title: Initiation of radical chain reactions of thiol compounds and alkenes without any added initiator: thiol-catalyzed cis/trans isomerization of methyl oleate.
abstract: 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.
authors.affiliation: Institute of Pure and Applied Chemistry, University of Oldenburg, Carl-von-Ossietzky Str. 9-11, 26129 Oldenburg, Germany.
authors.forename: Ursula
authors.initials: U
authors.lastname: Biermann
authors.forename: Werner
authors.initials: W
authors.lastname: Butte
authors.forename: Rainer
authors.initials: R
authors.lastname: Koch
authors.forename: Patrice A
authors.initials: PA
authors.lastname: Fokou
authors.forename: Oǧuz
authors.initials: O
authors.lastname: Türünç
authors.forename: Michael A R
authors.initials: MA
authors.lastname: Meier
authors.forename: Jürgen O
authors.initials: JO
authors.lastname: Metzger
mesh_terms: D000475:Alkenes / Q000737:chemistry*; D002384:Catalysis; D008956:Models, Chemical; D015394:Molecular Structure; D009829:Oleic Acids / Q000737:chemistry*; D013237:Stereoisomerism; D013438:Sulfhydryl Compounds / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D000475:Alkenes; D009829:Oleic Acids; D013438:Sulfhydryl Compounds; C005576:methyl oleate