SYNTHESIS OF AROMATIC CARBOXYLIC ACIDS
1845
methanol in the presence of cobalt carbonyl benzyl
, %
chloride base catalytic system. This system is the most
suitable for carbonylation of 5,6-dichloroacenaphthene
to acenaphthene dicarboxylic acid used for preparing
a valuable commercial product, 1,4,5,8-naphthalene-
tetracarboxylic acid [9].
To prepare more efficient catalytic systems for car-
bonylation of aryl halides, dicobalt octacarbonyl
was modified.
Since aryl halides are activated by strongly nucleo-
philic anionic cobalt complexes, we examined catalyt-
ic systems generating new reactive intermediate anion-
ic cobalt complexes.
, h
of -chloronaphthalene in the presence of
Conversion
It is known [10] that Co(CO) in alcoholic solu-
4
different cocatalysts: (1) ClCH COOCH , (2) PhCH Cl
2
3
2
tions containing a base undergoes nucleophilic attack
of epoxides to form immediately anionic alkylcobalt
carbonyl complexes in which the negative charge is
localized on the alkyl group. We suggest that catalytic
properties of these complexes will differ from those of
the anionic complexes formed with alkyl halides after
attack of methylate anion on the carbonyl group of the
neutral alkylcobalt carbonyl complex. Indeed, as seen
from the figure, the cobalt carbonyl olefin oxide sys-
tem is more active by a factor of approximately 2 than
our most active alkylcobalt carbonyl catalytic system
containing benzyl chloride [7 9]. Based on these
results, we developed a series of highly active catalyt-
ic systems [8, 11, 12] containing olefin oxides, epi-
chlorohydrin, and ethylenechlorohydrin as cocatalysts
(see figure). All these cocatalysts, as well as epoxides,
form intermediate anionic complexes, with epichloro-
hydrin being more active owing to the presence of two
functional groups. These cocatalysts are not consumed
by the parallel pathway of self-carbonylation (as does
benzyl chloride). These compounds may undergo
methanolysis but this reaction is insignificant in the
presence of weak bases such as alkali metal carbo-
nates. The methanolysis products, monomethyl ethers
of glycols can be readily separated from the target
products, arylcarboxylic acids.
(dashed line reflects the reaction course after introduction
of additional portion of benzyl chloride), (3) C H Br,
8
17
(4) ClCH CH OH, and (5)
as a function of
CH CH CH
2
2
3
2
O
time . T = 60 C, P
= 1 atm, [Co (CO) ] = 0.017 M.
CO
2 8
The mechanism of carbonylation of aryl halides in
the presence of Co (CO) epoxide catalytic system is
2
8
similar to that of the reaction catalyzed by cobalt car-
bonyl modified with benzyl chloride [4, 8]. The mech-
anism of carbonylation of aryl halides in the presence
of cobalt carbonyl epoxide base catalytic system can
be described by a catalytic cycle (see scheme).
In this system, Co(CO) anion reacts with epoxide
4
with opening of the three-membered epoxy ring to
form intermediate anionic complex. Probably, this
complex is subjected to cyclization into negatively
charged metallalactone M which is a true catalyst of
aryl halide carbonylation.
This complex is highly nucleophilic and activates
ArHal by the S 1 mechanism (which is confirmed
RN
by the fact that this reaction is inhibited by aromatic
nitro compounds and other one-electron acceptors) via
formation of arylalkylcobalt carbonyl complex. For-
mation of metallalactones from metal carbonyls has
been demonstrated with an example of alkylcobalt
carbonyl complexes of iron [13].
An important advantage of epoxide cocatalysts is
the absence of halogen atom in their composition.
Hence, the base is not consumed for binding the
halogen. Propylene oxide is the most available, cheap,
and convenient in use.
The main examples of highly efficient synthesis of
mono- and polyaromatic acids performed for the first
time by carbonylation of the corresponding aryl hal-
ides in methanol in the presence of Co(CO) propyl-
4
The activity of the examined cocatalysts decreases
in the following order:
ene oxide K CO catalytic system are presented in
2
3
Tables 1 and 2.
>
>
ClCH
CH
> OHCHRCH Cl >
2
3
2
As seen from Tables 1 and 2, Co(CO) propylene
4
O
O
O
oxide K CO catalytic system can be used for selec-
2
3
PhCH Cl > p-ClC H CH Cl > CH I > C H I >
tive synthesis of aromatic mono- and dicarboxylic
acids in a high yield in methanol under very mild
2
6
17
4
2
3
2 5
C H Br > n-C H Br > ClCH COOCH .
2
5
8
2
3
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 78 No. 11 2005