OXIDATIVE PHOTOFRAGMENTATION OF ALPHA,BETA-AMINO ALCOHOLS VIA SINGLE ELECTRON-TRANSFER - COOPERATIVE REACTIVITY OF DONOR AND ACCEPTOR ION RADICALS IN PHOTOGENERATED CONTACT RADICAL ION-PAIRS

The studies presented in this paper show that alpha,beta-amino alcohols undergo a very clean C-C bond cleavage upon SET (single electron transfer) oxidation by photoexcited electron acceptors in a process which generally culminates in two-electron reduction of the acceptors. For a number of different alpha,beta-amino alcohols, the oxidative fragmentation occurs in a high chemical yield (> 90%), yet with low to medium quantum efficiencies (0.0001-0.1) which vary strongly depending on the properties of electron donor (D), acceptor (A), and solvent. The net quantum efficiency reflects the competition between back electron transfer and the chemical redox process. Detailed mechanistic studies were carried out to investigate the visible light induced oxidative fragmentation of alpha,beta-amino alcohols in the presence of electron acceptors including thioindigo (TI), 9,10-dicyanoanthracene (DCA), 2,6,9,10-tetracyanoanthracene (TCA), and 1,4-dicyanonaphthalene (DCN). Cosensitized (biphenyl) photoredox leads to free ions, A- and D+, and moderately efficient unassisted fragmentation of D+. Quenching of 1A* by electron donor (D) to give a solvent separated radical ion pair (SSRIP) leads to a very inefficient reaction. In contrast, quenching to give a contact radical ion pair (CRIP) gives a relatively efficient reaction. This reaction is sensitive to the stereochemistry of the amino alcohol, suggesting a preferred anticoplanar configuration during the C-C bond cleavage process. The critical matching of reactivity of acceptor and donor ion radicals allows a rapid reaction to occur in the relatively narrow time window between formation and decay of the contact radical ion pair.