Sensitised photolysis of cis-cyclo-octene vapour

Abstract

The mercury-sensitised photolysis of cis-cyclo-octene (1c) gave octa-1,7-diene and three bicyclo-octanes as the major products. The reaction is interpreted in terms of three intramolecular hydrogen transfers, i.e.[1,2], [1,4], and [1,5] shifts. The most important primary process is the [1,5] hydrogen shift through a six-membered transition state resulting in a cyclic 1,4-diradical intermediate. The 1,4-diradical thus formed undergoes competitive cyclisation and β-cleavage, thus providing an analogy with the Norrish type II reaction of carbonyl compounds. The kinetics of the reaction indicate that the precursor to hydrogen abstraction is a vibrationally excited π,π* triplet of (1c). With benzene as sensitiser only the products formed by a [1,5] hydrogen shift are observed (together with formation of trans-cyclo-octene) and this is consistent with a ready type II-like reaction.