The photocycloaddition reactions of benzene with 1,2-, 1,3-, and 1,4-dienes are described. Propa-1,2-diene (allene) and cyclonona-l,2-diene are unique among hydrocarbon ethylenic systems since 1,2-para-cycloaddition to the benzene ring is the major mode of reaction. The rneta-cycloaddition of 1,Z-dienes is regiospecific and this is accounted for by a mechanism involving prior mefa-bonding in the arene leading to a polarised prefulvene species which undergoes 1,2-addition to the diene. The conformationally fixed cis-diene 12-dimethylenecyclohexane undergoes selective 1,4-para-and 1,4-meta-photocycloaddition to benzene whereas the reaction with 3-msthylenecyclohexene is of low yield and non-selective giving diene dimers, substitution products, and adducts. Penta-1 , 4-diene and cyclohexa-l,4-diene yield 12-rneta-cyclo-adducts with benzene, and evidence is presented for formation of 1,2-para-cycloadducts from each system: diene dimers are also produced in the latter case but solutions of bicyclo[2.2.1] hepta-2,5-diene in benzene are essentially inert photochemically. SINCE the early reports that benzene and its sirnyle 1,3-dienes, tlie rcactioris of acyclic and cyclic 1,2-and derivatives form ortho-(1), meta-(Z), and, less commonly, 1,4-dic.nes witli photoexcited benzene have been totally #am-pliotocycloadducts (3) with ethylenes and cisneglected. In the present paper we describe and discuss and trans-para-1 ,kphotocycloadducts (4) and (5) with tlie pliotoreactions of benzene with allene (propa-l,2diene) ,22 cyclonona-1,2-diene,22 peiita-l,4-diene, cycloliexa-I ,.l-diene, and bicyclo[2.2.1]liepta-2,5-die1ie and coin pare tlicse with those of hydrocarbon inonoenes. I;or comparison also with these systems we have investigated the reactions of benzene with certain 1,3-V ( 2 1 dienes but unlike previous studies with this arcne ours have involved conformat ionally fixed dienes since these avoid the problems of addend isoirierisation and sub-R e , scqiient side reactions which beset conformationally ( 3 1 flesi ble sys terns. 23 I< ESULTS AN D J)I SC USS I 0 N In all experiments the radiation source coinprised ( 4 ) R . = H (5 1 (15) R = Me low-pressure mercury-arc lamps. Nitrogen degassing generally had no significant effect on reaction efficiencies or product ratios within the time limits of the (6)

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 Me,C= also undergo ene-type acyclic para-addition to benzene to give cyclohexa-l,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 CH,OD 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,rneta-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 T, 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 S, benzene or initial meta-cycloaddition of the ethylene t o S1 benzene. The ortho-and para-cycloadditions are formally forbidden as concerted processes from S1 benzene plus So ethylene, and these are considered to involve intermediates having a degree of dipolar character.

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