The Inter- and Intramolecular Photocycloaddition of Ethylenes to Aromatic Compounds

Dienophilic ethylenes yield solely ortho photocycloadducts with benzene whereas alkenes and enol ethers give products arising from both ortho and meta cycloaddition. The relative efficiencies of the twoprocesses are largely predictable and this combined with reasonable chemical and quantum yields make these photoreactions realistic synthetic procedures towards bicyclo[4.2.Oloctane and tricyclo[3.3.0.02'8]octane systems. The ortho cycloaddition involves a measure of charge—transfer to or from the ethylene at some stage in the reaction pathway, whereas two routes are considered to account for the orientational selectivity of the meta process involving ethylenes and substituted benzenes. Positions and modes of intramolecular reaction between the chromophores of non-conjugated phenyl vinyl bichromophoric systems are understandable in terms of their favoured molecular conformations leading to preferred orientations of the addends.