Acid-Catalyzed Oxidation of
Benzaldehydes to Phenols by Hydrogen Peroxide

M. Matsumoto, K. Kobayashi, Y. Hotta
J. Org. Chem. 49, 4740-4741 (1984)

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Summary

A wide variety of benzaldehydes were oxidized to phenols by H2O2 in acidic methanol.

Table I
Acid-Cat. Oxidation of Benzaldehydes by H2O2a
Run Benzaldehyde Time Yieldc
Phenol Ester
a
2-methoxy 24 h
93%
b
3-methoxy 68 h
68%
c
4-methoxy 24 h
90%
d
2,3-dimethoxy 63 h
30%
14%
e
2,4-dimethoxy 14 h
90%
f
3,4-dimethoxy 5 h
60%
g
2,3,4-trimethoxy 1 h
97%
h
2,4,5-trimethoxy 4 h
89%
i
2,4,6-trimethoxy 2 h
89%
j
3,4-methylenedioxy 24 h
67%
8%
k
4-methyl 24 hb
28%
51%
l
4-chloro 24 hb
87%
m
4-nitro 24 hb
80%

a. Unless otherwise stated, benzaldehyde 1 (5 mmol) and 31% H2O2 (6.4 mmol) were stirred in the presence of H2SO4 (0.1 mL) in MeOH (10 mL) under Ar at room temp.
b. The reaction mixture was heated at refluxing temperature.
c. All the products were isolated by chromatography (SiO2).

When a solution of 2-methoxybenzaldehyde (a) (5.0 g, 36.7 mmol) and 31% aqueous H2O2 (5.3 g, 48 mmol) in methanol (50 mL) was stirred with sulfuric acid (0.5 mL) at room temperature for 24 h, 2-methoxyphenol was produced in a 94% yield. Similarly, a wide variety of other benzaldehydes 1 were oxidized to the corresponding phenols and/or methyl benzoates as cited in Table I.