Photochemical Cycloaddition of Singlet trans-Stilbene with α,β-Unsaturated Esters

The photochemical cycloaddition reactions of singlet trans-stilbene with five a,d-unsaturated esters and diesters are described. Adduct stereochemistry is influenced more by secondary -orbital overlap than by steric effects. The reactivity of the unsaturated esters with singlet stilbene increases with their electron affinity. This trend is consistent with the formation of a charge-transfer-stabilized exciplex intermediate in which singlet stilbene is the electron donor. Cycloaddition quantum yields, however, decrease with increasing electron affinity. This trend is readily explained by using simple frontier molecular orbital theory.