Iron-Promoted 1,5-Substitution (SN2") Reactions of Enyne Acetates and Oxiranes with Grignard Reagents

Acetate derivatives of 2-en-4-yne alcohols 1 and enyne oxiranes 4 regioselectively underwent 1,5-substitution (S(N)2 '') reactions with Grignard reagents in the presence of an iron compound to provide vinylallenes exclusively with the (E)-configuration. An alkali salt was needed to avoid the hydride-promoted reductive 1,5-substitution pathway for 1, whereas no such additive was needed for the effective conversion of 4 into the desired alkylated or arylated vinylallene structure.