2,4,6-triphenylpyrylium ion-mediated photooxidation of N,N-dibenzylhydroxylamine and its derivatives

The irradiation of an O-2-purged MeCN solution of N,N-dibenzylhydroxylamine and its derivatives (1) in the presence of 2,4,6-triphenylpyrylium tetrafluoroborate (TPT) with 366 nm light was found to give the corresponding N-benzylidenebenzylamine N-oxide derivative and 2,4,6-triphenyl-3,4-epoxypyran. Solvent effects on the fluorescence quenching of TPT by 1 and the quantum yield for the disappearance of TPT (Phi (TPT)) strongly suggested that electron transfer to both singlet and triplet TPT occurs, particularly on the low concentration sides of 1, affording the hydroxylamine radical cation (1(.+)) and the TPT-derived pyranyl radical (TP.). Furthermore, substituent effects on the limiting quantum yield (Phi (TPT,lim)) showed that the increased stability of 1(.+) lowers the return electron-transfer rate to enhance the relative reactivity of TP. toward O-2, that is, to increase Phi (TPT.lim). On the other hand, analysis of heavy atom effects on the TPT emission intensity and the photooxidation efficiency led us to conclude that the presence of 1,2-dibromoethane accelerates intersystem crossing to the excited triplet-state TPT through the enhanced spin-orbit coupling resulting in an increase in Phi (TPT). The observed heavy atom effects on Phi (TPT) were explained in terms of the increased contribution of a triplet 1(.+)-TP. pair intermediate in which spin-forbidden reverse electron transfer is responsible for a great enhancement in the reactivity of TP. toward O-2. (C) 2001 Elsevier Science B.V. All rights reserved.