Flash photolysis of (E)-1,2-bis(1-chloro-1-phenylmethyl)cyclopropane. Generation of 1,5-diphenylpentadienyl radical and 1,5-diphenylpentadienylium cation.

Abstract

The 1,5-diphenylpentadienyl radical (5) is generated from (E)-1,2-bis(1-chloro-1-phenylmethyl)cyclopropane (1) via a two-photon process, either in cyclohexane or in acetonitrile as solvent. Two-laser two-color flash photolysis experiments show that excitation of the benzylic radical generated by homolysis of the first C-Cl bond leads, after ring-opening and proton loss, to the stabilized radical 5. This radical is also generated by photolysis of either (1E,3E)-5-chloro-1,5-diphenyl-1,3-pentadiene (6) or (1E,4E)-1,5-diphenyl-1,4-)pentadiene (7) via one-photon or two-photon processes, respectively. On the other hand, laser flash photolysis of 1 in acetonitrile also produces some 1,5-diphenylpentadienylium cation (10) generated via a one-photon process. Its formation can be explained as due to competitive photoheterolysis leading to a benzylic cation which thermally ring-opens and dehydrohalogenates. Species 10 is more efficiently generated by photolysis of 6 in acetonitrile and undergoes photoisomerization after laser excitation.