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

The reaction of β-mercaptoethanol with methyl oleate in tert-butyl alcohol was investigated under photochemical conditions. The time-dependent Z/E isomerization of the unsaturated moiety as well as the thiol-adduct 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 × 10⁵ and 2.9 × 10⁵ M^-1 s^-1, respectively. For the β-elimination of the thiyl radical from the alkyl radical adduct, the rate constants were 1.7 × 10⁷ and 1.6 × 10⁸ 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.