Photoinduced electron transfer from enol silyl ethers to quinone. Part 1. Pronounced effects of solvent polarity and added salt on the formation of α-enones

The enol silyl ethers (ESE) of various ketones are efficiently oxidized to the corresponding α,β-unsaturated enones (E) when a dichloromethane solution containing equimolar amounts of chloranil is irradiated with filtered light (λexc > 380 nm). The 1 : 1 adduct (A) of enol silyl ether and quinone is a byproduct, the structure of which is established by X-ray crystallography. Solvent polarity and added salts play a major role in establishing the product distribution between E and A. Such a medium effect, coupled with the ready isomerization of the kinetic–thermodynamic isomers derived from the silylation of 2-methylcyclohexanone, points to the cation radical of the enol silyl ether (ESE˙+) as the reactive intermediate. A radical–ion pair mechanism involving the rapid one-electron oxidation of enol silyl ethers by photoactivated chloranil is discussed.