TUNNELING EFFECTS ON THE 1,3-SIGMATROPIC AND 1,5-SIGMATROPIC HYDROGENSHIFTS IN THE GROUND-STATE OF PHOTO-FRIES REARRANGED INTERMEDIATES OFPHENYL ACETATE STUDIED BY LASER FLASH-PHOTOLYSIS

The rate constants for the 1,3- and 1,5-sigmatropic hydrogen shifts in the ground state of the photo-Fries rearranged intermediates of pheny l acetate produced by laser flash photolysis at 266 nm were directly m easured 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-hydrog en 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 th e basis of the experimental results of temperature and isotope effects , it is shown that the intramolecular 1,3-hydrogen (or deuterium) shif t in MCH proceeds via tunnelling processes at two vibrational energy l evels: E = 0 (v = v(0)) and E = E(v) (= 3.9 kcal mol(-1) for the hydro gen shift or 4.4 kcal mol(-1) for the deuterium shift) (v = v(1)) unde r the experimental condition. The temperature and isotope effects on t he 1,3-shifts can be elucidated by the calculated rates according to t he tunnel effect theory proposed by Formosinho. The enhancement of the rates for the 1,3- and 1,5-sigmatropic shifts in polar solvents, espe cially in alcohols, is caused intermolecularly by a basic catalysis of the solvents. It is shown that the 1,3- or 1,5-sigmatropic hydrogen s hift proceeds via the intramolecular process at a low concentration of phenyl acetate (similar to 2 X 10(-3)M) in nonpolar MCH.