Photoinduced electron transfer from enol silyl ethers to quinone. Part 2. Direct observation of ion-pair dynamics by time-resolved spectroscopy

Abstract

Time-resolved spectroscopy in two time domains (encompassing picoseconds and nonseconds/microseconds) allows all the reactive intermediates involved in the photooxidation of enol silyl ethers (ESE) by chloranil (CA) to be identified, and each step in their temporal evolution to α-enone (E) and adduct (A) to be delineated. Thus, the radical–ion pair [ESE˙+, CA˙–] is the common intermediate formed in unit quantum yield via the highly efficient quenching of triplet chloranil by enol silyl ethers in both dichloromethane and acetonitrile. In the context of the Fuoss–Winstein formulation, the first-formed [ESE˙+, CA˙–] is a contact ion-pair, which in a non-polar solvent, such as dichloromethane, predominantly suffers an initial ion-pair collapse (internal return) by β-proton transfer, and the resultant radical pair ultimately leads to α-enone (E). The contact ion-pair formed in the polar solvent undergoes diffusive separation, and the free ion ESE˙+ suffers desilylation by acetonitrile and ultimately leads to adduct (A). Added electrolyte to dichloromethane solutions modulates the ion-pair behaviour via the ‘special salt effect’ to divert the enone pathway to adduct formation.