PHOTOINDUCED ELECTRON-TRANSFER FROM ENOL SILYL ETHERS TO QUINONE .1. PRONOUNCED EFFECTS OF SOLVENT POLARITY AND ADDED SALT ON THE FORMATIONOF ALPHA-ENONES

The enol silyl ethers (ESE) of various ketones are efficiently oxidize d to the corresponding alpha,beta-unsaturated enones (E) when a dichlo romethane solution containing equimolar amounts of chloranil is irradi ated with filtered light (lambda(exc) > 380 nm). The 1:1 adduct (A) of enol silyl ether and quinone is a byproduct, the structure of which i s established by X-ray crystallography, Solvent polarity and added sal ts 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-me thylcyclohexanone, points to the cation radical of the enol silyl ethe r (ESE(.+)) as the reactive intermediate, A radical-ion pair mechanism involving the rapid one-electron oxidation of enol silyl ethers by ph otoactivated chloranil is discussed.