Martin Gaplovsky, Yuri V. Il'ichev, Yavor Kamdzhilov, Svetlana Kombarova, Marek Mac, Markus A. Schwörer and Jakob Wirz, "2,5-Photochemical reaction mechanisms of 2-nitrobenzyl compounds: 2-Nitrobenzyl alcohols form 2-nitroso hydrates by dual proton transfer", Photochem. Photobiol. Sci. 2005, 4, 33-42.
Abstract: Irradiation of 2-nitrobenzyl alcohol (1, R=H) and 1-(2-nitrophenyl)ethanol (1, R=Me) in various solvents yields 2-nitroso benzaldehyde (4, R=H) and 2-nitroso acetophenone (4, R=Me), respectively, with quantum yields of about 60%. The mechanism of this reaction, known since 1918, was investigated using laser flash photolysis, time-resolved infrared spectroscopy (TRIR), and 18O-labeling experiments. The primary aci-nitro photoproducts 2 react by two competing paths. The balance between the two depends on the reaction medium. Reaction via hydrated nitroso compounds 3 formed by proton transfer prevails in aprotic solvents and in aqueous acid and base. In water, pH 3-8, the classical mechanism of cyclization to benzisoxazolidine intermediates 5, followed by ring opening to carbonyl hydrates 6, predominates. The transient intermediates 3 and 6 were identified by TRIR. Potential energy surfaces for these reactions were mapped by density functional calculations.