The kinetics of Z/E isomerization of methyl oleate catalyzed by photogenerated thiyl radicals

The reaction of beta-mercaptoethanol with methyl oleate in tert-butyl alcohol was investigated under photochemical conditions. The time-dependent VIE isomerization of the unsaturated moiety as well as the thiol-odduct formation were analyzed on the basis of radical generation rates and established rate constants. This provides precise room-temperature rate constants for the reversible thiyl radical addition to both Z and E isomers. The rate constants for HOCH2CH2S addition to the Z and E forms were found to be 1.6 x 10(5) and 2.9 x 10(5) M-1 s(-1), respectively. For the beta-elimination of the thiyl radical from the alkyl radical adduct, the rate constants were 1.7 x 10(7) and 1.6 x 10(8) s(-1), the faster associated with the formation of the E isomer. DFT-BB1K calculations on 2-butene assign this large preference of fragmentation for the E isomer to a lower activation barrier for the formation of the E transition state from the equilibrium radical adduct structure.