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 HOCH 2 CH 2 S . addition to the Z and E forms were found to be 1.6×10 5 and 2.9×10 5 M -1 s -1 , respectively. For the β-elimination of the thiyl radical from the alkyl radical adduct, the rate constants were 1.7×10 7 and 1.6×10 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.