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.
References
[
1] Gedye R, Smith F, Westway K, Ali H, Baldisera L, Laberge L, Rousell J (1986) Tetrahedron Lett
7: 279
2] For reviews see: Abramovitch RA (1991) Org Prep Proc Int 23: 683; Mingos DMP, Baghurst DR
1991) Chem Soc Rev 20: 1; Raner KD, Strauss CR, Trainor RW, Thorn JS (1995) J Org Chem
0: 2456
2
[
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[3] Majetich G, Hicks RJ (1995) Microwave Power & Electromagn Energy 30: 27; Varma RS,
Dahiya R (1998) J Org Chem 63: 9038
[4] Shari® A, Mojtahedi MM, Saidi MR (1990) Tetrahedron Lett 40: 1179