Theoretical modelling of the epoxidation of vinylallenes to give cyclopentenones

Epoxidation of vinylallenes (1,2,4-pentatrienes) can lead to cyclopent-2-enones. Various experimental results suggest that these reactions are concerted and that vinylallene oxides undergo a concerted and thermal ring-closing reaction to give cyclopentenones. In 1977, this view has been supported by the epoxidation of a non-racemic 4-methyl-2,3,5-hexatriene to give a non-racemic 2,5-dimethyl-2-cyclopenten-1-one. Indeed, for the cyclization of (3E)-4-methyl-2,3-epoxy-2,3,5-hexatriene or (4E)-4-methyl-3,4-epoxy-2,3,5-hexatriene into 2,5-dimethyl-2-cyclopenten-1-one, a transition structure for the concerted rearrangement was located, and IRC calculations showed it linked together. The activation barrier predicted at the B3LYP/6-311++G(3df,3pd)//B3LYP/6-31G(d) level of theory is 80.9 or 124.2 kJ mol⁻¹, respectively and the reaction is exothermic by 127.2 or 89.2 kJ mol⁻¹, respectively.