The kinetic anomalies observed in the formation of small ring systems explained by sigma-assistance and sigma-resistance. Part 4. Carbanion cyclizations

Abstract

Cyclization of carbanions to give cyclopropane, cyclobutane, and cyclopentane derivatives shows a behaviour that deviates from that predicted by the Ruzicka hypothesis. This adds to the anomalies, generally observed in the formation of 3-, 4-, 5-, and 6-membered saturated ring systems, i.e., the enthalpy of activation for the formation of the odd-membered rings is consistently found to be lower than that of the less strained next higher even-membered ring. MNDO calculations were performed on the reaction profiles of the carbanion cyclizations for n= 3,4, and 5. The results of these calculations support the earlier conclusion that the anomalies in the ΔH^‡ values can be adequately explained by the influence of sigma-assistance and sigma-resistance (i.e., by rules for σ-systems analogous to those described extensively for π-electron systems), mediated by through-bond interaction between the reacting centres.