Mechanistic and kinetic aspects of the photoinduced OCH3 substitution in 3,5-dinitroanisole. A probe for solvent effects in thermal reactions

Abstract

Nucleophiles (N) such as triethylamine (TEA) and OH^– react with 3,5-dinitroanisole (3,5-DINA) in its lowest triplet state in aqueous or in mixed aqueous solutions and cause the formation of 3,5-dinitrophenolate (3,5-DNP). The quantum yield (Φ) of 3,5-DNP and the kinetics of the conversion have been investigated as a function of the concentration of N. The kinetics have been studied spectroscopically in the period between 50 ns and 120 s after flash excitation. The absorption spectra at three intermediate stages {X}, {Y} and {Z} are given and the transient species present at each stage are identified. The hydrogen-bonded triplet state is involved in three parallel reactions with N and yields an exciplex (E) and two σ-complexes with either a C(2)—N or a C(4)—N bond. The two σ-complexes return to 3,5-DINA. It is argued that binding in E arises from overlap of lone-pair orbitals on OCH₃ and on N and that E is the immediate precursor of a thermally more stable exciplex (X^u) and of the anion 3,5-DINA^–. Clustering of H₂O molecules around the H-bonded NO₂ groups in E lowers the activation energy required for its dissociation into 3,5-DINA^– and an ^˙OH radical. This explains why an asymptotic value Φ= 0.37 is reached at high concentrations of NaOH in aqueous solution, while Φ can become as large as 0.8 in CH₃CH + H₂O (1:1 by volume). Exciplex X^u is attacked on C(1) by N and H₂O. In the case of OH^– these two reactions produce a complex σ{C(1)—OH^–}, which is subsequently converted into 3,5-DNP. When TEA replaces NaOH in the solutions, there are two paths to 3,5-DNP. One starts with the reaction of the triplet-state molecule with OH^– present and is essentially the same path encountered in the case of NaOH. The second path starts with the reaction of the triplet-state molecule with TEA, which yields an exciplex E(TEA). At low concentration of TEA this route is of relatively little importance, presumably because E(TEA) dissociates efficiently into the anion of 3,5-DINA and TEA⁺.