Alkene Metathesis Pathway for Well-Defined Catalysts

4 Alkene Metathesis Pathway for Well-Defined Catalysts 

DFT calculations on the reactivity of d catalysts, carried out on simplified models of bisalkoxy imido molybdenum alkylidene complexes, suggested that the separated reactants achieve metathesis via a Chauvin-type mechanism, with a single step for the metallacyclobutane formation and one for the cycloreversion [5, 36]. Although the two metallacyclobutanes (TBP and SP) are minima of the potential energy surface, only the TBP intermediate is on the reaction pathway. Moreover, calculations showed that the preferential attack of ethene occurs trans to one of 

Recently, combined experimental/computational studies have highlighted the specificity of each system, in particular, the formation of trisubstituted alkenes 

Deactivation pathways arise from the decomposition of any of the intermediates and lead to non-reactive species (i.e., species that cannot readily re-enter the catalytic cycle). Moreover, the reaction intermediate can also lead to the formation of by-products, which may or may not be connected to deactivation processes. Computational studies of such reactions are challenging because they can involve many intermediates and reaction pathways. In addition, there is often a lack of information about such processes in terms of their nature and the spectroscopic signatures of the chemical species containing the metal center and/or of the by-products, as well as kinetic data of their mode of formation. For these reasons, studies of deactivation and by-product formation are rarely carried out, even if they are probably indispensable for a full understanding of the efficiency of a catalytic process and the rational design of better catalysts. 

For well-defined silica-supported catalysts (Y = OSi=) deactivation occurs via degraftrng, is first-order in ethene, and does not involve bimolecular decomposition pathways [42]. The formation of by-products, mainly 1-butene as a primary product and alkanes in the case of propene metathesis, when X = alkyl ligand, has also been observed [42]. Of note, the silica-supported, alkyl-alkylidene complexes deactivate faster and form larger amounts of by-products than the pyrrolyl-contarning complexes [23, 32]. 

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