Direct photolysis of cycloalkenes

Abstract

The direct photolyses at 184.9 nm of cyclohexene (1a), cycloheptene (1b), and cyclo-octene (1c) were investigated in the vapour and liquid phases. The vapour-phase photolysis of these cyclo-alkenes gave the decomposition products (ethylene and butadiene from cyclohexene) or α,ω-dienes (2) and vinylcycloalkanes (3), the yields of which decrease with increasing pressure, along with minor amounts of the rearrangement products, methylenecycloalkanes (4) and bicyclo[n.1.0]alkanes (5: n= 3–5). Liquid-phase photolysis on the other hand afforded only the latter two rearrangement products and the cis–trans-isomer, when sterically allowed. On the basis of the pressure dependence of the product yields and other evidence, it is shown that the precursor of the decomposition products. α,ω-dienes, and vinylcycloalkanes is a vibrationally excited ground state of the cycloalkenes, which may be produced in a rapid internal conversion process from the excited singlet state generated initially. The rearrangement products were proved to be formed via a carbene intermediate (8) by the use of the deuteriated cycloheptane. Upon direct irradiation -in the liquid phase, the trans-isomers of cyclohexene and cycloheptene were also generated, and trapped only in the presence of added acidic methanol to form the corresponding methoxycycloalkenes (6). The lifetime of trans- cycloheptene generated photochemically was determined to be 23 min at –10 ± 2 °C in a low-temperature experiment.