Theoretical reaction paths for cation radical cycloadditions : The [3+2] or role-reversed cation radical diels-alder

An extended basis set ab initio reaction path calculation for the prototype role-reversed cation radical Diels-Alder reveals that even this symmetry-forbidden cyclization mode proceeds without activation, at least in the gas phase. An intermediate, termed a long-bond complex, is encountered on the path, but the energy minimum is so shallow that the reaction is considered likely to be concerted when entropy effects are considered. The calculation further reveals that the factor which importantly distinguishes the role-reversed cycloaddition mode from the allowed cycloaddition mode, namely, the presence of charge predominantly on the diene moiety as contrasted to the dienophilic moiety, is maintained well along the reaction path and even in the long-bond intermediate. Competition between the role-reversed Diels-Alder and a competing [2+1] cyclobutane cycloaddition is envisioned.