Manganese porphyrins as efficient catalysts in solvent-free cyclohexane oxidation

Abstract

Novel non-symmetric third-generation manganese metalloporphyrins, chlorido(5-(4-nitrophenyl)-10,15,20-triphenyl-2,3,7,8,12,13,17,18 octobromoporphyrinate)manganese(III) (MnBr8P1) and the mixture of isomers chlorido(5,10-(4-nitrophenyl)-15,20-diphenyl-2,3,7,8,12,13,17,18-octabromoporphyrinate)manganese(III) and chlorido(5,15-(4-nitrophenyl)-10,20-diphenyl-2,3,7,8,12,13,17,18-octabromoporphyrinate)manganese(III) (cis/trans-MnBr8P2) were obtained from their second-generation analogues (MnP1 and cis/trans-MnP2). These four catalysts were used in cyclohexane oxidation reactions with iodosylbenzene (PhIO) or iodobenzene diacetate (PhI(OAc)2) in the presence or absence of solvent (CH2Cl2). Cyclohexanol (Cy-ol) and cyclohexanone (Cy-one) were the sole reaction products. Regardless of the system, third-generation catalysts MnBr8P1 and cis/trans-MnBr8P2 performed better than second-generation catalysts (MnP1 and cis/trans-MnP2, respectively). In solvent systems, cis/trans-MnBr8P2 afforded a greater total yield (%Cy-ol + %Cy-one) than systems using MnBr8P1, with an increase from 80 to 89% in PhIO and from 47 to 57% in PhI(OAc)2. The systems without solvent, showed to be viable and presented significant yields with PhIO as oxidant, with total yield of 30% for MnBr8P1, and 55% for cis/trans-MnBr8P2. Additionally, in systems with third-generation catalysts without solvent, selectivity was close to 100% for Cy-ol. Keywords: Manganese porphyrins, Cyclohexane oxidation, Solvent-free reactions, Iodosylbenzene, Iodobenzene diacetate