Factors Affecting the Selectivity of the Oxidation of Methyl p-Toluate by Cobalt(III)

The anaerobic oxidation of methyl p-toluate by cobalt(III) in acetic acid was investigated. Observed products were 4-carbomethoxybenzaldehyde (2), 4-carbomethoxybenzoic acid (3), 4-carbomethoxybenzyl acetate (1), 4,4′-dicarbomethoxybibenzyl (6), methyl 2,4-dimethylbenzoate (8), and methyl 3,4-dimethylbenzoate (9). Deuterium isotope labeling showed that 2 was not formed from 1, but appeared to be formed directly from methyl p-toluate via 4-carbomethoxybenzyl alcohol (5). The ratio of (2 + 3) to 1 was 0.5 with [py3Co3O(OAc)5OH[PF6] and 1.0 with cobaltic acetate. Cobaltic acetate was generated in situ by the reaction of cobaltous acetate and peracetic acid. When the oxidation was carried out in the presence of chromium (0.05 equiv based on cobalt), the ratio increased dramatically and no 6 was observed. Other transition metals such as vanadium, molybdenum, and manganese had a similar effect, but were not as effective as chromium. Chromium was observed to form a mixed-metal cluster complex with cobalt. Treatment of an acetic acid solution of cobaltous acetate and methyl isonicotinate with K2CrO4 produced a solid tentatively identified as [(MIN)3Co2CrO(OAc)6][CrO4H] (MIN = methyl isonicotinate). The selectivity for the oxidation of methyl p-toluate exhibited by the mixed-metal cluster complex was similar to that observed by the addition of chromium to oxidations using [py3Co3O(OAc)5OH[PF6].