Mechanisms for (Porphinato)manganese(III)-Catalyzed Oxygenation and Reduction of Styrenes in Benzene–Ethanol Containing Sodium Borohydride
作者:Masayuki Takeuchi、Koji Kano
DOI:10.1246/bcsj.67.1726
日期:1994.6
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.
在有氧苯-乙醇体系中,通过(卟啉)锰(III)催化反应,苯乙烯与NaBH4反应生成1-苯乙醇、乙酰苯、2,3-二苯基丁烷和乙苯。所用的催化剂为氯(5,10,15,20-四苯基卟啉)锰(III)([MnIIICl(tpp)])和氯[5,10,15,20-四(2,4,6-三甲基苯基)卟啉]锰(III)([MnIIICl(tmp)])。据推测,[MnIII(tpp)]催化的苯乙烯与NaBH4反应生成1-苯乙基自由基和[MnII(tpp)]。1-苯乙基自由基与二氧反应生成过氧自由基(C6H5)(CH3)CHOO·,该自由基通过与[MnII(tpp)]配位得到稳定。形成的[MnIII[OOCH(CH3)(C6H5)](tpp)]可能生成[MnIIIOH(tpp)]和乙酰苯,后者容易被NaBH4还原为1-苯乙醇。1-苯乙基自由基的耦合和歧化反应分别生成2,3-二苯基丁烷和乙苯。在[MnIII(tmp)]+催化下,苯乙烯反应速率较慢,这可以用自由基和[MnII(tmp)]形成时的空间位阻来解释。同样,所有关于α-甲基苯乙烯的结果都可以通过形成1-甲基-1-苯乙基自由基来解释,该自由基的反应活性低于1-苯乙基自由基,并且不受与锰配合物相互作用的影响。自由基反应机理得到了TEMPO效应结果的支持。