Microwave assisted selective oxidation of benzylic alcohols with calcium hypochlorite under solvent-free conditions

Article abstract of DOI:10.1007/s007060170103

A remarkably fast microwave assisted selective oxidation of benzylic alcohols with calcium hypochlorite under solvent-free conditions is described.

Full text of DOI:10.1007/s007060170103

Monatshefte f uÈ r Chemie 132, 655±658 (2001)
Microwave Assisted Selective Oxidation of
Benzylic Alcohols with Calcium Hypochlorite
under Solvent-Free Conditions
1
1;Ã
Mohammad M. Mojtahedi , Mohammad R. Saidi
2
,
2
Mohammad Bolourtchian , and Jafar S. Shirzi
1
Department of Chemistry, Sharif University of Technology, Tehran, Iran
Chemistry and Chemical Engineering Research Centre of Iran, Tehran, Iran
2
Summary. A remarkably fast microwave assisted selective oxidation of benzylic alcohols with
calcium hypochlorite under solvent-free conditions is described.
Keywords. Oxidation; Benzylic alcohol; Calcium hypochlorite.
Introduction
Microwave assisted organic transformation coupled with solvent-free conditions
have attracted much attention due to enhanced reaction rates, easier work-up, and
facilitated puri®cation [1±7].
Oxidation of alcohols using stoichiometric or catalytic amounts of hazardous
metallic reagents is a matter of economic and environmental concern [8]. Calcium
hypochlorite is a versatile, mild, and inexpensive oxidizing agent [9±13]. In con-
tinuation of our research under environmentally benign solvent-free and micro-
wave assisted conditions [14], we now report an operationally simple and fast
method for the selective oxidation of benzylic alcohols to aromatic aldehydes or
ketones based on the use of calcium hypochlorite under solvent-free conditions
(Scheme 1).
Scheme 1
à Corresponding author

656
M. M. Mojtahedi et al.
Table 1. Oxidaton of aromatic alcohols with calcium hypochlorite under microwave irradiation
a
Without microwave irradiation; b total yield
Results and Discussion
The reaction was carried out by irradiation of well-mixed moist alumina (basic,
acidic, or neutral), calcium hypochlorite, and alcohol for one minute. The resulting
mixture was extracted with ether. After evaporation of the solvent, the crude product
was obtained.
An additional bene®t of this reaction is the selective oxidation of benzylic
alcohols in the presence of aliphatic alcohols. When benzyl alcohol and 1-cyclo-
hexanol were irradiated together under this conditions, only benzyl alcohol was
oxidized, and benzaldehyde was the sole product (Scheme 2). Also, the calcium
hypochlorite/moist alumina system oxidizes 2-hydroxy-1,2-diphenyl ethanone
(benzoin) to diphenylethane-dione (benzil) in 65% yield. Irradiation of 1-phenyl-

Selective Oxidation of Benzylic Alcohols
657
Scheme 2
1
,2-dihydroxyethane, and calcium hypochlorite/neutral alumina (Table 1, entry 8)
for 20 seconds gave benzaldehyde as the sole product. Mixing this diol with
hypochlorite/basic alumina for 5 seconds in a mortar gave 53% 1-phenyl-2-
hydroxy-1-ethanone, 20% benzaldehyde, and 27% of starting material (Table 1,
entry 9). Mixing the diol for longer periods shows little change in the above ratio.
The results and optimum irradiation times for each reaction are shown in Table 1.
In conclusion, calcium hypochlorite/moist alumina or montmorillonite K10 are
useful, low cost, and selective reagents for the solventless oxidation of benzyl
alcohols.
Experimental
A commercial microwave oven (900 W) was used for irradiation of the reaction mixture. All
compounds were identi®ed by their NMR and IR spectra.
General procedure
Alumina (2.5 g) and calcium hypochlorite (143 mg, 1 mmol, 1.0 equivalents) were mixed thoroughly
in a mortar. Then the aromatic alcohol (2 mmol) and H2O (36 mg, 2 mmol) were added and mixed
3
well. The reaction mixture was transferred to a 25 cm beaker and irradiated in a conventional
microwave oven (900 W) for the time given in Table 1 at which GC analysis indicated no further
3
change of product composition. The resultant mixture was extracted with diethyl ether (2 Â 10 cm ).
The combined organic extracts were dried over Na2SO4, and the solvent was removed with a rotary
evaporator. It should be mentioned that prolong heating in the microwave oven will cause
overoxidation of the alcohols.
Acknowledgments
We thank the Iranian National Research Council for partial ®nancial support.
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Received August 28, 2000. Accepted September 21, 2000
È
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