Breaking the Rules: On the Relative Stability of Some Methylencyclopropane and Methylcyclopropene Derivatives

Abstract

The structure of the spirocyclic product obtained by reacting catechol with 1,1‐dichloro‐2‐(chloromethyl)cyclopropane is shown by NMR and X‐ray analysis to be that of a 2‐methylcyclopropene (MeCP), instead of the previously reported 2‐methylenecyclopropane (MCP) one. The study of the equilibration between the two isomeric forms by experimental and computational means (including both Density Functional Theory – DFT – and Coupled Cluster with single, double, and perturbative triple excitations – CCSD(T) – calculations) revealed that, at variance with most of the alkylidenecyclopropane/alkylcyclopropene systems described to date, for the compounds of the present study the MeCP derivative is more stable by approx. 2.5‐3.0 Kcal mol‐1 than the MCP one. The extension of the DFT and CCSD(T) study to other spiro‐MCP/MeCP pairs suggests that the origin of the unexpected shift of the equilibrium position can be tracked back to a combination of electronic and ring‐strain effects. These findings lead to re‐think a long‐standing, and substantially undisputed belief in the area of unsaturated cyclopropane derivatives.