ELECTROCHEMICAL OXIDATION AND REDUCTION OF ALPHA-DIMETHYLSILYL ESTERS. A NOVEL SILICON GAMMA-ARYL EFFECT

摘要

A series of alpha-dimethylsilyl esters was prepared by treatment of the corresponding esters successively with lithium diisopropylamide and chlorodimethylsilane. Anodic oxidation of methyl alpha-dimethylsilyl dihydrocinnamate (5d) was found to occur at substantially less positive potential than silyl esters not bearing a phenyl group beta to the ester group. Preparative scale oxidation of this ester in trifluoroacetic acid afforded methyl beta-trifluoroacetoxydihydro-cinnamate, which is presumably produced by a process involving rearrangement of an intermediate alpha-carbocation. A series of semi-empirical and density functional ab initio quantum mechanical calculations have led to the conclusion that the ease of oxidation of 5d is associated with participation of the benzene group in the initial electron transfer process via overlap of the rear lobe of the carbon-silicon bond with the benzene pi-system. Cathodic reduction of the silyl esters in the presence of oxygen results in desilylation by electrochemically generated superoxide ion.