TUNNELLING EFFECTS ON THE 1,3- AND 1,5-SIGMATROPIC HYDROGEN SHIFTS IN THE GROUND STATE OF PHOTO-FRIES REARRANGED INTERMEDIATES OF PHENYL ACETATE STUDIED BY LASER FLASH PHOTOLYSIS

摘要

The rate constants for the 1,3- and 1,5-sigmatropic hydrogen shifts in the ground state of the photo-Fries rearranged intermediates of phenyl acetate produced by laser flash photolysis at 266 nm were directly measured in several solvents. The rate constant for the intramolecular 1,3-hydrogen shift (3.6 s(-1)) is greater than that for the 1,5-hydrogen shift (6.5 x 10(-2) s(-1)) in the ground state in methylcyclohexane (MCH) at 293 K, contrary to the expectation by the Woodward-Hoffmann rule, showing that the heteroatom of the corresponding carbonyl oxygen plays an important role for the intramolecular hydrogen shifts. On the basis of the experimental results of temperature and isotope effects, it is shown that the intramolecular 1,3-hydrogen (or deuterium) shift in MCH proceeds via tunnelling processes at two vibrational energy levels: E = 0 (v = v(0)) and E = E(v) (= 3.9 kcal mol(-1) for the hydrogen shift or 4.4 kcal mol(-1) for the deuterium shift) (v = v(1)) under the experimental condition. The temperature and isotope effects on the 1,3-shifts can be elucidated by the calculated rates according to the tunnel effect theory proposed by Formosinho. The enhancement of the rates for the 1,3- and 1,5-sigmatropic shifts in polar solvents, especially in alcohols, is caused intermolecularly by a basic catalysis of the solvents. It is shown that the 1,3- or 1,5-sigmatropic hydrogen shift proceeds via the intramolecular process at a low concentration of phenyl acetate (similar to 2 X 10(-3)M) in nonpolar MCH. [References: 46]