Mechanistic study of decomposition of cyclohexyl hydroperoxide catalysed by manganese(III) tetraarylporphyrins

Abstract

The reaction between manganese(III) tetraarylporphyrins and cyclohexyl hydroperoxide has been investigated. Since pyridine increases the decomposition rate of cyclohexyl hydroperoxide, all experiments were performed in the presence of pyridine. Experiments with 2,6‐di‐tert‐butylpyridine showed that pyridine increases the reaction rate by ligation to the manganese porphyrin as well as by acting as a base. Cyclohexyl hydroperoxide is decomposed into cyclohexanol and cyclohexanone. Since neither 3,3,5,5‐tetramethylcyclohexanol nor cyclohexanol‐d12 are oxidized under these reaction conditions, cyclohexanone is formed directly from the peroxide and not by oxidation of the alcohol. During the reaction, a manganese(V)oxo porphyrin complex is formed. This complex may react with (i) the peroxide under formation of cyclohexanol and molecular oxygen, (ii) the solvent cyclohexane, or (iii) manganese(III) porphyrin. The latter reaction leads to destruction of the catalyst. This destruction is prevented by introduction of bulky groups on the ortho positions of the phenyl. The scission of the hydroperoxide is suggested to be heterolytic. It is also base‐catalysed. The rate‐determining step in the decomposition of cyclohexyl hydroperoxide is scission of the oxygenoxygen bond of the manganese peroxyl porphyrin complex. A mechanism in line with the kinetic data is proposed. Copyright © 1993 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim