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385
The presence of powdered potassium hydroxide was
found to be vital in these reactions Since the oxidation
of benzhydrol to benzophenone with molecular oxygen
using 5 mol% of cobalt phthalocyanine as catalyst in the
absence of powdered potassium hydroxide was not
complete even after 24 h, whilst it was complete within
45 min in the presence of an equimolar amount of
potassium hydroxide. Other bases such as potassium
carbonate and tertiary amines were found to be less
effective than powdered potassium hydroxide.
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To evaluate the effect of solvents, the aerobic oxidation
of benzhydrol was carried out under similar reaction
conditions but using different solvents: xylene, acetoni-
trile, methanol, toluene and 1,2-dichloroethane. Xylene
was found to be the most suitable. The oxidation of
secondary alcohols to the corresponding ketones was
found to be very slow at room temperature and could be
carried out more efficiently in refluxing xylene. The
catalytic effect of the cobalt phthalocyanine was also
investigated using benzhydrol as the substrate. In the
absence of catalyst, the reaction was found to be very slow
and was not complete even after 8 h whilst with 0.5 mol%
cobalt phthalocyanine as catalyst, the reaction was
complete within 1.5 h. An increase in catalyst from 0.5
to 10 mol% has only a marginal effect in decreasing the
reaction time, indicating the effectiveness of catalyst even
at low concentration. Non-activated alcohols such as
borneol and cyclohexanol could not be oxidized in the
absence of catalyst under the same reaction conditions.
The use of cobalt phthalocyaninetetrasulphonamide16
and cobalt phthalocyaninetetracarboxylic acid17 as cata-
lyst in place of cobalt phthalocyanine gave comparable
results. The mechanism of this reaction is not clear at this
stage and further studies in this direction are being carried
out.
10. Kaneda, K.; Yamashita, T.; Matsushita, T.; Ebitani, K. J.
Org. Chem. 1998, 63, 1750.
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D.; Noel, J. P.; Meunier, B. J. Am. Chem. Soc. 1996, 118,
7418; (d) Hadasch, A.; Sorokin, A.; Rabion, A.; Fraisse,
L.; Meunier, B. Bull. Soc. Chim. Fr. 1997, 134, 1025; (e)
Sorokin, A.; Meunier, B. J. Chem. Soc., Chem. Commun.
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1995, 268, 1163; (g) Kasuga, K.; Mori, K.; Sugimori, T.;
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14. (a) Jain, S. L.; Sain, B. Chem. Commun. 2002, 1040; (b)
Jain, S. L.; Sain, B. J. Mol. Catal. 2001, 176, 101; (c) Rao,
T. V.; Sain, B.; Kumar, K.; Murthy, P. S. N.; Prasada Rao,
T. S. R.; Joshi, G. C. Synth. Commun. 1998, 28, 319; (d)
Sain, B.; Murthy, P. S. N.; Rao, T. V.; Prasada Rao, T.
S. R.; Joshi, G. C. Tetrahedron Lett. 1994, 35, 5083; (e)
Rao, T. V.; Sain, B.; Murthy, P. S.; Prasada Rao, T. S. R.;
Jain, A. K.; Joshi, G. C. J. Chem. Res. (S) 1997, 300; (f)
Rao, T. V.; Sain, B.; Murthy, P. S. N.; Joshi, G. C.; Prasada
Rao, T. S. R. Stud. Surf. Sci. Catal. 1998, 113, 921.
15. Typical experimental procedure: To a stirred solution of
benzhydrol (184 mg, 1 mmol) in xylene (5 ml) was added
powdered potassium hydroxide (56 mg, 1 mmol), cobalt
phthalocyanine (28 mg, 5 mol%) and the reaction mixture
was refluxed for 45 min under an oxygen atmosphere. The
reaction progress was monitored by TLC (SiO2). At the end
of reaction cobalt phthalocyanine and powdered potas-
sium hydroxide were removed by filtration. The filtrate
obtained was evaporated under vacuum and the residue
thus obtained was purified by column chromatography on
silica gel using ethyl acetate/hexane (1:4) as eluent.
Evaporation of the solvent yielded benzophenone
In summary, we have developed a simple and eco-friendly
catalytic aerobic oxidation procedure for the oxidation
of both activated and non-activated alcohols to the
corresponding ketones. The simplicity of the system, the
non-hazardous nature of the catalyst and the versatility
of the method towards a range of activated and non-acti-
vated alcohols make cobalt phthalocyanine catalyzed
oxidation an attractive, environmentally friendly syn-
thetic tool for the oxidation of secondary alcohols to
ketones by molecular oxygen.
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