S. Rajasekhara Reddy et al. / Tetrahedron Letters 45 (2004) 3561–3564
Table 2. Recycling 1 for the aerobic oxidation of benzyl alcohol to
3563
References and notes
benzaldehyde
Runa
PhCHO (%, yield)b
Recovery 1 (%)
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a Benzyl alcohol (1 mmol) and complex 1 (20 mg for 1st run) were
stirred under an oxygen balloon in toluene (3 mL) at ca. 100 °C for
9 h.
b Determined by GC.
OH
OH
IV O
Ln
V
V O
.
O
Ln
R
V
OH
O2
O
O
H
H
R
H
a
b
OH
V
IV
Ln
R
RCH2OH
O
V
IV
O
Ln
H
H2O2
H2O + 1/2O2
H
O
V
O
IV
Ln
R
O
V
IV
Ln
H
HO
OH
H
O
H
H
R
H
R
R
H
Scheme 3.
The formation of a mixture of an aldehyde, carboxylic
acid and ester in the homogeneous process may be due
to the acidic nature of the reaction medium (pH ¼ 4.3),
which can facilitate the formation of a hemiacetal fol-
lowed by oxidation to the ester (Scheme 2).6d Such a
functional transformation does not take place in the
case of the heterogeneous process because its pH is 6.6.
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The catalyst 1 can be filtered and recycled without loss
of activity (Table 2). Regarding the mechanism, a cat-
alytic cycle is proposed in Scheme 3. The alcohol fol-
lowed by molecular oxygen may coordinate with the
catalyst 1 to provide the superoxo complex a that can
readily undergo intramolecular hydrogen abstraction to
give the intermediate b and elimination of H2O2 to
complete the catalytic cycle.
In conclusion, an oxidation of alcohols to aldehydes and
ketones is described using a recyclable polyaniline sup-
ported vanadium catalyst 1 under molecular oxygen.
The reactions are efficient, function in the absence of
any additive and generate water as the only by-product.
Acknowledgements
This work was supported by the Department of Science
and Technology (Sanction No. SR/S1/OC-092002), New
Delhi and the Council of Scientific and Industrial Re-
search (Sanction No. 01(1804)/02/EMR-II), New Delhi.