Photochemistry of substituted cyclic enones. Part 12.1 Photocycloaddition of 3-phenylcyclopentenone and 3-phenylcyclohexenone to (E)- and (Z)-1-phenylpropene

Abstract

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 ¹H-NMR spectroscopy in solutions of d₆-benzene. The adducts formed for each photoreaction have been shown to be independent of the starting 1-phenylpropene isomer. For 3-phenylcyclopentenone 1, a photostationary state of 32∶68 (E)- to (Z)-1-phenylpropene is achieved during the course of the reaction. This isomerisation reaction may occur through reversion of triplet 1,4-biradical intermediates to ground state molecules or by an energy transfer process. However for 3-phenylcyclohexenone 2 isomerisation occurs only through reversion of triplet 1,4-biradical intermediates. The mechanism of photoadduct formation and that of the isomerisation reaction are discussed for both phenylenones.

Rate constants for quenching of the 3-phenylcyclopentenone 1 and 3-phenylcyclohexenone 2 triplet states by (E)- and (Z)-1-phenylpropene, 3 and 4, in cyclohexane and benzene solutions have been recorded using laser flash photolysis. In cyclohexane at high quencher concentration, complex decay kinetics are observed consistent with observation of both the enone triplet state and either a triplet exciplex or 1,4-biradical intermediates.