Pericyclic Reactions of Radical Cations



Electron transfer catalysis is an efficient method for the catalysis of symmetry-forbidden or slow pericyclic reactions. Accurate quantum mechanical calculations are an important tool for gaining insights into the mechanistic details of these fast reactions involving radical cations. The current “state of the art” of computational studies of pericyclic reactions of radical cations is reviewed. In particular, four parent reaction types are discussed: (i) the ring-opening of the cyclobutane radical cation; (ii) the [2+2] cycloreversion of the cyclobutane radical cation; (iii) the radical cation Diels−Alder reaction of 1,3-butadiene and ethylene; and (iv) the [1,3] methylene shift in the vinylcyclopropane radical cation. The transfer of these findings to chemically more relevant substituted systems is also briefly discussed. The potential energy hypersurfaces obtained are very flat and have activation barriers that are significantly lower than the ones for the corresponding neutral reactions, which is in agreement with the large rate acceleration observed experimentally. Many of the located radical cation structures closely resemble their biradical counterparts in the neutral, stepwise pathways. The reactions generally follow a lower symmetry pathway, due to Jahn−Teller distortions induced by the unpaired electron. Finally, the results from computationally efficient B3LYP/6-31G* calculations are found to be in good agreement with those from highly correlated MO calculations.