REGIOSELECTIVE PHOTOPROTONATION OF 1,3-DIMETHOXYBENZENES IN 1,1,1,3,3,3-HEXAFLUOROISOPROPYL ALCOHOL - H-D ISOTOPE-EXCHANGE AND LASER FLASH-PHOTOLYSIS STUDIES

Laser flash photolysis of 1,3-dimethoxybenzene and five substituted derivatives (2-Me, 4-Me, 5-Me, 2-MeO, 4-MeO) in 1,1,1,3,3,3-hexafluoroisopropyl alcohol (HFIP) results in a single transient with lambda(max) in the range 395-460 nm, dependent on the additional substituent. This transient forms in a monophotonic process, is unaffected by oxygen, but quenched by methanol and bromide, and decays by returning to the ground-state aromatic compound. These are characteristics of an intermediate cyclohexadienyl cation, produced by protonation of the aromatic excited state by the solvent. The lambda(max) values of the cyclohexadienyl cations produced by protonation of these dimethoxybenzenes in concentrated H2SO4 however lie in the range 315-340 nm. The 400-MHz H-1 NMR of the latter solutions reveals that the cations obtained here are 2,4-dimethoxybenzenonium ions, obtained by protonation at the 4-position. Photolysis at 254 nm in HFIP-d reveals that, for the four compounds that are unsubstituted at C2, there is a relatively efficient exchange of the hydrogen at this carbon; with the two compounds bearing 2-substituents there is no net photochemical reaction. Thus, the cyclohexadienyl cations observed as transient intermediates in the photochemical experiments are 2,6-dimethoxybenzenonium ions, obtained by protonation at C2 of the excited state. The change in selectivity between ground and excited states is explained by a shift of pi-electron density upon excitation. The different spectra of 2,4- and 2,6-dimethoxybenzenonium ions have their origins in very different effects of o- and p-methoxy substituents, the former causing a approximately 40-nm bathochromic shift in a benzenonium ion and the latter a approximately 55-nm hypsochromic shift. Photolysis of 1-methoxynaphthalene in HFIP-d results in exchange at C5, and to a lesser extent at C8, as has been previously observed in acidic aqueous solutions. A transient is observed upon laser flash photolysis of this compound in HFIP with two lambda(max) at 360 and 550 nm. This transient is assigned to the cyclohexadienyl cation obtained by photoprotonation at C5.