ChemInform Abstract: PHOTOCHEMISTRY OF ALKENES IN SOLUTION

Abstract

The direct and sensitized photochemical behaviour of cyclic and acyclic olefins is compared. Under either set of conditions cyclo-hexenes and -heptenes undergo photoprotonation in hydroxylic media, perhaps via a trans intermediate. By contrast acyclic, exocyclic, and larger-ring cyclic olefins fail to undergo photoprotonation on either direct or sensitized irradiation. However, both cyclohexenes and cycloheptenes, as well as larger-ring cyclic and acyclic olefins, undergo slow isomerization to positional and skeletal isomers on direct irradiation in either protic or aprotic media. Unique behaviour has been observed for tetraalkyl-substituted olefins, which exhibit behaviour on direct irradiation in alcoholic solvents suggesting the intervention of a cation radical intermediate. Mechanistic implications of these various types of photochemical behaviour are discussed. INTRODUCTION One of the most commonly occurring organic chromophores is that of the simple carbon—carbon double bond and yet, despite its ubiquity, the ethylene chromophore has received relatively little attention by photochemists. In a parallel sense, despite its apparent simplicity, this chromophore has confounded spectroscopists and theoreticians alike, and even today there exist many questions conce'rning the location and configurational assignments of the various electronically excited states of ethylene and -its simple alkyl derivatives'. In view of the common occurrence of the ethylene chromophore in organic systems, its photochemical behaviour offers the potential of new, unexplored reactions which should have broad synthetic utility, Moreover, a systematic study of its photochemical behaviour should shed additional light on the nature and location of its various excited states. It was with these thoughts in mind that several years ago we undertook such an investigation.