A non-least motion pathway for 1,2-shift in cyclic vinyl cations

Abstract

Qualitative molecular orbital arguments are presented to support a novel non-least motion pathway for 1,2-migrations in cyclic vinyl cations. Ab initio calculations using a split valence 3-21G basis set on the bridged vinyl cation with varying HCC angle θ (5) show that the bridging hydrogen moves out of the C₂H₂ plane at θ less than 170°. This is due to the decreased interaction of the bridging group or atom with the distorted π bond. Semi-empirical calculations using the MINDO/3 method were carried out on the classical and two types of bridged structures for the cyclopentenyl cation. The transition state corresponding to the non-least motion pathway is found to be of lower energy than that for the least motion pathway.