DIMERIZATION OF 1-PHENYLCYCLOALKENE CATION RADICALS DEPENDENT ON THEIR STRUCTURE

Irradiation of 1-phenylcyclopentene (1a) with 1,4-dicyanobenzene (DCNB ) in acetonitrile gave [2 + 4] cycloadducts, tetralin-type dimers (2-4 ), together with a 2:1 adduct of 1a with DCNB (5), through an electron transfer reaction. However, similar irradiation of 1-phenylcyclohexen e (1b) gave 1-cyano-2-phenylcyclohexane (7) and 1:1 adducts of 1b with DCNB (8 and 9), instead of yielding dimeric products. Transient absor ption spectra obtained by pulse radiolysis and laser flash photolysis of 1a showed simultaneous formation of a new absorption band with lamb da(max) around 480 nm, attributable to dimer cation radicals, and decr easing absorption bands for monomer cation radicals (1a(.+), lambda(ma x): 400 and 670 nm). In the case of 1b, by contrast, only absorption b ands for monomer cation radicals 1b(.+) were observed in a region simi lar to those of 1a(.+). The optimized structure of the monomer olefin cation radicals, calculated by the PM3 method, suggested that, unlike the chairlike structure of 1b(.+), the fully planar structure of 1a(.) would, on interacting with another neutral 1a, promote the formation of dimer cation radicals.