Group 6 metal alkylidene and alkylidyne N-heterocyclic carbene complexes for olefin and alkyne metathesis

Olefin metathesis: what have we learned about homogeneous and heterogeneous catalysts from surface organometallic chemistry?

“…This concept, that showed the importance of dissymmetry at the metal centre, was soon after generalized to d 0 Mo and W catalysts, [M(E)( CHR)(X)(Y)] with M(E) = Mo/W(imido/oxo) ( Scheme 1 ), 68,69 and shown to be a predictive guiding principle for the metathesis catalyst design not only in heterogeneous but also in homogeneous systems such as MAP, 25,70–78 MAC, 79,80 or NHC-stabilized cationic catalysts. 81–85 …”

“…This concept, that showed the importance of dissymmetry at the metal centre, was soon aer generalized to d 0 Mo and W catalysts, [M(E)(] CHR)(X)(Y)] with M(E) ¼ Mo/W(imido/oxo) (Scheme 1), 68,69 and shown to be a predictive guiding principle for the metathesis catalyst design not only in heterogeneous but also in homogeneous systems such as MAP, 25,[70][71][72][73][74][75][76][77][78] MAC, 79,80 or NHC-stabilized cationic catalysts. [81][82][83][84][85] Metallacyclobutane intermediates of W-based catalysts are usually more stable than those of their Mo-based congeners and are oen observed; the Mo analogues are rarely reported for either homogeneous or well-dened silica-supported systems. 25,86 Computational results indicate that weaker s-donating ligands X and Y lead to more stable metallacyclobutane intermediates, and the isomerization between the two metallacyclobutane isomers (TBP and SP) via a turnstile process tends to be more facile for W as compared to Mo and for weaker s-donor ligands.…”

“…This principle has been recently exploited in designing Mo and W catalysts containing N-heterocyclic carbenes (NHC)-very strong s-donor ligands (see Section 2.2). [81][82][83][84][85] The effect of the ancillary X ligand on the relative stability of metallacyclobutanes derived from the reaction of such silica-supported W imido alkylidenes with ethylene has been studied in detail by solid-state NMR spectroscopy. With X ¼ OtBu, a strong s-donor ligand, the SP metallacyclobutane was formed exclusively.…”

Molecular-level insight in supported olefin metathesis catalysts by combining surface organometallic chemistry, high throughput experimentation, and data analysis

“…Research in academic and industrial laboratories over the last several decades has produced impressive advances in the field of alkene metathesis. 1 – 6 This research has helped establishing detailed structure–activity relationships (SAR) for well-defined Mo-, W- and Ru-based molecular catalysts, 7 , 8 and thereby aided in the rational development of catalytic systems with improved activity, selectivity and stability. 9 – 11 Within this, high-throughput experimentation (HTE) can accelerate building robust SAR as it allows for a rapid and systematic acquisition of data on large libraries of compounds and formulations enabling the identification of catalysts.…”

Silica‐Supported Cationic Tungsten Imido Alkylidene Stabilized by an N‐Heterocyclic Carbene Ligand Boosts Activity and Selectivity in the Metathesis of α‐Olefins

“…Alkene metathesis is a robust and atom‐economic synthetic method for the creation of carbon‐carbon double bonds; it has thus found ample use in both academia and industry [1–3] . Over the last decades, substantial efforts have been put in developing efficient molecular catalysts, [4–15] but immobilized analogs have received increasing attention because of their benefits for process intensification [16,17] and their improved performance with the development of well‐defined supported systems, mostly prepared through surface organometallic chemistry (SOMC) [18–23] . SOMC, a molecular approach to generate well‐defined heterogeneous catalysts, in which the surface is exploited as a ligand to covalently anchor molecular complexes, has been particularly successfully employed for the preparation of silica‐supported Schrock‐type metathesis catalysts of the general formula [(≡SiO)M(E)(=CHR)(X)] (M=Mo or W, E=NR or O), that display high activity, selectivity and stability with performances often surpassing those of their homogeneous counterparts ( Scheme 1).…”