Electron Transfer Photochemistry of a Bridged Norcaradiene:  A Mechanistic Probe for Radical Cation Nucleophilic Capture

The radical cation of tricyclo[4.3.1.01,6]deca-2,4-diene (3) can be generated by electron transfer to 1,4-dicyanobenzene (DCB) in acetonitrile/methanol; the species is captured by regiospecific nucleophilic attack 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 attack, even though the secondary cyclopropane carbon has spin and charge density. The observed reaction is compatible with a theoretical model proposed by Shaik and Pross, which correlates radical cation reactivity with the spin density of the corresponding triplet state. The capture of 3 • + occurs in the position where both SOMO and LUMO have significant orbital coefficients.