Stereochemistry of the Vinylic S(RN)1 Reaction of Triarylvinyl Halides. The Structure of the Intermediate alpha-Arylvinyl Radical.

ABSTRACT

Evidence for the intermediacy of a vinyl radical in the vinylic S(RN)1 reaction (S(RN)1(V)) of 2-anisyl-1,2-diphenylvinyl bromide 2 is obtained. The photostimulated S(RN)1(V) reaction of pinacolone enolate ion with (E)-2 and (Z)-2, which are used as stereoindicators, gives complete loss of the original stereochemistry of the two precursors in the substituted and hydrodehalogenated products; i.e., stereoconvergence is found. It is concluded that in the reaction of 2 a beta-substituted alpha-phenylvinyl radical is a reactive intermediate and that it has either a linear structure or an average linear structure due to a rapidly interconverting E,Z mixture of bent radicals. This conclusion is supported by comparing the stereochemical course of the S(RN)1(V) reaction with those of other reactions of the same precursor, which unambiguously give rise to the same alpha-phenylvinyl radical intermediate by alternative mechanisms. Among the reactions investigated, the hydrodehalogenation of precursor 2 by LAH appears to take place by an ET mechanism.