Reactions between Benzene and Alkenes

These are all stereospecific with respect to alkenes. Though reaction requires excitation of benzene chromophore, but involves initial excitation of alkene. Sometimes charge transfer complex between alkene and benzene also undergoes excitation. All the three cycloadditions involve singlet excited states of benzene ring. 

7i-molecular orbitals of benzene and their m-plane symmetry is depicted in Fig. 5.8. 

symmetry properties of reactants, i.e., benzene and ethylene match with that of product in ground state reaction is thermally feasible but photochemical ortho cycloaddition between benzene and ethene is unfavourable. 

In meta-addition either direct concerted cycloaddition or initial meta-bonding in excited singlet state to give prefulvene type intermediate takes place. Formation of bond (i) can be before (ii) and (iii) or vice versa or subsequent to (ii) and (iii). This reaction is stereospecific and insensitive to proton donor and solvent which suggests that (ii) and (iii) may be formed is concerted step. 

Ortho-cycloaddition takes place with a olefin which has low ionization potential in comparison to benzene where polar nature of reaction overpowers the symmetry imposed barrier to this reaction. Polar nature of ortho-cyclo-addition is supported by the fact that in case of doner substituted ethylenes, reaction is promoted by polar solvent, but in meta-addition no solvent effect is there, o-and p-photocycloadditions are disallowed to occur as concerted addition between of benzene and Sq of alkene until mixing of charge-transfer states occurs. 

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