α-Halo carbanion intermediates : I. Synthesis of α-chlorobibenzyls and stilbenes via the α-metalation of benzyl chlorides with n-butyllithium

Abstract

The reactions of variously substituted benzyl chlorides with n-butyllithium in tetrahydrofuran at low temperatures (−100°) leads to the formation of the correspondingly substituted α-chlorobibenzyl (1,2-diarylchloroethane) in good-to-excellent yields. The corresponding symmetrically substituted stilbenes are readily formed in good yields by thermal dehydrochlorination of the α-chlorobibenzyl. The data indicate that in tetrahydrofuran at low temperatures the reactions of benzyl chlorides with n-butyllithium proceed almost exclusively via α-hydrogen-metal interconversion (α-metalation), leading to the formation of the α-chlorobenzyllithium as the active intermediate, which is rapidly consumed by subsequent displacement on excess benzyl chloride to produce the observed product, viz. ▪ R=H, o-Cl, p-Cl, 2,4-Cl 2, o-CH 3, p-CH 3