Mechanistic role of benzylic bromides in the catalytic autoxidation of methylarenes.

Different pathways for benzylic bromide transformations were examined under conditions of cobalt-bromide catalysis in acetic acid. It has been shown that benzylic bromides participate in the catalytic cycle through their catalyzed and noncatalyzed oxidation, through their reaction with Co(III), and through cobalt(II)-catalyzed solvolysis. The rates of the direct reduction of Co(III) by several benzylic bromides were measured under an argon atmosphere; the reaction occurs by a mechanism involving two forms of Co(III). The same reaction under an oxygen atmosphere initiates the cobalt-bromide-catalyzed oxidation of benzyl bromide, thus leading to the regeneration of inorganic bromide and the fast reduction of Co(III). Solvolysis of benzylic bromides plays only a minor role in the regeneration of inorganic bromide in glacial acetic acid.