Ring‐Closing Metathesis Reactions: Interpretation of Conversion–Time Data

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 (-dc(S)/dt). 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.

The Key Role of the Nonchelating Conformation of the Benzylidene Ligand on the Formation and Initiation of Hoveyda–Grubbs Metathesis Catalysts

“…Mechanistic studies play a dominant role in development of catalytic systems, and olefin metathesis is one of the main areas of the frontier research 1. For the family of Hoveyda–Grubbs complexes (e.g., 1 ), numerous studies that focused on the catalytic cycle2 revealed the importance of the initiation step as a key process3 responsible for release of active species, and involved in the bimolecular deactivation pathway 4. In kinetic studies Plenio demonstrated that the initiation proceeds by dissociative (D) or interchange mechanism (I a ), depending on structure of the substrate and coordinating alkoxy group of the catalyst 5.…”

The Influence of Structure on Reactivity in Alkene Metathesis

“…Relative rates of RCM of several substrates, as measured by Plenio. 175 The deconvolution of kinetic data, with the aim of quantifying rate constants for important steps, remains a considerable challenge in the quantitative study of metathesis chemistry. Insights into how the rates of key steps of the metathesis mechanism are affected by catalyst and substrate structure are potentially very valuable, and could contribute to the design of reactions and pre-catalysts.…”

“…Ethene is not treated at any point in the model, due to the complexities of modelling both the chemical and physical processes in which it is involved. A more detailed and useful kinetic model for RCM remains to be constructed.Plenio has developed an alternative way in which to treat concentration/time data from RCM reactions mediated by Hoveyda-type complexes 175. The authors conducted a series of kinetic experiments (at 303 -323 K in toluene-d8) using a range of pre-catalysts, at different concentrations, and with different pre-catalyst loadings.…”

Studies on synthesis of quinonylidene Hoveyda‐type complexes

“…As steric hindrance increases for a series of ligands, the destabilization reaches the limit required for complex isolation, as displayed by 4 (Scheme ). At the same time the presence of sterically demanding NHC ligand (SiPr) acts in the opposite direction, stabilizing the complexes, and interplay of both factors controls activity profiles of the metathesis catalysts …”