried out.
Both possible pathways were computed using the density
functional theory (DFT) method.
27
by Kim et al., but with a second benzene molecule solvating
the system. Structures 2 and 3 represent, respectively, the
complexes of Al2Cl6 with benzene, while structures 4 and 5
correspond to the organoaluminum compounds hypothetically
formed as intermediates in the reaction.
Structure 2 represents the bridged Al2Cl6 solvated by two
benzene molecules. This species keeps basically the same
structure of the isolated bridged Al2Cl6 (Figure 4), as expressed
by the small changes in the geometric parameters.
Aluminum chloride is known to dimerize in noncoordinating
2
4
25
2 6
This indicates that the solvation of the bridged Al Cl in
solvents and in the gas phase up to temperatures of 440 °C.
At temperatures between 440 and 800 °C, it is a mixture of the
monomer and the dimer. On the basis of this behavior, AlCl3 is
expected to be a dimer in the benzene solution. By the method
of isothermal distillation, aluminum chloride was found to be
benzene is due mainly to dispersive (van der Waals) forces. It
is known that aluminum chloride, in the absence of HCl, does
3
0
not react with aromatic hydrocarbons. However, complexes
of benzene and Al2Br6 are formed and can even be crystallized.31
Crystal structure obtained by X-ray diffraction for the benzene-
present as a dimer (Al2Cl6) in a saturated solution in benzene
at room temperature.26 Nevertheless, it is known that the
interaction of benzene with aluminum chloride could lead to
3
1
Al Br complex is similar to our computed complex 2 by the
2
6
present DFT calculations.
Structure 3 is also predicted to be a minimum on the PES,
corresponding to the complex formed between benzene and the
monobridged Al2Cl6. In this complex, the aluminum chloride
dimer acts as an activated Lewis acid, interacting with the
π-system of benzene. The second benzene molecule solvates
the complex essentially by long-range interactions, as expressed
by the interatomic distances of this molecule to the benzene-
Al2Cl6 complex, roughly 3.3 Å (Figure 3). This is essentially
complex 1, which is further complexed by a second AlCl3
molecule (forming the monobridged structure). The further
the formation of a weak π-complex between benzene and
monomer AlCl3.27 Support for this claim comes especially from
the solubility measurements of aluminum chloride in aromatic
solvents.28 Kim et al. have shown, by ab initio calculations, that
the benzene-AlCl3 complex is a minimum on the potential
energy surface (PES), thus being a possible (stable) intermediate
2
7
in this media. Their calculations indicate that this species
consists of an oriented π-complex, where one of the carbon
atoms of benzene interacts with the aluminum atom of the AlCl3.
π-Complexes of aromatics were not considered to be intermedi-
ates in the Friedel-Crafts reactions.7 However, there is
increasing evidence that in certain electrophilic aromatic
substitutions of the Friedel-Crafts type, the transition state may
a
complexation of the π-complex (structure 1) by another AlCl3
molecule giving 3 promotes a stronger interaction of the
electrophilic aluminum atom with the π-system, as expressed
by the shorter interatomic distance Al-C of this complex (2.273
Å). The analogous interatomic distance in structure 1 is 2.420
Å. This indicates that the AlCl3 is being superelectrophilically
29
be represented as an oriented π-complex.
Nevertheless, the above-mentioned ab initio studies on this
system considered the interaction of the only monomeric AlCl3
3
4
activated by the second AlCl3 molecule. This species is
predicted to be an oriented π-complex, because the electrophilic
center, which coincides with the aluminum atom in AlCl3, lies
on the top of a carbon atom in the benzene molecule. The
π-complex between benzene and a monobridged Al2Br6 was
(
21) (a) Eisch, J. J. In ComprehensiVe Organometallic Chemistry; Wilkinson,
G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, 1982;
Vol. 1, p 555. (b) Eisch, J. J. In ComprehensiVe Organometallic Chemistry,
2
nd ed.; Abel, E. W., Wilkinson, G., Stone, F. G. A., Eds.; Pergamon
Press: Oxford, 1995; Vol. 1, p 431.
(
22) Grosse, A. V.; Mavity, J. M. J. Org. Chem. 1940, 5, 106.
23) Olah, G. A.; Rasul, G.; Aniszfeld, R.; Prakash, G. K. S. J. Am. Chem. Soc.
3
2
previously proposed by Brown et al. to explain the instability
of the intermediate Al2Br6:C6H6 observed by several experi-
mental methods. The relative enthalpies of the complexes 1-5
and other isomeric systems of interest are shown in Table 7.
It can be seen that the most stable species is the Al2Cl6
sandwiched by two benzene molecules, that is, structure 2. Such
(
1
992, 114, 5608.
(
24) (a) Norris, J. F.; Rubinstein, D. J. Am. Chem. Soc. 1939, 61, 1163. (b)
Mao, X.; Xu, G. Huaxue Wuli Xuebao 1988, 1, 347. (c) Smirnov, V. V.;
Perlovskaya, O. R. Zh. Fiz. Khim. 1998, 72, 1092. (d) Mains, G. J.; Nantsis,
E. A.; Carper, W. R. J. Phys. Chem. A 2001, 105, 4371.
(
(
(
25) Aarset, K.; Shen, Q.; Thomassem, H.; Richardson, A. D.; Hedberg, K. J.
Phys. Chem. A 1999, 103, 1644.
26) Nagy, F.; Dobis, O.; Litvan, G.; Telcs, I. Acta Chim. Acad. Sci. Hung.
1
959, 21, 397.
27) (a) Tarakeshwar, P.; Lee, J. Y.; Kim, K. S. J. Phys. Chem. A 1998, 102,
253. (b) Tarakeshwar, P.; Kim, K. S. J. Phys. Chem. A 1999, 103, 9116.
(30) Brown, H. C.; Pearsall, H. W.; Eddy, L. P.; Wallace, W. J.; Grayson, M.;
Nelson, K. L. Ind. Eng. Chem. 1953, 45, 1462.
2
(
c) Tarakeshwar, P.; Kim, K. S. J. Phys. Chem. A 1999, 103, 11486.
(31) Eley, D. D.; Taylor, J. H.; Wallwork, S. C. J. Chem. Soc. 1961, 3867.
(32) Brown, H. C.; Wallace, W. J. J. Am. Chem. Soc. 1953, 75, 6265.
(33) Ryzhva, G. L.; Zibareva, L. N.; Bratchikov, A. V.; Slizhov, Y. G.;
Nekhoroshev, V. P. Zh. Fiz. Khim. 1980, 54, 3275.
(
28) Fairbrother, F.; Scott, N.; Prophet, H. J. Chem. Soc. 1956, 1164.
(
29) (a) Olah, G. A.; Kuhn, S. J.; Flood, S. H. J. Am. Chem. Soc. 1961, 83,
4571. (b) Olah, G. A.; Kuhn, S. J.; Flood, S. H. J. Am. Chem. Soc. 1961,
83, 4581.
(34) Olah, G. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 767.
11384 J. AM. CHEM. SOC.
9
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Chemoselective Carboxylation of Aromatics
A R T I C L E S
Figure 3. Optimized geometries of 1-5 at the B3LYP/6-31+G* level.
Figure 4. Optimized geometries, at B3LYP/6-31+G* level, for monomeric and dimeric aluminum chloride, and for the π-complex 1:1 with benzene.
species were previously identified by X-ray structure determi-
nation of crystals formed from AlBr3 and benzene.31 Equilibria
data of benzene with AlCl3 indicate that the ∆H and ∆S of
complex formation are -3.23 kcal/mol and -6.41 cal/mol K,
J. AM. CHEM. SOC.
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VOL. 124, NO. 38, 2002 11385

A R T I C L E S
Olah et al.
Scheme 3. Complex Formation by the Interaction of CO2 with
Table 7. Reaction Enthalpies (298.15 K and 1 atm) for the
Equilibria Involving Aluminum Chloride and Benzene, Calculated at
B3LYP/6-311+G*//B3LYP/6-31+G* + ZPE Level
Monomeric AlCl3
o
∆H (298.15 K)
reaction
(kcal/mol)
Al2Cl6 + 4C6H6 ) (2) + 2C6H6
Al2Cl6 + 4C6H6 ) 2 (1)
Al2Cl6 + 4C6H6 ) (3) + 2C6H6
Al2Cl6 + 4C6H6 ) (4) + (1) + HCl + C6H6
Al2Cl6 + 4C6H6 ) (5) + HCl + 3C6H6
Al2Cl6 ) 2AlCl3
-2.4
8.6
5.4
48.7
35.2
20.5
Scheme 4. Equilibria Involving CO2 and Al2Cl6 Dimer
Table 8. Relative Energies (298.15 K and 1 atm) for the
Complexes Involving Aluminum Chloride and Benzene, Calculated
at B3LYP/6-311+G*//B3LYP/6-31+G* + ZPE Level
o
relative energy (δ∆H 298.15 K)
isomer
(kcal/mol)
Al2Cl6 + 4C6H6
2.4
11.0
0.0
7.8
51.1
37.6
2
(1)
(
(
(
(
2) + 2C6H6
3) + 2C6H6
4) + (1) + HCl + C6H6
5) + HCl + 3C6H6
respectively.33 Our calculations for the enthalpy of interaction
between aluminum chloride with benzene giving 2 gave a value
of -2.4 kcal/mol, thus being in good agreement with the
experimental value of -3.23 kcal/mol. Interaction energies of
various structural possibilities are given in Table 8.
It is shown that the interaction of benzene with monomeric
AlCl3 giving 1 is endothermic by 8.6 kcal/mol, significantly
higher than the experimental value. Nevertheless, 2 is not
expected to be an effective Lewis acid to react with CO2 because
the π-orbital of aluminum atoms is involved in bonding with
the chlorine atoms. This led us to conclude that this species is
significantly involved in the media, due to its high stability.
However, its involvement in the reaction is unlikely; there are
possible preequilibria involving other species. This species could
be either the monobridged species 3 (as the nonionized or
+
-
intimate pair of AlCl2 ‚AlCl4 ) or the organometallic complex
. Our DFT results indicate that a thermodynamically favorable
4
species, the monobridged cluster 3, could be involved in a
preequilibrium and is 7.8 kcal/mol less stable than species 2.
The organometallic complex 5 (PhAl2Cl5) is 37.6 kcal/mol
higher in energy than 2, which suggests that this pathway is
energetically unfavorable.
Besides the benzene-AlCl3 interaction, aluminum chloride
complexation with CO2 in aromatics could also lead to an
activated intermediate, which could react with the aromatic
hydrocarbons. To obtain better insight into the activation
process, the possible interactions of CO2 with aluminum chloride
were studied at the B3LYP/6-311+G*//B3LYP/6-311+G* level.
As a first approach, we investigated the interactions of CO2
with the (AlCl3)n species (n ) 1-4). We focused, however, on
the interaction of CO2 with the monomeric (AlCl3) and the
dimeric (Al2Cl6) aluminum chloride, because these species are
known to exist in these solutions.
The interaction of CO2 with the dimeric Al2Cl6 shows a more
complex picture, with the formation of different possible
intermediates (Scheme 4).
Complexation of CO2 with AlCl3 leads to the O-coordinated
structure Cl3Al-OdCdO (6a), which was found to be a stable
minimum (Figure 5).
Linear CO2 becomes slightly bent in complex 6a, with the
bond angle of 178.7°. The Al-O bond distance in structure 6a
is 2.155 Å. The C-O (AlCl3) and C-O bond distances are 1.172
and 1.147 Å, respectively, only 0.011 Å longer and 0.014 Å
shorter than C-O bond length in CO2. Enthalpy of complex
formation for this case was found to be exothermic by 6.0 kcal/
mol (Table 9).
Superelectrophilic activation34 of the complex 6a by alumi-
num chloride is possible. To investigate this possibility, calcula-
tions of 6a further complexed with one or two AlCl3 molecules
were investigated. This would decrease the back-donation
(Scheme 5) of the nonbonded electron pairs of the chlorine
atoms to the aluminum, increasing the electron deficiency of
the empty p-orbital on the carbon.
The study of the system indicates that the equilibria involving
CO2 and the monomer (AlCl3) and the dimer (Al2Cl6) of
aluminum chloride, shown respectively in Schemes 1 and 2,
are feasible. The first possibility deals only with the complex
formation between monomeric aluminum chloride with CO2
This would enhance the electrophilicity of the carbon atom,
leading to superelectrophilic activation (Scheme 6).
(Scheme 3)
11386 J. AM. CHEM. SOC.
9
VOL. 124, NO. 38, 2002

Chemoselective Carboxylation of Aromatics
A R T I C L E S
Figure 5. Optimized geometries of complexes formed by interaction of (AlCl3)n, n ) 1-4, with CO2 at the B3LYP/6-311+G* level.
Table 9. Calculated Enthalpies of Complexation of CO2 with
Scheme 5. Valence Bond Representation of Back-Donation of
Nonbonded Electrons of Chlorine to the Aluminum Atom
+
(
AlCl3)n, n ) 1-4, and AlCl2 at 298 K (B3LYP/6-311+G*//B3LYP/
6
-311+G* + ZPE level)
∆
H
rel (298 Κ)
ν
CO,terminal
(cm-1)
ν
CO,complexed
(cm-1
)
reaction
(kcal/mol)
CO2 + AlCl3 f CO2‚AlCl3 (6a)
-6.0
-27.5
+5.9
+6.8
+2.4
2448
2488
2468
2476
2464
1383
1397
1397
1388
1373
+
+
CO2 molecule is just slightly bent in the complexes, as expressed
by the OdCdO bond angle of 178.7°, 177.0°, 177.6°, and
179.8°, respectively, for species 6a, 7, 8, and 9. The structure
of the CO2‚(AlCl3)4 complex is interesting, because the alumi-
num atom expands its valence to accommodate one extra pair
of electrons from one of the chlorine atoms, becoming penta-
coordinated. Pentacoordinated aluminum is also a feature
predicted for the (AlCl3)3 and (AlCl3)4 clusters (Figure 6).
Table 9 shows the enthalpies of complex formation between
CO2 and AlCl3, Al2Cl6, Al3Cl9, and Al4Cl12 at 298.15 K and
CdO vibrational bands for each complex.
CO2 + AlCl2 f CO2‚AlCl2 (6c)
CO2 + Al2Cl6 f CO2‚Al2Cl6 (7)
CO2 + Al3Cl9 f CO2‚Al3Cl9 (10)
CO2 + Al4Cl12 f CO2‚Al4Cl12 (11)
CO2
-x-
2464
1373
Figure 6 shows the optimized geometries of the (AlCl3)n
n ) 1-4) aluminum chloride clusters.
(
Geometries of these complexes show interesting features. In
all of the CO2‚(AlCl3)n (n ) 1-4) complexes, the Al-O bond
length does not change significantly. In all of the cases, the
J. AM. CHEM. SOC.
9
VOL. 124, NO. 38, 2002 11387

A R T I C L E S
Olah et al.
Figure 6. Optimized geometries of AlCl3, Al2Cl6, Al3Cl9, and Al4Cl12 at the B3LYP/6-311+G* level.
Scheme 6. Possible Activation of CO2 by Superelectrophilic Aluminum Chloride
It can be seen that the CO2 complexation is an endothermic
reaction in all cases, except the reaction involving monomeric
AlCl3 or AlCl2 species. Dimeric aluminum chloride, however,
Complexation of CO2 with 6b leads to O-coordinated structure
6c, which was found to be a minimum. Carbon dioxide remains
linear in the C2V symmetric complex 6c. The Al-O bond
distance in 6c is 1.913 Å. This is much shorter than the Al-O
distance in 6a (2.155 Å) (Figure 5). Enthalpy of complex
formation from CO2 and 6b was found to be exothermic by
27.5 kcal/mol. The complex formation between neutral (AlCl2)2
and CO2 was also calculated; however, no minimum was found
on the PES.
+
is also capable of ionizing under suitable conditions.
+
-
4
Al Cl h AlCl + AlCl
2
6
2
+AlCl2 is a doubly electron-deficient superelectrophile, and
its interaction with carbon dioxide was also calculated. The Al-
Cl bond distance in optimized structure 6b is 2.008 Å.
11388 J. AM. CHEM. SOC.
9
VOL. 124, NO. 38, 2002

Chemoselective Carboxylation of Aromatics
A R T I C L E S
Figure 7. Geometries of the complexes formed by interaction of Al2Cl6 with CO2 at the B3LYP/6-311+G* level.
Table 10. Relative Enthalpies at 298 K and CdO Vibrational
Frequencies for the CO2‚Al2Cl6 Complex Isomers (B3LYP/
6-311+G*//B3LYP/6-311+G* Level)
Nevertheless, the more superelectrophilically activated alu-
minum chloride seems to favor complexation with CO2, which
behaves as a weak nucleophile. This suggests that the surface
of aluminum chloride, which can be regarded as the (AlCl3)4
expanded in two dimensions, can work as a superelectrophilic
surface, favoring the complexation of CO2. This result also
points out the low basicity of the CO2 molecule, because the
chlorine in aluminum chloride is predicted to be more basic
than CO2, leading to a slightly endothermic enthalpy of complex
formation of this molecule in this system.
∆
H
rel (298 Κ)
ν
CO,asym
(cm-1)
ν
CO,sym
(cm-1
)
species
(kcal/mol)
CO ‚Al Cl (7)
2
2
6
1.55
2.16
0.0
17.91
11.11
9.6
2468
2458
1661
1871
1579
2448
1393
1385
1385
1073
1376
1390
CO2‚Al2Cl6 (10)
CO2‚Al2Cl6 (13)
CO2‚Al2Cl6 (15)
CO2‚Al2Cl6 (16)
Cl3Al-OdCdO-AlCl3 (17)
CO + Al Cl
2
2
6
7.45
Optimization of all structures shown in Scheme 4 was
attempted, but some of the species did not appear to be minima
on the PES. Figure 7 shows the optimized structures of species
Table 10 contains the relative enthalpies (∆H) for the species
7 and those shown in Figure 7. The most stable species found
for this system was species 13. This species is similar to the
chloroformate aluminum salt with internal coordination with
10, 13, 15, 16, and 17, which were found as minima on the
PES.
J. AM. CHEM. SOC.
9
VOL. 124, NO. 38, 2002 11389

A R T I C L E S
Olah et al.
Scheme 7. Mechanistic Pathway for the Reaction of Benzene
with CO2 Activated by AlCl3
presently available data, direct CO2 activation appears to be the
preferred pathway.
Conclusions
In conclusion, the direct carboxylation of aromatic compounds
was achieved in high yields with the CO2-AlCl3/Al reagent
system. A variety of substituted benzenes were carboxylated,
giving their corresponding carboxylic acids. Added aluminum
metal plays a significant role in the reaction. Although it may
not necessarily actively contribute to the activation of carbon
dioxide, its binding of HCl formed in the reaction impedes side
reactions and the formation of byproducts. As it also forms
additional aluminum chloride, it leads to an increase of the yields
of carboxylic acids. To evaluate the mechanistic aspects of the
reaction, the effect of several experimental variables was also
studied, including the reaction of organoaluminum compounds
with carbon dioxide. Density functional theoretical calculations
were also carried out to probe the energy requirements for
various possible activation processes and pathways. In ac-
cordance with the experimental and computational results,
superelectrophilic activation of carbon dioxide by aluminum
chloride complexation and its reaction with aromatics is
proposed as the most viable pathway for the studied carboxy-
lations.
Experimental Section and Calculations
3
In this study, AlCl and Al as well as the aromatic reactants were
purchased either from Fluka or from Aldrich. The high purity carbon
dioxide (Coleman grade, 99.995% purity) was purchased from Mathe-
son. Triphenylaluminum was commercially available from Organo-
metallic, Inc., while phenylaluminum dichloride and phenylaluminum
the other aluminum atom in the dimer. Nevertheless, structures
7
and 10 are close in energy as compared to 13, 1.6 and 2.2
kcal/mol higher in energy, respectively.
2
2
dibromide were synthesized according to the literature reports. The
carboxylation reactions were carried out in a Parr monel autoclave,
following a general procedure. The product identification was carried
out by GC-MS (HP 5890 GC coupled with a HP-5971 mass selective
Geometric parameters of structures 13-16 clearly show that
2
these species contain a typical sp carbonyl functional group.
2
This is supported by the characteristic sp CdO vibrational
-
1
1
13
modes around 1660 cm . Structure 17 can be considered as
di-O-(AlCl3) complexes of CO2.
detector) using a 30m long DB-5 column; H and C NMR spectra
were obtained on a 300 MHz superconducting NMR spectrometer in
3
CDCl solvent with tetramethylsilane as the internal standard.
The possibility of species 13 (a six-member ring) as the more
stable intermediate among the structures proposed in Scheme
The General Procedure for the Carboxylation of Aromatic
Compounds. Preparation of Mesitylene Carboxylic Acid. First, 2.5
4
raises the mechanistic possibility of its involvement in the
g (18.75 mmol) of AlCl
a magnetically stirred Parr autoclave under argon atmosphere, and then
0 mL of mesitylene was added. After closing the autoclave, it was
pressurized by CO up to 57 bar pressure, and the mixture was heated
3
and 0.6 g (22.2 mmol) of Al were placed into
reaction of benzene with CO2 and AlCl3. This mechanistic
possibility is delineated in more detail in Scheme 7.
4
In the above mechanistic proposal, an initial activation of
CO2 by two molecules of aluminum chloride affording the
intermediate 13 should be expected. After this species is formed
in the reaction media, it would react with benzene (similar to a
chloroformic species). Nevertheless, this mechanism does not
exclude the possibility of other competing pathways, involving
species 7 and 10 or even an organometallic like PhAlCl2
intermediate.
On the basis of both the experimental and the theoretical data,
a differentiation of possible mechanistic pathways can be
proposed. There is a significant energy difference between the
organometallic path and that involving carbon dioxide activation.
Considering the formation of the most preferable intermediate,
we found that calculations suggest that the CO2-AlCl3 complex
is the least energy demanding process. Although several isomeric
structures can be assumed and calculated for CO2-AlCl3 (or
Al2Cl6) complexes, the energy requirement for their formation
is significantly (38-28 kcal/mol) lower than that for the
formation of PhAlCl2. Consequently, on the basis of the
2
to 40 °C and allowed to react for 18 h (external oil bath) under
continuous stirring (1000 rpm). The reaction was then stopped by
cooling and depressurizing, and water was added to the reaction mixture.
The products were extracted with ether, and the acid was transferred
to the aqueous phase by extraction with 10% NaOH. The pH of the
collected NaOH extract was adjusted to pH ) 1 by adding aqueous
HCl and placed into an ice bath. Precipitated white crystals were filtered
and dried. Finally, 2.55 g (83%) of 2,4,6-trimethyl-benzoic acid was
obtained.
It should be noted that, during our studies, aluminum chloride
samples from different batches (same catalog number and grade and
each from Aldrich) were used. We observed that different samples
showed different activities. These changes in activity could be taken
into account, and the reaction temperature was adjusted to the individual
samples.
2
Phenylaluminum Dichloride (PhAlCl ). A mixture of triphenyl-
aluminum (1.0 g, 3.75 mmol) and anhydrous aluminum chloride (1.0
g, 7.5 mmol) was placed into a high pressure Carins tube under argon
atmosphere. The tube was closed, and the mixture melted at 180 °C
11390 J. AM. CHEM. SOC.
9
VOL. 124, NO. 38, 2002

Chemoselective Carboxylation of Aromatics
A R T I C L E S
for 1 h. The reaction mixture was then cooled slowly, and a white
were used to characterize stationary points as minima (number of
imaginary frequencies (NIMAG) ) 0) and to evaluate zero-point
vibrational energies (ZPE), which were scaled by a factor of 0.98.
Energies were obtained at the B3LYP/6-311+G*//B3LYP/6-311+G*
+ ZPE and B3LYP/6-311+G*//B3LYP/6-31+G* + ZPE levels.
Calculated energies are given as Supporting Information.
crystalline solid was obtained upon filtration. mp 93-94 °C, lit. 94-
2
2
9
5 °C.
Phenylaluminum Dibromide (PhAlBr
aluminum (1.0 g, 3.75 mmol) and anhydrous aluminum bromide (2.0
2
). A mixture of triphenyl-
g, 7.5 mmol) was reacted according to the preparation of PhAlCl
white solid was obtained.
2
. A
3
5
Acknowledgment. Support of our work by the Loker
Hydrocarbon Research Institute and National Science Founda-
tion is gratefully acknowledged.
Calculations. Density functional theory (DFT) calculations were
3
6
performed with the Gaussian 98 package of programs. Optimized
3
7
geometries were obtained at the B3LYP /6-311+G*//B3LYP/6-
38
3
11+G* levels. Vibrational frequencies at the B3LYP/6-31+G* level
Supporting Information Available: Computational data
concerning the absolute energies, zero-point energies, and
thermal corrections for the calculated species (PDF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
(
35) Ziegler, T. Chem. ReV. 1991, 91, 651.
(
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(37) Becke’s Three Parameter Hybrid Method Using the LYP Correlation
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