Selective oxidation of substituted xylenes to toluic acids by hypochlorite–Ru
system under phase transfer conditions
Yoel Sasson,* Abed El-Aziz Al Quntar and Ami Zoran
Casali Institute of Applied Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
Instantaneous aqueous extraction of toluic acid salts is the
basis for a novel selective process for the oxidation of a single
methyl group of various xylenes; aqueous hypochlorite is
inert towards methylbenzenes at pH higher than 9.0,
however, in the presence of an organic solvent, a Ru catalyst
and a phase transfer agent, rapid oxidation to benzoic acids
is observed at 25 °C.
were dissolved in 1,2-dichloroethane (25 ml) in a 300 ml flask
equipped with a mechanical stirrer, a thermometer, a pH meter
and a dual dropping funnel, one filled with commercial aqueous
hypochlorite and the other with 20% aqueous NaOH. Aqueous
hypochlorite (200 ml) was added into the flask with stirring at
a rate of 1.5 ml min21 while keeping the system at 25 °C. The
pH was maintained throughout the process at 9.0 by manual
gradual addition of 20% aqueous NaOH (28 ml in total was
added). After two hours the aqueous phase was separated,
acidified with 20% aqueous sulfuric acid to pH 3 and the
precipitate was filtered and dried to give 4-chloro-2-methyl-
benzoic acid (5.50 g, 98%), mp 170 °C, were obtained. The
structure of the product was confirmed by comparison with an
authentic sample.
The combination of ruthenium tetroxide as catalyst with sodium
hypochlorite as primary oxidant was shown to be effective in
the oxidation of olefins, alkynes and aromatic rings. The system
could be improved by the addition of a phase transfer catalyst
(PTC).1 We have shown that methylbenzenes can readily be
transformed into benzoic acids at room temperature upon
exposure to aqueous sodium hypochlorite at pH 9–10 in the
presence of RuCl3 and tetra-n-butylammonium bromide
(TBAB) catalysts in a two phase system [eqn. (1)].2
Interestingly, when 4-nitro-o-xylene was reacted under an
identical procedure, the meta methyl group was oxidised to
yield 2-methyl-5-nitrobenzoic acid (mp 176 °C) in 90% yield.
This is shown in eqns. (2) and (3).
CH3
CO2Na
RuCl3•3H2O (1%),
2 h, TBAB (5%),
25 °C, pH 9
CH3
CO2Na
CH3
RuCl3•3H2O (1%),
2 h, TBHS (5%),
25 °C, pH 9
+ 3NaOCl + NaOH
+ 3NaCl + 2H2O (1)
CH3
+ 3NaOCl + NaOH
+ 3NaCl + 2H2O (2)
X
X
Yield 92–98%
X = H, Cl, Br, NO2, CN
Cl
Cl
In a recent study we subsequently observed that this system
is also capable of converting alkanes and cycloalkanes into
ketones accompanied by some chlorinated products,3 similar to
other ruthenium catalyzed oxidations.4
98%
CH3
CH3
RuCl3•3H2O (1%),
2 h, TBHS (5%),
25 °C, pH 9
CH3
CO2Na
A major feature of the Ru–NaOCl–PTC system in the
oxidation of methylbenzenes is the instantaneous transport of
the benzoic acid product as it forms into the basic aqueous phase
thus allowing the facile separation of the product from the
catalysts which abide in the organic phase ready for reuse in a
succeeding reaction batch. This unique characteristic can also
be the foundation for the design of a continuous catalytic liquid–
liquid oxidation process.
The instantaneous extraction of benzoic acids (as sodium
salts) into the aqueous phase can also be utilized for a selective
oxidation of one methyl group in polymethylbenzenes such as
xylenes or trimethylbenzenes. This is the subject of this
communication. We have found that when xylenes are reacted
under the above conditions complete conversion is achieved
after two hours with only one methyl group selectively oxidized
to the corresponding carboxylic acid.
Toluic acids are obtained in the autoxidation of xylenes in the
presence of a cobalt(ii) catalyst in acetic acid solvent but high
selectivity to the desired products is obtained only at relatively
low conversions.5,6 At higher conversions the major product is
the dicarboxylic acid. Thus alternative methods were proposed
for the preparation of substituted toluic acids. Typical examples
are the hydrolysis of toluonitriles7,8 (obtained by diazotization
of anilines in presence of CuCN) and the oxidation of
methylacetophenones via the haloform reaction.9,10
Our new oxidation system is demonstrated with the following
typical procedure. 4-Chloro-o-xylene (4.6 g, 33 mmol), tetra-
n-butylammonium hydrogen sulfate11 (TBHS, 0.47 g, 1.6
mmol) and ruthenium chloride trihydrate (70 mg, 0.33 mmol)
+ 3NaOCl + NaOH
+ 3NaCl + 2H2O (3)
NO2
NO2
90%
The reaction was found to be of a general nature with
numerous substituted xylenes. These experiments have revealed
the following observations. (a) When unsubstituted xylenes or
xylenes bearing electron donating groups (such as methoxy-
xylenes) are reacted under the above conditions, the major
reaction taking place is ring chlorination accompanied with
some side chain chlorination. The ring chlorinated xylenes were
further oxidized to the corresponding toluic acids resulting in
a mixture of toluic acids and side-chain chlorinated xylenes.
(b) In oxidizing xylenes bearing an electron withdrawing
substituent which also has a lone pair of electrons available to
stabilize the carbonium ion, such as bromine or chlorine, the
methyl group para or ortho to the substituent is oxidized. (c) In
reactions of xylenes with electron withdrawing substituents not
containing an unshared pair of electrons such as nitro, sulfonate
or carboxylate, the methyl group meta to the substituent is
selectively oxidized. (d) The oxidation rate is slower when
substituents with stronger 2I effect are present. (e) No
oxidation reaction is taking place in the aqueous phase thus once
the toluic acid product is extracted into the water no further
oxidation is observed. (f) Non-selective oxidation was realized
when 4-nitro-m-xylene was reacted with formation of both
possible toluic acids.
Chem. Commun., 1998
73
Sasson, Yoel
