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 1 H-NMR spectroscopy in solutions of d 6 -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.

Time-resolved IR studies of cyclic enone triplet excited states and their reactions with alkenes

“…The observed lifetime is consistent with previously reported values. 22 The analogous TRIR spectrum of 18 O-labeled 1 reveals C=O vibrational modes in the ground state (at 1648 cm À1 ) and the triplet excited state (at 1474 cm À1 ).…”

Reversal of reaction type selectivity by Lewis acid coordination: the ortho photocycloaddition of 1- and 2-naphthaldehyde

“…Based on NOESY data for 18 and for the alcohol obtained from 18 by reduction (LiAlH 4 in THF, 88% yield, see ESI† for further details), the relative configuration of the two ethyl groups within the cyclobutane ring is trans . Formation of this product from ( Z )- 17 requires the involvement of an intermediate in which free rotation is possible, potentially a triplet 1,4-diradical 20…”

Photochemistry of Cyclobutanes: Synthesis and Reactivity

“…The outcome of this addition is different than that for the irradiation of enone 211b in the presence of the same alkene when only 214 is formed. A detailed mechanistic examination of the reaction has sought to resolve the difference in the observed reactivities153 .…”