Why is the concerted (2+2) mechanism of the reactions of SO3 with alkenes favored over the (3+2) mechanism? Density functional and correlated ab initio calculations and a frontier MO analysis

A study was conducted comparing the (2+2) mechanism with the (3+2) mechanism influencing the cycloaddition reactions or ring formation between sulphur trioxide (SO3) and alkenes or olefins using density functional analysis and the Hartree-Fock, MP2 and QCISD(T) analysis methods. Findings suggested that the cycloaddition reactions between SO3 and olefins followed a concerted but asynchronous reaction (2+2) with substantial zwitterionic character, while regioselectivity followed the Markovnikov Rule. Moreover, the frontier molecular orbital theory explains the SO3 and OsO4 reaction.