1,3-HYDROGEN SHIFT IN PROPENE RADICAL CATION - A FACILE ANTARAFACIAL REARRANGEMENT

The 1,3-hydrogen shifts in both the pi-ionized and neutral forms of propene are studied by ab initio MO calculations. Stationary points are located at the (U)MP2 level and relative energies calculated at the (U)MP4 level with the 6-31 G(d, p) basis set. As in neutral propene, the migratory process in the propene radical cation takes place antarafacially, via a C(s)-transition structure. Upon pi-ionization, the barrier height is reduced from 358 to 139 kJ/mol. Such a remarkable effect arises from a weakened carbon-hydrogen bond and a more favourable orbital interaction in the ionized transition structure. The distribution of localized orbital centroids in the transition structures suggests that the 1,3-sigmatropic rearrangement involves a hydride transfer in pi-ionized propene and a proton transfer in neutral propene.