Ab initio studies of the radical cation Diels-Alder reaction

The radical cation Diels-Alder reaction of the Id-butadiene radical cation with ethylene, yielding the cyclohexene radical cation, was studied by B3LYP hybrid functional and QCISD(T)//QCISD calculations using the 6-31G* basis set. The intermediates and transition states involved in three different mechanisms, a concerted C-s-symmetric and a stepwise unsymmetric anti [4 + 2] pathway and a stepwise unsymmetric out-gauche pathway leading to vinylcyclobutane, have been considered. The synchronous C-s-symmetric pathway is prevented by a pseudo-Jahn-Teller distortion and is 19 kcal/mol higher in energy than the stepwise pathways. The stepwise anti pathway was found to be the lowest-energy pathway with an activation energy of 0.3 kcal/mol relative to the initially formed ion-molecule complex. The gauche-out pathway, leading to vinylcyclobutane, is 3.5 kcal/mol higher in energy than the anti pathway, leading to cyclohexene. In contrast to earlier calculations by Bauld at the MP2/6-31G*//3-21G level of theory, an ion-molecule complex was found to be part of the reaction pathway and no in-gauche intermediate could be located. The transition structures and intermediates involved in both stepwise pathways closely resemble the previously described species involved in the stepwise mechanism of the neutral Diels-Alder reaction.