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Ring‐Closing Metathesis Reactions: Interpretation of Conversion–Time Data
Author(s) -
Thiel Vasco,
Wannowius KlausJürgen,
Wolff Christiane,
Thiele Christina M.,
Plenio Herbert
Publication year - 2013
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.201204150
Subject(s) - chemistry , substrate (aquarium) , metathesis , reaction rate constant , catalysis , reaction rate , salt metathesis reaction , kinetic energy , kinetics , stereochemistry , organic chemistry , polymerization , physics , ecology , quantum mechanics , polymer , biology
Abstract Conversion–time data were recorded for various ring‐closing metathesis (RCM) reactions that lead to five‐ or six‐membered cyclic olefins by using different precatalysts of the Hoveyda type. Slowly activated precatalysts were found to produce more RCM product than rapidly activated complexes, but this comes at the price of slower product formation. A kinetic model for the analysis of the conversion–time data was derived, which is based on the conversion of the precatalyst (Pcat) into the active species (Acat), with the rate constant k act , followed by two parallel reactions: 1) the catalytic reaction, which utilizes Acat to convert reactants into products, with the rate k cat , and 2) the conversion of Acat into the inactive species (Dcat), with the rate k dec . The calculations employ two experimental parameters: the concentration of the substrate ( c (S)) at a given time and the rate of substrate conversion (−d c (S)/d t ). This provides a direct measure of the concentration of Acat and enables the calculation of the pseudo‐first‐order rate constants k act , k cat , and k dec and of k S (for the RCM conversion of the respective substrate by Acat). Most of the RCM reactions studied with different precatalysts are characterized by fast k cat rates and by the k dec value being greater than the k act value, which leads to quasistationarity for Acat. The active species formed during the activation step was shown to be the same, regardless of the nature of different Pcats. The decomposition of Acat occurs along two parallel pathways, a unimolecular (or pseudo‐first‐order) reaction and a bimolecular reaction involving two ruthenium complexes. Electron‐deficient precatalysts display higher rates of catalyst deactivation than their electron‐rich relatives. Slowly initiating Pcats act as a reservoir, by generating small stationary concentrations of Acat. Based on this, it can be understood why the use of different precatalysts results in different substrate conversions in olefin metathesis reactions.