Quadricyclane radical cation isomerizations

Isopropylidene quadricyclane 1 upon oxidation with catalytic amounts of the electron transfer catalyst tris(p-tolyl)aminium hexafluoroantimonate (TTA(.+) SbF6-) gives bicycloheptadiene 2 spontaneously and in quantitative yields. Epoxidation of the isopropylidene group drastically changes the reactivity of the quadricyclane framework. Under the same reaction conditions (catalytic amounts of TTA(.+) SbF6-) norbornadiene 4 is formed spontaneously and quantitatively according to an NMR study. Theoretical calculations on the model compounds quadricyclane 5a and isopropylidene quadricyclane 5b at the B3LYP/3-21G and MP2/3-21G level of density functional theory (DFT) and ab initio theory reveal the mechanism of both reactions. In the parent quadricyclane system the concerted (but not synchronous) three-electron cycloreversion is favored, and by hyperconjugation with the pi* orbital the isopropylidene derivative prefers a simultaneous cleavage of a "lateral" bond and a cyclobutane bond to give the intermediate 9b. Starting from 9b there are two pathways to the product bicycloheptadiene 12b. The pathway involving the trimethylenemethane intermediate 11b turns out to be a dead end because the system has to overcome a very high activation barrier to give the bicycloheptadiene. Much more favorable and consistent with the reaction conditions is a 1,2-shift, which has a barrier of only 1.7 kcal mol(-1), leading directly to the bicycloheptadiene radical cation 10b and subsequently upon reduction to the neutral product 12b. A number of known quadricyclane rearrangements can be explained by these mechanisms.