Mechanisms for (Porphinato)manganese(III)-Catalyzed Oxygenation and Reduction of Styrenes in Benzene–Ethanol Containing Sodium Borohydride

Abstract 1-Phenylethanol, acetophenone, 2,3-diphenylbutane, and ethylbenzene are produced in the (porphinato)manganese(III)-catalyzed reaction of styrene with NaBH4 in aerobic benzene–ethanol. Chloro(5,10,15,20-tetraphenylporphinato)manganese(III) ([MnIIICl(tpp)]) and chloro[5,10,15,20-tetrakis(2,4,6-trimethylphenyl)porphinato]manganese(III) ([MnIIICl(tmp)]) have been used as the catalysts. It is suggested that the [MnIII(tpp)]-mediated reaction of styrene with NaBH4 yields 1-phenylethyl radical and [MnII(tpp)]. The 1-phenylethyl radical reacts with dioxygen to generate a peroxyl radical, (C6H5)(CH3)CHOO·, which is stabilized by coordinating to [MnII(tpp)]. [MnIII[OOCH(CH3)(C6H5)](tpp)] thus formed may yield [MnIIIOH(tpp)] and acetophenone which is readily reduced to 1-phenylethanol with NaBH4. The coupling and disproportionation reactions of the 1-phenylethyl radicals afford 2,3-diphenylbutane and ethylbenzene, respectively. Slower reaction of styrene observed in the catalysis by [MnIII(tmp)]+ can be interpreted in terms of a steric hindrace in the formation of the radical and [MnII(tmp)]. Similarly, all results obtained for α-methylstyrene can be explained by the formation of the 1-methyl-1-phenylethyl radical which is less reactive than the 1-phenylethyl radical and is free from the interaction with the manganese complex. The reaction mechanism involving the free radicals has been supported by the results on the effects of TEMPO.