Oxidation of Benzyl Alcohol with MPPC/Alumina
1215
secondary benzylic alcohols (Table 1) and therefore can be used for selective
oxidation of benzylic alcohols in the presence of other hydroxy groups.
In conclusion, a safe oxidant on solid support, which is a good alternative to
pyridinium chlorochromate, is introduced. It has all the advantages of pyridinium
chlorochromate and, in addition, is selective for benzylic alcohols.
Experimental
All oxidation products are known and were identi®ed by comparison of their physical and
spectroscopic data with those of authentic samples. Yields refer to isolation of the 2,4-
dinitrophenylhydrazones (DNPH). N-Methylpiperidine was obtained commercially.
Preparation of N-methyl piperidinium chlorochromate adsorbed on alumina
To a solution of 20 g CrO3 (0.2 mol) in 36.8 g 6 N HCl (0.22 mol) at 40ꢀC, 19.8 g N-methylpiperidine
(0.2 mol) were added within 15 min. The resulting solution was stirred at 10ꢀC for 2 h by which time
a yellow-orange solid precipitated. Isolation of this material and drying gave MPPC (found: C 30.10,
H 5.70, N 5.60, Cr 21.9%; calcd.: C 30.64, H 6.00, N 5.96, Cr 22.1%). Reheating of the above MPPC
suspension to 40ꢀC resulted in a homogeneous solution. Alumina (100 g, 100±200 mesh) was then
added while stirring at 40ꢀC. After evaporation under reduced pressure, the yellow-orange solid was
dried in vacuum. MPPC and MPPC/alumina can be preserved for at least 3 months in air and at room
temperature without losing their activity.
Oxidation of benzylalcohol
1 g of the above reagent (2 mmol) was added to a solution of 0.147 g benzylalcohol (1 mmol) in
10 cm3 of dioxane. The reaction mixture was stirred for 100 min at room temperature. After
completion of the reaction (monitored by TLC) the solid was ®ltered off and washed with 10 cm3 of
CHCl3. The combined ®ltrates were evaporated and vacuum distilled to give 83% benzaldehyde.
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