Zinc-Promoted Reactions. 2. Ionic and Nonionic Pathways in the Reduction of Acetophenone and 2,2-Dimethyl-1-phenylpropan-1-one

Article abstract of DOI:10.1021/jo00024a026

The Clemmensen reduction of acetophenone and related substrates (PhCHXCH3, X = Cl, OH, OAc) was investigated in anhyd AcOH and in the presence of LiCl, HCl, or TFA.Ethylbenzene, 1-acetoxy-1-phenylethane, 2,3-diphenyl-2,3-butanediol, and 2,3-diphenyl-2-butene were formed in yields strongly dependent on the experimental conditions.The formation of the hydrocarbon was favored in neat AcOH and in AcOH/HCl/LiCl.Ionic and nonionic pathways were recognized.In the presence of Cl^-, the proposed mechanism involves the intermediacy of PhCH(CH3)Cl.The process may proceed via ^*- and the carbon radical PhC^*(CH3)OZnX.The second SET may lead to a carbanion, quenched by the acid to give PhCH(CH3)OZnX, a precursor of PhCHXCH3 (X = OAc, Cl).Coupling reactions, involving different radical species, account for the formation of the dimeric compounds.In the reduction of 2,2-dimethyl-1-phenylpropan-1-one, the preponderance of addition reactions, mainly involving Cl^-, can be explained by the difficult approach of zinc metal to the hindered carbonyl group.Only traces of neopentylbenzene were obtained, the main product being 2-methyl-3-phenyl-2-butene.The proposed mechanism requires that the first SET occurs on adduct PhCCl(OH)C(CH3)3 to produce 2,2-dimethyl-1-phenylpropan-1-ol, the precursor of the alkene.