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

Abstract

Dedicated to Prof. Antonio Togni for his contribution to coordination and fluorine chemistry A well-defined silica-supported cationic W imido alkylidene was prepared through surface organometallic chemistry. This catalyst shows preferential activity towards αover internal olefins, which is atypical for W-based catalysts, but consistent with the strong σ-donating ability of the NHC ancillary ligand. Complementing the studies on tungsten-based d 0 metathesis catalysts, the silica-supported cationic W imido alkylidene displays the highest activity among W imido catalysts for α-olefins and shows improved selectivity for this class of olefins compared to Mo-based catalysts.

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

“…3d ). 121 Similarly, for the oxo systems replacement of the aryloxide ligand in [( SiO)W( O)( CH t Bu)(OHMT)] by a more σ-donating thiolate (SHMT) gives a substantially more active catalyst ( Fig. 3d ).…”

Olefin Metathesis Catalysts Generated In Situ from Molybdenum(VI)‐Oxo Complexes by Tuning Pendant Ligands

“…Metathesis from these systems initiates rapidly through cross‐metathesis of the starting alkylidene (usually a neopentylidene (R= t Bu), neophylidene (R=CMe 2 Ph) or, more recently, adamantylidene) and the olefin substrate [16–21] . These catalysts display very high turnover frequencies (TOFs) and turnover numbers (TONs) when the right ligand set is chosen, with often even higher activities upon immobilization on an oxide support [17,19,22,23] . Detailed studies have shown that, even with well‐defined high oxidation state alkylidenes, low valent Mo(IV) olefin complexes or dimeric compounds are generated during metathesis; [24,25] these have been shown to display poor albeit measurable activity.…”

Boosting the Metathesis Activity of Molybdenum Oxo Alkylidenes by Tuning the Anionic Ligand σ Donation

“…Since its early mechanistic proposal, continuous efforts have been directed at gaining a detailed understanding of what makes specific metal alkylidenes good for this catalytic reaction . For instance, for early-transition-metal (d 0 ) metathesis catalysts that share a common structural motif(X)­(Y)­M­(CHR)­E (M = Mo or W, E = imido or oxo ligand, and X and Y are anionic ligands)each ligand has a tremendous influence on the stability of the reaction intermediates and the overall catalytic activity. , In that context, surface organometallic chemistry, , in combination with computational chemistry, has played a key role in the identification and understanding of the effect of each ligand . In particular, these studies, complemented by molecular chemistry, have shown that (i) the trigonal-bipyramidal (TBP) isomer is a key intermediate in the metathesis pathway while the square-pyramidal (SP) isomer is usually more stable and corresponds to an off-cycle intermediate and (ii) the anionic ligands play a major role in the stability of the TBP and SP metallacyclobutane intermediates and, in turn, drive the overall catalytic performance (Scheme A) .…”