Radical Ions in Photochemistry. 44. The Photo-NOCAS Reaction with Acetonitrile as the Nucleophile.

Studies on the photoinduced electron transfer (PET) reactions of isobutylene (2-methylpropene, 1) in the absence of methanol have identified a new photochemical nucleophile-olefin combination, aromatic substitution (photo-NOCAS) reaction. Under these conditions acetonitrile was found to act as the nucleophile and to combine with the alkene radical cation. The resulting distonic radical cation then adds to the radical anion of 1,4-dicyanobenzene (2(-*)). The final product (6) results from cyclization into the ortho postion of the phenyl group. This product formation is rationalized on the basis of the relatively high oxidation potential of the alkene (i.e., one-electron oxidation yields a reactive radical cation), the fact that addition of the nucleophile (acetonitrile) to the radical cation is relatively unhindered, and the relatively low acidity of the radical cation due to the low radical stability of the allylic radical formed upon deprotonation. High-level ab initio molecular orbital calculations were used to determine the structures and relative energies of the possible intermediate distonic and bridged radical cations. The scope and mechanism of this type of photo-NOCAS reaction are discussed.