Organometalliques portant une fonction eliminable en β : II. Competition en α et β eliminations a partir de β alkoxy et silyloxy α,α-dichlorocarbenoides

Abstract

β-Alkoxy and -silyloxy α,α-dichlorocarbenoids, stable at low temperature (−100°C), undergo a rapid decomposition upon warming, either by α-elimination of ClLi or β-elimination of ROLi. β-Elimination is generally observed and leads to the formation of a dichloroalkene which reacts with excess of butyllithium (or lithium dialkylamide) to give the corresponding mono-substituted alkyne (or chloroalkyne) with good yields. α-Elimination is followed by the migration of a group from the alcoholic carbon to the carbenoid center; alkyl (or trimethylsilyl) α-chlorovinyl ethers are thus formed. The silyl derivatives are further cleaved by an excess of the metalating agent, to alkali α-chloroenolates of aldehydes or ketones. α-Elimination of LiCl is always observed when steric hindrance prevents any syn or anti conformation for the two groups (Ro - Li) involved in a β-elimination. Thus, when the bulky trimethylsilyloxy group leads to α-elimination, its replacement by a methoxy group affords the products of β-elimination.