Photochemical reaction mechanisms of 2-nitrobenzyl compounds in solution I. 2-nitrotoluene: Thermodynamic and kinetic parameters of the aci-nitro tautomer

The largely reversible, light-induced tautomerization of 2-nitroluene (1) t o the quinonoid aci-nitro tautomer aci-1 was studied by flash photolysis as a benchmark for comparison with the widely used nitrobenzyl phototriggers ('caged compounds'). The pH-rate profile for the decay of aci-1 in aqueous solution exhibits downward curvature at pH 3-4, which is attributed to pre- equilibrium ionization of the nitronic acid aci-1 to its anion 1(-) (pK, 3. 57). Two regions of upward curvature, at pH ca. 6 and <0 (H-0 approximate t o - 1), each indicate a change in the reaction mechanism. The elementary re actions that dominate between the curved regions are assigned on the basis of kinetic isotope effects and the observation of general acid catalysis: H ydronium ions regenerate 2-nitrotoluene by C-protonation of I in the PH ran ge of 0 - 6, and H2O is the proton Source at pH > 6. A hird, irreversible N ef-type isomerization of aci-1 prevails in highly acidic solutions (pH < 0) . The equilibrium constant for the thermal tautomerization of 1 to aci-1 is estimated as pK(T) = 17.0 +/- 0.2 based on kinetic data.