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

FRANCISCO AMAT-GUERRI, ADRIANA PAJARES,JOSE GIANOTTI,GUILLERMO STETTLER, S. BERTOLOTT, CRIADO,NORMAN A. GARCíA

JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY A-CHEMISTRY

ELSEVIER

Año: 1999 vol. 126 p. 59 - 64

1010-6030

(kt) singlet molecular oxygen [O2 (1Äg)] 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 (1Äg)-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 (1Äg) addition to the 2,3-positions of the pyridine ring.