ELECTRON-TRANSFER PHOTOCHEMISTRY OF A BRIDGED NORCARADIENE - A MECHANISTIC PROBE FOR RADICAL-CATION NUCLEOPHILIC CAPTURE

The radical cation of tricyclo[4.3.1.0(1,6)]deca-2,4-diene (3) can be generated by electron transfer to 1,4-dicyanobenzene (DCB) in acetonit rile/methanol; the species is captured by regiospecific nucleophilic a ttack of methanol in the 2- (and 5-) position. On the other hand, the reaction shows little stereochemical preference. The resulting allylic radical reacts by aromatic substitution on the radical anion of DCB, the 3-position serving as the principal site of coupling. Remarkably, the cyclopropane function of 3(.+) is not a target of nucleophilic att ack, even though the secondary cyclopropane carbon has spin and charge density. The observed reaction is compatible with a theoretical model proposed by Shaik and Press, which correlates radical cation reactivi ty with the spin density of the corresponding triplet state. The captu re of 3(.+) occurs in the position where both SOMO and LUMO have signi ficant orbital coefficients.