MINDO/3 study of the isomerization of 1-methylcyclohexylium ion to 1-ethylcyclopentylium ion. An alternative mechanism for this branching rearrangement

Abstract

A MINDO/3 study of the rearrangement mechanism of 1-methylcyclohexylium ion to 1-ethylcyclopentylium ion is presented. The structures at three intermediates and three transition states have been calculated and characterized. The results show that a ring-contraction mechanism via a corner-protonated cyclopropane intermediate is a more adequate description of this process than one based on an H-bridged edge-protonated cyclopropane intermediate. The value calculated for the barrier height for the ring-expansion process (14.5 kcal mol^–1) is in good agreement with the experimental value (15.4 kcal mol^–1) for the activation energy. The proposed mechanism resembles the Wagner–Meerwein ring-contraction reaction.