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Initiation of Radical Chain Reactions of Thiol Compounds and Alkenes without any Added Initiator: Thiol‐Catalyzed cis / trans Isomerization of Methyl Oleate
Author(s) -
Biermann Ursula,
Butte Werner,
Koch Rainer,
Fokou Patrice A.,
Türünç Oǧuz,
Meier Michael A. R.,
Metzger Jürgen O.
Publication year - 2012
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201103252
Subject(s) - isomerization , chemistry , thiol , homolysis , photochemistry , catalysis , radical , kinetic isotope effect , organic chemistry , physics , deuterium , quantum mechanics
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.