Theory of Cycloadditions of Excited Aromatics to Alkenes

A qualitative molecular orbital rationalization of the selectivities in ortho, meta, and cycloadditions of singlet excited states of substituted benzenes to alkenes, and of triplet state di—ir—methane rearrangements of substituted benzonorbornadienes is presented. As the name suggests, excited states of benzene and substituted benzenes are prone to aroused reactivity tendencies, rearranging to high energy isomers, or combining with a variety of reagents toward which they are inert in the ground states.1'2 This generalization is particularly clearly manifested in the photoreactions of aromatic molecules with alkenes.1 In a cycloaddition between benzene and ethylene, the two ethylene carbons may become united to two carbons of benzene disposed ortho, meta, or to each other, as shown in Fig. 1.