Singlet molecular oxygen-mediated photooxidation of 2-substituted 3-hydroxypyridines

The kinetics of the Rose Bengal sensitised photooxidation of six 2-substitu ted 3-hydroxypyridines and the parent compound 3-hydroxypyridine (3-OHP), a ll with the basic molecular structure of known pesticides, have been studie d in solution in neutral or alkalinised water or acetonitrile-water 4:1. Th e kinetic constants of the involved chemical (k(r)) and overall (k(t)) sing let molecular oxygen [O-2 ((1)Delta(g))] quenching processes have been dete rmined by comparative methods based on absorbance changes,oxygen uptake, or time-resolved phosphorescence detection. The results indicate that both ra te constants are in the order range 10(5)-10(8) M-1 s(-1), except in the ca se of 2-carboxy-3-hydroxypyridine in water, where no chemical reaction was detected. The k(t) value increases with electron-releasing substituents at position 2, as well as in water or alkaline media, pointing to a quenching process through a polar complex [O-2 ((1)Delta(g))-substrate]. All 3-hydrox ypyridines are photooxidised, with quantum efficiencies as high as 0.66 for 2-methyl-3-hydroxypyridine. The photooxidation products from 3-OHP in wate r have been analysed by GC/MS. Products from the pyridine ring cleavage, as well as 2,3-dihydroxypyridine, have been detected. All can be derived from a 1,2-dioxetane intermediate formed by O-2 ((1)Delta(g)) addition to the 2 ,3-positions of the pyridine ring. (C) 1999 Elsevier Science S.A. All right s reserved.