Tunneling effects on the 1,3-sigmatropic hydrogen shift in the photo-Fries rearranged intermediate of 2,4-dimethoxy-6-(p-tolyloxy)-s-triazine

Abstract

Kinetic studies on the 1,3-sigmatropic hydrogen shift in the photo-Fries rearranged intermediate of 2,4-dimethoxy-6-(p-tolyloxy)-s-triazine (PTTH) were carried out in various solvents by using laser flash photolysis techniques. The intrinsic rate constants for the intramolecular [1,3] hydrogen and deuterium shifts in dehydrated nonpolar methylcyclohexane at 293 K were determined to be 1.7 s⁻¹ and 5.7 × 10⁻¹ s⁻¹, respectively. The rates for the 1,3-hydrogen shift in alcoholic solvents were significantly enhanced by the basic catalytic action of solvent molecule(s). The experimental results of temperature and isotope effects showed that the intramolecular [1,3] hydrogen and deuterium shifts in the photo-Fries rearranged intermediates of PTTH and PTTD proceeded via quantum mechanical tunneling at two vibrational levels [v = v₀ and v = v₁ (ΔE^H = 3.4₃ kcal mol⁻¹, ΔE^D = 3.8₀ kcal mol⁻¹)]. According to the tunnel effect theory (TET) proposed by Formosinho, the magnitude of the tunneling frequency factor for this system was found to be smaller than that for the previously reported system (phenyl acetate), because of the presence of a sterically bulky triazine ring. Furthermore, it was suggested from the TET that the migrating hydrogen for the intramolecular [1,3] hydrogen shift directly transferred to the carbonyl oxygen (the direct intramolecular hydrogen shift) without the basic catalytic action of the adjacent triazine ring (the indirect intramolecular hydrogen shift).