A novel catalysis of N-hydroxyphthalimide (NHPI) combined with Co(acac) n ( n = 2 or 3) in the oxidation of organic substrates with molecular oxygen

Abstract

A novel class of catalysts for aerobic oxidation for a wide variety of organic substrates was developed. Benzylic compounds such as fluorene were oxidized with molecular oxygen in the presence of N-hydroxyphthalimide (NHPI) to give the corresponding oxygenated products such as fluorenone. For the oxidation of cycloalkanes, polycyclic alkanes and alkylbenzenes, which are difficult to be oxidized by NHPI alone, a combined catalytic system of NHPI with Co(acac) n ( n = 2 or 3) was found to be efficient for the oxidation of these substrates. Thus cycloalkanes were successfully oxidized with dioxygen in the presence of a catalytic amount of NHPI (10 mol%) and Co(acac) 2 (0.5 mol%) in acetic acid at 100°C to give cycloalkanones and dicarboxylic acids as principal products. Alkylbenzenes were also oxidized by this catalytic system to form the corresponding carboxylic acids and/or carbonyl compounds in good yields. Exposure of NHPI to atmospheric oxygen in benzonitrile at 80°C generated phthalimide N-oxyl (PINO), a key intermediate, in this oxidation. The formation of PINO from the NHPI and dioxygen was significantly accelerated by adding Co(acac) 2 to the reaction system.