Application of activation hardness in perturbed pericyclic reactions: a case study involving electrocyclic ring opening reactions of heterocyclobutenes

Abstract

Cyclobutene undergoes electrocyclic ring opening through conrotatory mode to give 1,3-butadiene and similarly its skeletally substituted analogues viz. phosphetene, thietene, oxetene and azetene isomerize to give their respective heterodienes. Quantum mechanical investigations on these perturbed pericyclic isomerizations at B3LYP level with 6-31G(d) and 6-311 + G(d,p) basis sets reveal that hetero - atoms significantly lower the barrier and alter the reaction free energies. Interestingly frontier orbital correlation diagram shows that σ and σ levels are perturbed significantly while π and π* levels undergo little changes during the reaction; the lone pairs on hetero - atoms are slightly stabilized in the TS. This results in s becoming HOMO in the TSs of phosphetene and thietene isomerizations and σ* becoming LUMO in all the cases. This necessitates defining hardness values based on closely interacting frontier orbitals for reactants and TSs and use them to compute activation hardness values to interpret relative reactivity.