Photochemical Reaction Mechanisms of 2-Nitrobenzyl Compounds in Solution, I. 2-Nitrotoluene: Thermodynamic and Kinetic Parameters of theaci-Nitro Tautomer

Abstract

Dedicated to Prof. Dr. Edgar Heilbronner on the occasion of his 80th birthdayThe largely reversible, light-induced tautomerization of 2-nitrotoluene (1) to 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 À (pK a 3.57). Two regions of upward curvature, at pH ca. 6 and`0 (H 0 %À 1), each indicate a change in the reaction mechanism. The elementary reactions that dominate between the curved regions are assigned on the basis of kinetic isotope effects and the observation of general acid catalysis: Hydronium ions regenerate 2-nitrotoluene by C-protonation of 1 À in the pH range of 0 ± 6, and H 2 O is the proton source at pH b 6. A hird, irreversible Nef-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 AE 0.2 based on kinetic data.

The Primary Steps in Excited‐State Hydrogen Transfer: The Phototautomerization of o‐Nitrobenzyl Derivatives

“…16. [j] In water; adapted from ref12a. [k] Determined by picosecond transient absorption spectroscopy in THF, ref.…”

Photoprocesses of Molecules with 2‐Nitrobenzyl Protecting Groups and Caged Organic Acids

“…It has been argued that the absorption at 320 nm of 2‐nitrosobenzaldehyde is due to the dimer and at 750 nm due to the monomer (4,13,17), which however also absorbs around 300 nm (6). Arguments against thermal dimer formation under the spectroscopic conditions of nitrosobenzenes have been given (4,19–21). Here, no dimers could be detected by UV–Vis spectroscopy and HPLC for any photoproduct in acetonitrile under the applied conditions.…”

Photoremovable Protecting Groups in Chemistry and Biology: Reaction Mechanisms and Efficacy

“…Parent o -nitrotoluene ( 47 , oNT, Scheme 30) and several derivatives have been studied by time-resolved spectroscopy. 153 The most recent investigation of oNT and the analogous o -nitrobenzaldehyde by Gilch and co-workers, 154 who used femtosecond transient absorption and stimulated Raman spectroscopy, has finally provided a convincing and complete picture of the early events. The results reported by these authors about oNT 154a are summarized in Scheme 30, which bears a striking similarity to that for the photoenolization of o -methylacetophenone (Scheme 4 in section 2.2).…”