Oxidation of alcohols with copper(II) salts mediated by nitroxyl radicals immobilized on ultrafine silica and ferrite surface

Abstract

The oxidations of alcohols with copper(II) salts mediated by 2,2,6,6-tetramethylpiperidinyloxy radical (TEMPO) moieties immobilized on ultrafine silica and ferrite surface were investigated. The immobilization of TEMPO structure on the silica and ferrite surface was achieved by the reaction of 4-hydroxyl TEMPO with acid anhydride groups on these surfaces, which were introduced by the treatment of surface carrying hydroxyl groups with 4-trimethoxysilyl-1,2,5,6-tetrahydrophthalic anhydride. The oxidations of primary alcohols, secondary alcohols, and diols with copper(II) salts scarcely proceeded in the absence of mediator. On the contrary, in the presence of TEMPO-supported silica and ferrite (Silica-TEMPO and Ferrite-TEMPO) as mediator, primary alcohols, secondary alcohols, and diols were oxidized with copper(II) salts to give the corresponding aldehydes, ketones, and lactones, respectively. Based on the above results, the oxidations of alcohols were considered to proceed as follows: alcohols are oxidized with oxoaminium moieties on silica and ferrite surface, which were formed by the reaction of surface TEMPO moieties with copper(II) salts, and oxoaminium moieties on the surface itself are reduced to the corresponding hydroxylamine moieties after the oxidation. Then the hydroxylamine moieties are oxidized with copper(II) salts to regenerate TEMPO moieties on the surface. For example, in the case of the oxidation of benzyl alcohol, Silica-TEMPO was recycled about 45 times. Silica-TEMPO and Ferrite-TEMPO were readily recovered from reaction mixture by centrifugation.