Formation of 3-Oxa- and 3-Thiacyclohexyne from Ring Expansion of Heterocyclic Alkylidene Carbenes: A Mechanistic Study.

The rearrangement pathways of two alkylidene carbenes appended to an oxa or thiacyclopentane into the corresponding heterocyclohexynes were elucidated using 13 C-labeling experiments. Both carbenes exhibited a preference for migration of the allylic carbon bound to the heteroatom. Anomeric interactions involving a heteroatom lone pair and antibonding orbital of the migrating bond and inductive destabilization of the minor migratory pathway are discussed as plausible reasons for the observed trends.