Arene-alkene photocycloaddition reactions mechanism

The photocycloaddition mechanism, and consequently the reaction selectivity, may vary considerably depending on the structure of the initial material and reaction conditions. In general, an excited arene and a ground-state alkene may react with initial polarization to form an exciplex.802 In [2 + 2] photocycloaddition reactions, biradical intermediates are often involved (Scheme 6.80a), although excitation of a ground-state charge-transfer (CT) complex (Section 2.2.3) has also been discussed in some cases, such as the [2 + 2] photocycloaddition of benzene with maleic anhydride (Scheme 6.80b).817 Here a zwitterion intermediate 194 collapses to the adduct 195 only in the absence of an acid. 

Another important photoreaction is the 1,3 cycloaddition of ethene to benzene. Although a 1,2 and a 1,4 photocycloaddition can also take place, the 1,3 photochemical addition is the most important, since it yields a variety of natural products depending on the substitution pattern of the alkene and arene. The reaction mechanism which was obtained by experimental results indicates an exciplex mechanism, whereas a SINDOl calculation favors the prefulvene mechanism.Since the experiments were done in solution and the calculations were performed in a vaccum, this is not at all surprising. According to the SINDOl calculations, there is a conical intersection near the prefulvene minimum, which is very flat. The semiempirical calculations are supported by MCSCF ab initio calculations, which show a remarkable similiarity in the important structures for both the exciplex and the prefulvene mechanisms.This demonstrates that semiempirical programs on the Cl level are well suited for photochemical problems. 

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