DIRECT OBSERVATION OF DIENOLS PRODUCED BY PHOTOCHEMICAL ENOLIZATION OF ALPHA,BETA-UNSATURATED KETONES - RATES AND ACTIVATION PARAMETERS FOR DIENOL REKETONIZATION VIA A 1,5-HYDROGEN SHIFT

The production of stable solutions of Z-dienols by ultraviolet light irradiation of α,β-unsaturated ketones at low temperatures (ca. -76°C) in d4-methanol is reported. The rates of reketonisation of the dienols via a 1,5-sigmatropic hydrogen shift were determined at various temperatures between - 43°C and + 2°C by monitoring the proton nmr spectra of the dienols. From the data the activation parameters for the reaction were calculated. For the dienol Z-2-hydroxy-4-methyl-2,4-pentadiene, 2, derived from photoenolisation of 4-methyl-3-penten-2-one, 1, the activation energy from the Arrhenius plot is 62 ± 4 kJ/mol, and the activation entropy and enthalpy from the Eyring plot are -87 ± 15 J/mol K and 60 ± 4 kJ/mol, respectively. For the dienol Z-4-tert-butyl-2-hydroxy-2,4-pentadiene, 4, obtained from photoenolisation of 4,5,5-trimethyl-3-hexen-2-one, 3, the activation energy, entropy, and enthalpy were found to be 47 ± 5 kJ/mol, - 135 ± 19 J/mol K, and 45 ± 5 kJ/mol, respectively. Copyright © 2014 - Canadian Science Publishing.