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

Article abstract of DOI:10.1246/bcsj.67.1726

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) (IIICl(tpp)>) and chloro<5,10,15,20-tetrakis(2,4,6-trimethylphenyl)porphinato>manganese(III) (IIICl(tmp)>) have been used as the catalysts. It is suggested that the III(tpp)>-mediated reaction of styrene with NaBH4 yields 1-phenylethyl radical and II(tpp)>. The 1-phenylethyl radical reacts with dioxygen to generate a peroxyl radical, (C6H5)(CH3)CHOO, which is stabilized by coordinating to II(tpp)>. III(tpp)> thus formed may yield IIIOH(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 III(tmp)>⁺ can be interpreted in terms of a steric hindrance in the formation of the radical and II(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.