Photochemical reactions of trans-stilbene and 1,1-diphenylethylene on silica gel: Mechanisms of oxidation and dimerization

The photochemical reactions of trans-stilbene and 1,1-diphenylethylene have been studied at a silica gel-air interface. Irradiation of trans-stilbene leads to the formation of a substantial amount of two dimers, along with the formation of cis-stilbene, phenanthrene and a small amount of benzaldehyde. Photochemical dimer formation from trans-stilbene on silica is promoted by inhomogeneous surface loading and slow surface diffusion. Benzaldehyde arises from a Type II oxidation mechanism. Irradiation of 1,1-diphenylethylene at a silica gel-air interface leads primarily to the formation of benzophenone. The oxidation of 1,1-diphenylethylene occurs by a Type I mechanism, as previously proposed [J. Org. Chem. 50 (1985) 149]. The differences in observed photochemistry for these two substituted phenylethylenes on silica is attributed to two factors. Isomerization of trans-stilbene competes efficiently with oxidation, whereas this pathway appears not to be important for 1,1-diphenylethylene. Singlet molecular oxygen is shown to be quenched in solution and at the silica gel-air interface at a faster rate by cis-stilbene than by trans-stilbene or 1,1-diphenylethylene, thus providing an enhanced Type II oxidation pathway for stilbene.