Electron transfer photochemistry of bifunctional strained-ring and unsaturated systems

Abstract: Organic radical cations, generated by electron transfer from bi-or trifunctional donor terpenes to photo-excited acceptors, undergo a variety of intra- and intermolecular reactions, including nucleophilic capture and sigmatropic shifts, deprotonation, intramolecular nucleophilic capture, and various cyclizations. The structures and reactions of organic radical cations have been the focus of much interest for the past two decades (ref. 1,2). A rich variety of substrates have been investigated by physical and chemical techniques to delineate changes in geometry upon oxidation, to assess the spin density distribution in the radical cations, and to elucidate parameters affecting their structures (ref. 2,3). Particularly the interactions between adjacent functions, conjugated or nonconjugated, pairs of olefinic a-systems or strained rings, or of strained ring moieties with olefinic fragments has attracted considerable interest (ref. 3). We discuss attempts to utilize light-induced radical cation reactions for synthetidpreparative purposes. We have studied photosensitized reactions of various donors with two or three functional groups, including unsaturated and strained ring moieties as well as an alcohol function as an intramolecular nucleophile. The sensitizers were chosen to give the putative electron transfer reaction either a marginal (-AG- 0.1 eV) or a more substantial driving force (-AG> 0.5 eV). The driving force for radical ion formation can be furthe