Photochemistry of substituted cyclic enones. Part 12. Photocycloaddition of 3-phenylcyclopentenone and 3-phenylcyclohexenone to (E)- and (Z)-1-phenylpropene

The quantum yields of adduct formation and isomerisation of (E)- and (Z)-1- phenylpropene by 3-phenylcyclopentenone 1 and 3-phenylcyclohexenone 2 have been measured by H-1-NMR spectroscopy in solutions of d(6)-benzene. The add ucts formed for each photoreaction have been shown to be independent of the starting 1-phenylpropene isomer. For 3-phenylcyclopentenone 1, a photostat ionary state of 32 : 68 (E)- to (Z)-1-phenylpropene is achieved during the course of the reaction. This isomerisation reaction may occur through rever sion of triplet 1,4-biradical intermediates to ground state molecules or by an energy transfer process. However for 3-phenylcyclohexenone 2 isomerisat ion occurs only through reversion of triplet 1,4-biradical intermediates. T he mechanism of photoadduct formation and that of the isomerisation reactio n are discussed for both phenylenones. Rate constants for quenching of the 3-phenylcyclopentenone 1 and 3-phenylcy clohexenone 2 triplet states by (E)- and (Z)-1-phenylpropene, 3 and 4, in c yclohexane and benzene solutions have been recorded using laser flash photo lysis. In cyclohexane at high quencher concentration, complex decay kinetic s are observed consistent with observation of both the enone triplet state and either a triplet exciplex or 1,4-biradical intermediates.