Ab initio study and NBO analysis of the allylic rearrangements (hetero Claisen and Cope rearrangements) and decarboxylation reactions (retro carbonyl ene reaction) of allylformate and allyldithioformate

Based on the optimized ground state geometries and using B3LYP/6-31G** method, the NBO analysis of donor-acceptor (bond-antibond) interactions also reveal that the resonance energies associated with the electronic delocalization from σ2-3 bonding orbitals toπ4-5* anti-bonding orbitals, increase from compounds 1 to 2. The increase of σ2-3→π4-5* resonance energy could decrease the corresponding TS energies of the concerted reactions from compounds 1 to 2, due to the increase of the aromatic character in the TS structures. Also, the decrease of σ2-3 bonding orbitals occupancies and increase of the π4-5* anti-bonding orbitals occupancies via σ2-3→π4-5* delocalizations, could facilitate the corresponding reactions of compound 2, compared to compound 1.