Biomimetic oxidation of cyclic and linear alkanes: high alcohol selectivity promoted by a novel manganese porphyrin catalyst

Abstract

A novel third-generation metalloporphyrin, chloro-5,10,15,20-tetrakis-(3′-bromine-4′-methoxyphenyl)-2,3,7,8,12,13,17,18-octabromoporphyrinatomanganese(III), [Mn^IIIBr₁₂T4(-OMe)PPCl], designated as MnBr₁₂Por, was synthesized and characterized. The catalytic activity of the new manganese porphyrin was investigated in the oxidation of cyclohexane, adamantane and n-hexane by iodosylbenzene (PhIO) or iodobenzene diacetate (PhI(OAc)₂) in comparison to the catalytic activity of the second-generation catalyst [Mn^IIIT4(-OMe)PPCl], designated as MnPor. All yields were based on oxidants. The MnBr₁₂Por/PhIO system led to higher yields of cyclohexane oxidation products (45%) with high selectivity for cyclohexanol (80%) as compared to the MnPor/PhIO system (23% and 77%, respectively). Addition of imidazole to MnPor/PhIO increased the total product yield from 23 to 43%; addition of imidazole to MnBr₁₂Por/PhIO did not alter the total product yield at all. For the MnPor/PhI(OAc)₂ and MnBr₁₂Por/PhI(OAc)₂ systems, addition of imidazole increased the product yields from 19 to 45% and from 35 to 66%, respectively. Addition of water increased the total product yields during cyclohexane and adamantane oxidation for all the studied systems. In all cases, MnBr₁₂Por performed better than MnPor.