Photochemical reactions of 1,2-dicyanonaphthalene with alkenes and dienes

The photoreaction of 1,2-dicyanonaphthalene (7) with 2-methyl-1-propene (2) in acetonitrile gave a [3 + 2]adduct at the 1,8-position of the naphthalen e ring of 7 with loss of hydrogen cyanide, a [2 + 2 + 2]adduct consisting o f 7, 2 and acetonitrile, and a cyclobutene-fused derivative, while that in benzene afforded a [3 + 2]adduct without loss of hydrogen cyanide and the c yclobutene-fused derivative. Yields of the products largely depended on the composition of the acetonitrile-benzene mixed solvent. The results suggest ed a polar nature of the [2 + 2 + 2]addition and a less polar nature of the formation of the cyclobutene-fused derivative. A maximal total yield (32%) of the [3 + 2]adducts was obtained in solvents containing 20-30 volume per cent of acetonitrile. [3 + 2]Photocycloadditions at the 1,8-position of the naphthalene ring of 7 were found to be general in the reactions with cyclo pentene, styrene, 1,3-butadiene, and isoprene. No photoreactions with l-hex ene were observed, possibly due to the poor electron-donating ability of l- hexene. The generality of the [3 + 2]photocycloaddition was discussed in te rms of the free energy change associated with single-electron transfer from alkenes or dienes to the singlet excited state of 7.