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A kinetic study of the dodecanethiol-catalyzed cis/trans isomerization of methyl oleate (cis-2) without added initiator was performed by focusing on the initiation of the radical chain reaction. The reaction orders of the rate of isomerization were 2 and 0.5 for 1 and cis-2, respectively, and an overall kinetic isotope effect k(H)/k(D) of 2.8 was found. The initiation was shown to be a complex reaction. The electron-donor/-acceptor (EDA) complex of dodecanethiol (1) and cis-2 formed in a pre-equilibrium reacts with thiol 1 to give a stearyl and a sulfuranyl radical through molecule-assisted homolysis (MAH) of the sulfur-hydrogen bond. Fragmentation of the latter gives the thiyl radical, which catalyzes the cis/trans isomerization. A computational study of the EDA complex, MAH reaction, and the sulfuranyl radical calculated that the activation energy of the isomerization was in good agreement with the experimental result of E(A)=82 kJ  M(-1). Overall, the results may explain that the thermal generation of thiyl radicals without any initiator is responsible for many well-known thermally initiated addition reactions of thiol compounds to alkenes and their respective polymerizations and for the low shelf-life stability of cis-unsaturated thiol compounds and of mixtures of alkenes and thiol compounds.

Keywords: Diels-Alder reaction / Cyclization / Methyl calendulate / Methyl α-eleostearate / Maleic anhydride / Renewable feedstockThe thermal, solvent-free addition of maleic anhydride to methyl calendulate (methyl 8,10-trans,12-cis-octadecatrienoate) (1a) occurs with very high regio-and stereoselectivity at C-8 and C-11 of the fatty compound giving the endo-DielsAlder adduct 4 with retention of the cis-configured double bond. Analogously, the Diels-Alder addition of maleic anhydride to methyl α-eleostearate (methyl 9-cis,11,13-trans-octadecatrienoate) (2a) takes place regio-and stereoselectively