A VERSATILE PROCEDURE FOR SYNTHESIS OF ORGANIC ACIDS
aliphatic (malonic) acids prepared by carbonylation
947
of the corresponding halides by the above procedure
are represented in Table 1.
Of particular interest is a modification of the meth-
od for carbonilation of benzyl chloride with calcium
hydroxide in isopropanol serving as a base:
Co2(CO)8/Isopropanol
PhCH2Cl + CO
(PhCH2COCOO)2Ca
Ca(OH)2
1
+ (PhCH2COO)2Ca
2
We found [2] that the product of double carbonyla-
tion of benzyl chloride (calcium phenylpyruvate) is
formed in large amounts along with PAA under mild
conditions (atmospheric CO pressure and 30 40 C)
(Fig. 1). Phenylpyruvic acid can be used for prepar-
ing an important amino acid, phenylalanine, which is
a precursor for production of a valuable sweetener,
aspartame (Table 1).
Fig. 1. Yield of (1) phenylpyruvic and (2) phenylacetic
acids vs. temperature T (P = 1 atm).
Some alklylcobalt carbonyl complexes RCo(CO)4
formed in situ in the course of carbonylation of alkyl
halides RX can catalyze carbonylation of low-active
aryl halides [5]. In this case, alkyl halides are co-
catalysts (A) (activators) of cobalt carbonyl in car-
bonylation of aryl halides. The reaction occurs under
very mild conditions (CO pressure 1 atm, 60 65 C):
Carbonylation of aromatic halides. It is known
that, unlike fatty aromatic halides, nonactivated aryl
halides do not enter into nucleophilic substitution.
That is why, apparently, these compounds are not
activated with the Co(CO)4 anion by the SN2 mech-
anism under the conditions of benzyl chloride car-
bonylation.
Co2(CO)8 + A
(I)
ArHal + CO
ArCOOR
ROH + B
We have shown previously [2, 6] that the most re-
active catalytic complex, PhCH2Co(CO)4, is formed
in the reaction with benzyl chloride as a cocatalyst
[A, scheme (I)].
First we activated more reactive aryl bromides and
aryl iodides by oxidative addition of low-valent me-
tal complexes [1]. We studied the preparation of
acenaphthenedicarboxylic acid by carbonylation of
5,6-dibromoacenaphthene [3] under the conditions of
phase-transfer catalysis by palladium halophosphine
complexes. As determined by IR spectroscopy, the ac-
tive catalytic species was the intermediate zero-valent
palladium complex PdCOP3 formed in the presence
of carbon monoxide. The mechanism of this reaction
was described in our previous paper [3]. It should be
noted that palladium complexes have also been recently
used to catalyze carbonylation of less active aryl chlo-
rides with acceptor substituents. In this case, complex
ligands (e.g., cyclohexyl-substituted ferrocenylphos-
phines) were developed to activate palladium [4].
However, these catalytic systems are difficultly avail-
able and expensive and can be used only for fine
synthesis of small amounts of expensive compounds.
The results of a study of the catalytic effect of
acceptor substituents on carbonylation of substituted
bromobenzenes and of the inhibition of aryl halide
carbonylation with one-electron acceptors (nitroben-
zene, anthraquinone) [7] suggest the SRN1 radical anion
mechanism of aryl halide activation. The active cat-
alytic species of carbonylation is the anionic complex
[RCo(CO)3COOCH3] [5, 7] formed by nucleophilic
attack of the methylate anion on the carbon atom of
the carbonyl group of alkylcobalt carbonyl. The nu-
cleophilic power of this complex is very high. Because
benzyl chloride is also carbonylated under these con-
ditions [scheme (I)], we determined the mode and rate
of feeding the cocatalyst, benzyl chloride, to maintain
the catalyst in the active form, thus providing an almost
complete conversion [7]. The catalytic cycle of the re-
action in the presence of benzyl chloride as a cocat-
alyst has been suggested and analyzed [7].
Although aryl halides are completely inactive in
carbonylation catalyzed by cobalt carbonyl under
ordinary conditions [in alcohols in the presence of
bases (B)], the making possible use of cobalt carbonyl
(the cheapest and the most available catalyst of oxo
synthesis) in carbonylation of aryl halides is a very
attractive practical and theoretical task.
A series of practically significant aromatic acids
was prepared for the first time by carbonylation in
the presence of a new effective catalytic system,
cobalt carbonyl benzyl chloride base, in methanol
(Table 1). This catalytic system is the most suitable
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 80 No. 6 2007