The Mechanistic Puzzle of Transition‐Metal‐Catalyzed Skeletal Rearrangements of Enynes

Abstract

Abstract Three pathways actually compete in metal‐catalyzed cyclizations of enynes in which the metal selectively activates the alkyne: an endocyclic process and two exo ‐cyclizations, one proceeding by anti attack of the alkene and a second one resulting in a syn addition. Although cyclobutenes may be formed in transition‐metal‐catalyzed cyclization of some enynes, particularly, 1,7‐enynes, these compounds are not necessarily the intermediates in the skeletal rearrangement. Cyclobutenes are formed by ring expansion of syn ‐cyclopropyl metal–carbenes formed in the syn pathway.