Etude théorique des réactions d'hydrosilylation des cétones par un catalyseur cuivre-diphosphine
(2012)
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- Supervisors
- Peeters, Daniel
- Abstract
- (en) This work consists in a theoretical study of the hydrosilylation reactions of ketones by a copper-diphosphine catalyst. The active species in these systems is a copper hydride coordinated by a diphosphine ligand. While the reaction is well known to experimentalists, many questions remain. Our work consists in an analysis of the postulated catalytic cycle and a study of the various factors that affect it. For this, we use quantum chemical methods and more precisely the density functional theory (DFT) to determine the different species occurring throughout the various steps. For each step, the geometric structure of extrema, the electronic distribution (NBO method) and the thermochemistry of the reaction have been determined using model compounds. The reaction process occurs in two parts: the formation of the catalyst and the catalytic cycle. The reaction between the pre-catalyst FCu[diphosphine] and the silane leads to the catalyst. The catalytic cycle is made up of two steps: the reduction of the ketone by the catalyst followed by a regeneration of the catalyst and the release of a silyl ether. These steps would be realized by an asynchronous concerted mechanism in which the transfer of the copper bounded atom is more advanced than the transfer to the metal centre. The nature and structure of the diphosphine ligand, ketone and hydrosilylant agent are the parameters influencing the process. The stabilization of the various species and the height of activation barriers for each step are a compromise between stabilizing electronic interactions (hydrogen like bond) and destabilizing steric interactions. The nature of the biaryl skeleton and substituents on the phosphorus atoms of the diphosphine ligand are very important. These determine the stability of complexes and the activation barriers. In addition, the nature of the ligand also determines the value of the bite angle P-Cu-P which influences the activation barrier of the first step of the catalytic cycle. The nature of the ketone mainly influences the first step of the catalytic cycle while the hydrosilylant agent modifies the second stage and the formation of the catalyst. The presence of a monophosphine ligand in the coordination sphere of copper allows the copper to keep its tetrahedral configuration. Indeed, the distance between the ligand and the copper atom varies along steps. The movement of the monophosphine facilitates the crossing of the transition state by an in and out motion which stabilizes the products. Finally, kinetics of the catalytic cycle are also discussed. This appear to be relatively complex, the rate-determining step depends on the nature of the reactants.
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Citations
Gathy, T. (2012). Etude théorique des réactions d’hydrosilylation des cétones par un catalyseur cuivre-diphosphine. https://hdl.handle.net/2078.5/163336