Photoinduced Wolff rearrangement and tautomerization of 3-chloro-2-hydroxypyridine isolated in an Ar matrix

Abstract

The photoreaction mechanism of 3-chloro-2-hydroxypyridine isolated in a low-temperature argon matrix was investigated by IR spectroscopy and density functional theory (DFT) calculations. Two tautomeric forms, keto and enol I, with an intramolecular N⋯H–O hydrogen bond, existed in the matrix before UV irradiation. Tautomerization from the keto form to enol I occurred completely upon the first UV irradiation (λ ≥ 300 nm), while the second light irradiation (λ ≥ 250 nm) induced the reverse tautomerization from enol I to the keto form as well as photodissociation to yield a five-membered-ring ketene, 2H-pyrrol-2-ylidenemethanone, and HCl. The latter is similar to the photolysis of 2-chlorophenol to yield its corresponding five-membered-ring ketene by Wolff rearrangement after dissociation of HCl by UV irradiation. Kinetic analysis indicates the existence of a short-lifetime precursor, which is expected to be the less stable enol tautomer with an intramolecular Cl⋯H–O hydrogen bond. A trace of the intermediate was spectroscopically detected only during UV irradiation.