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 B3LY P hybrid functional and QCISD(T)//QCISD calculations using the 6-31G* basis set. The intermediates and transition states involved in three different m echanisms, a concerted C-s-symmetric and a stepwise unsymmetric anti [4 + 2 ] pathway and a stepwise unsymmetric out-gauche pathway leading to vinylcyc lobutane, have been considered. The synchronous C-s-symmetric pathway is pr evented by a pseudo-Jahn-Teller distortion and is 19 kcal/mol higher in ene rgy than the stepwise pathways. The stepwise anti pathway was found to be t he lowest-energy pathway with an activation energy of 0.3 kcal/mol relative to the initially formed ion-molecule complex. The gauche-out pathway, lead ing to vinylcyclobutane, is 3.5 kcal/mol higher in energy than the anti pat hway, 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 stepw ise pathways closely resemble the previously described species involved in the stepwise mechanism of the neutral Diels-Alder reaction.