A New Oxidizing System for Aromatic Alcohols by the Combination of N -Oxoammonium Salt and Electrosynthesized Tetraalkylammonium Tribromide

Abstract

A combination of 2,2,6,6-tetramethyl-1-piperidinyloxyl and tetraalkylammonium tribromides (R4NBr3), which are available from the corresponding tetraalkylammonium bromides via electrooxidation with potassium bromide, has proved to be useful for oxidations of primary and secondary alcohols to the corresponding aldehydes and ketones, respectively. The oxidation reaction proceeds smoothly even with a 0.5–1.0 mol% of N-oxyl compounds and 1.5–2.0 equivalents of tetraalkylammonium tribromides in an aqueous-organic two-phase solution buffered at pH 8.0–8.6. This recyclable oxidant/co-oxidant combination system may involve the formation of N-oxoammonium salts, actual oxidizing agents of alcohols, by the action of hypobromite species generated from R4NBr3 in the binary solution. Utility of the method is highlighted by the selective oxidation of benzylic alcohols bearing electron-releasing groups on the aromatic nucleus to the corresponding aldehydes or ketones without any bromination and overoxidation.