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

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Summary *

The kinetics of the Rose Bengal sensitised photooxidation of six 2-substituted 3-hydroxypyridines and the parent compound 3-hydroxypyridine (3-OHP), all with the basic molecular structure of known pesticides, have been studied in solution in neutral or alkalinised water or acetonitrile-water 4:1. The kinetic constants of the involved chemical (kr) and overall (kt) singlet molecular oxygen [O2(1Dg)] quenching processes have been determined by comparative methods based on absorbance changes,oxygen uptake, or time-resolved phosphorescence detection. The results indicate that both rate constants are in the order range 105-108 M-1 s-1, except in the case of 2-carboxy-3-hydroxypyridine in water, where no chemical reaction was detected. The kt 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 [O2(1Dg)-substrate]. All 3-hydroxypyridines are photooxidised, with quantum efficiencies as high as 0.66 for 2-methyl-3-hydroxypyridine. The photooxidation products from 3-OHP in water 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 O2(1Dg) addition to the 2,3-positions of the pyridine ring. Information provided by the agent in SIGEVA