6810
J . Org. Chem. 1997, 62, 6810-6813
Ca ta lytic Oxid a tion of Alk ylben zen es w ith Molecu la r Oxygen
u n d er Nor m a l P r essu r e a n d Tem p er a tu r e by
N-Hyd r oxyp h th a lim id e Com bin ed w ith Co(OAc)2
Yasushi Yoshino, Yoshiaki Hayashi, Takahiro Iwahama, Satoshi Sakaguchi, and
Yasutaka Ishii*
Department of Applied Chemistry, Faculty of Engineering & High Technology Research Center,
Kansai University, Suita, Osaka 564, J apan
Received May 6, 1997X
A practical catalytic method to convert alkylbenzenes into the corresponding carboxylic acids under
atmospheric dioxygen at ambient temperature using a combined catalytic system consisting of
N-hydroxyphthalimide (NHPI) and Co(OAc)2 was developed. For instance, the oxidation of toluene
was completed by NHPI combined with Co(OAc)2 under an oxygen atmosphere at room temperature
to give benzoic acid in 81% yield. Under these conditions, o- and p-xylenes were selectively converted
into the corresponding monocarboxylic acids without the formation of the dicarboxylic acids. ESR
measurements showed that Co(II) species assists in the formation of phthalimide-N-oxyl (PINO),
which is a key species in this oxidation, from NHPI.
Although there have been major advances in the
oxidation of alkylbenzenes can be done under normal
pressure and temperature, it will vastly contribute to
industrial chemistry from technical, economical and
environmental aspects since undesired reactions due to
the higher pressure and temperature will be minimized.
However, there is yet no way to carry out such oxidations
under ambient conditions.
Although there are many reports on the catalytic
oxidation of alkanes with dioxygen (1 atm), these reac-
tions must be carried out in the presence of reducing
reagent.4-9 Recently, it is reported that metalloporphy-
rins are efficient catalysts for the autoxidation of organic
substrates in the absence of any reducing reagents.10-12
3-Methylpentane was oxidized by dioxygen (1 atm) in the
presence of 1/7500 equiv of the iron complex of haloge-
nated porphyrin as a catalyst at 23 °C for 3 h, forming
3-hydroxy-3-methylpentane in 192% yield based on the
catalyst.12a
oxidation of organic substrates using molecular oxygen,
the development of effective and selective methods for
the catalytic functionalization of saturated hydrocarbons
still remains a major challenge in synthetic chemistry.1
Effective large-scale processes for the transformation of
alkanes with molecular oxygen have been limited because
of the exceedingly low reactivity of alkanes. Typical
examples of such processes are the autoxidation of
cyclohexane and p-xylene. The reactions involve a free
radical chain and hence are carried out under relatively
harsh conditions, i.e., higher oxygen pressure and more
severe temperature (usually >150 °C).2 As a conse-
quence, the reactions are often difficult to control and
exhibit poor product selectivity.
The selective catalytic oxidation of alkylbenzenes with
molecular oxygen is a very important reaction for the
production of bulk and fine chemicals such as benzoic acid
and terephthalic acid.2,3 Practically, the oxidation of
toluene is carried out in the presence of a catalytic
amount of cobalt(II) 2-ethylhexanoate under a pressure
of 10 atm of air at 140-190 °C.2e If the direct aerobic
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