Enhanced Basicity of the 2-Position of 1,3-Dialkoxybenzenes in S1: Acid Catalyzed Photochemical Proton/Deuteron Exchange

Article abstract of DOI:10.1021/jo00107a021

The photochemistry of several 1,3-dialkoxy-substituted benzenes 1-8 has been studied in aqueous H2SO4 (D2SO4) solution.In contrast to ground state behaviour, all of these compounds (with the exception of 7) are photoprotonated efficiently (via S₁) and almost exclusively at the 2-position (and to a much lesser extent at the 5-position) to give dialkoxy-substituted cyclohexadienyl cation intermediates (2,6-dialkoxybenzenonium ions), resulting in proton/deuteron exchange of the 2-position of the benzene ring for all systems except 4 (and 7).In the ground state, protonation takes place predominantly at the 4-position for 1-4 and at the 6-position for 5 and 6.In the case of 4, photoprotonation at the 2-position is implicated by observation of fluorescence quenching by acid but deuterium exchange at this position is not possible.Photoprotonation (quenching) rate constants (k_H) were calculated by Stern-Volmer analysis of steady state fluorescence quenching by proton and are in the range 0.9-3E9 M^-1 s^-1.Quantum yields for deuterium incorporation in 4:1 aqueous D2SO4-CH3CN are reported for several substrates at selected acidities.The sigmoid dependence of plots of fluorescence quenching vs acidity is used to give estimated pK_BH(1+)^* of ca. 0.1-0.5 for these substrates, indicating that they are much more basic in S₁ than in S₀.