The photoaddition of ethylenic hydrocarbons to benzene

Abstract

The photoreactions of a range of acyclic and cyclic ethylenic hydrocarbons with benzene are described. The processes which occur depend on the ethylene, and in particular on its ionisation potential relative to that of benzene, but in general, meta-cycloadditions occur in every case and often predominate. ortho-Cycloaddition tends mostly to occur with the ethylenes of lowest ionisation potential relative to that of benzene, and para-cycloaddition occurs with cyclobutene to a significant extent, but not with cyclopropene or cyclopentene. Acyclic ethylenes containing the group Me₂C also undergo ene-type acyclic para-addition to benzene to give cyclohexa-1,4-dienes.

The ortho-, meta-, and para-cycloadditions are stereospecific with respect to the ethylene, but the ene-addition is non-stereospecific. The former are mostly insensitive to solvent polarity, the exception being ortho-cycloaddition of 2,3-dimethylbut-2-ene which is definitely promoted by polar solvents. The ene-addition of 2,3-dimethylbut-2-ene was promoted by methanol but not by acetonitrile: the use of CH₃OD led non-stereospecifically to incorporation of deuterium in the 4-position of the adduct.

Cyclohexene was anomalous in giving mainly a mixture of three cyclobutane dimers, with only minor proportions of the endo and exo, meta-adducts and possibly a trace of the para-cycloadduct. The photocyclisation of benzene to give a range of essentially saturated polymers up to ca. (C₆H₆)₂₄ can be initiated by traces of cis-cyclo-octene.

All the products, with the exception of the cyclobutane dimers from cyclohexene, are considered to be formed via S₁ benzene, although T₁ benzene-sensitised cis–trans isomerisation of the ethylenes also slowly occurs: thus prolonged irradiation of cis-cyclo-octene and benzene leads eventually to the formation of trans-cyclo-octene and the corresponding cycloadducts.

The mechanisms of these processes are discussed with reference to orbital symmetry considerations, and the observed effects of polar solvents and proton donors. Only the meta-cycloadditions appear to occur by concerted processes involving either initial meta-bonding in S₁ benzene or initial meta-cycloaddition of the ethylene to S₁ benzene. The ortho- and para-cycloadditions are formally forbidden as concerted processes from S₁ benzene plus S₀ ethylene, and these are considered to involve intermediates having a degree of dipolar character.