Silica‐Supported Molybdenum Oxo Alkylidenes: Bridging the Gap between Internal and Terminal Olefin Metathesis

Abstract

Grafting am olybdenum oxoa lkylidene on silica (partially dehydroxylated at 700 8 8C) affords the first example of awell-defined silica-supported Mo oxoalkylidene,whichisan analogue of the putative active sites in heterogeneous Mobased metathesis catalysts.Incontrast to its tungsten analogue, which shows poor activity towards terminal olefins because of the formation of as table off-cycle metallacyclobutane intermediate,t he Mo catalyst shows high metathesis activity for both terminal and internal olefins that is consistent with the lower stability of Mo metallacyclobutane intermediates.This Mo oxo metathesis catalyst also outperforms its corresponding neutral silica-supported Mo and Wi mido analogues.Olefin metathesis is an atom-economical catalytic reaction that is used both in academia and in industry for the synthesis of ab road range of organic compounds that contain carbon = carbon double bonds.T his reaction is catalyzed by both early (molybdenum or tungsten) and late (ruthenium) transition-metal-based alkylidene complexes that perform well for aw ide range of organic substrates, which now include (for Mo) olefins that are directly functionalized with Cl, CF 3 ,orCNgroups. [1] Most of the early-transition-metal (d 0 )c atalysts share the same structural motif:(X)(Y)M( = CHR)E, [2] where X and Yare anionic ligands,Eis an imido or oxo ligand, and MisMoorW(Scheme 1). Early studies focused on generating metal oxo complexes-the proposed active sites of the industrial supported metal oxide catalysts. [3] Six-coordinate W-based oxo alkylidene complexes of the type W(O)(CHR)L 2 Cl 2 (L = ap hosphine ligand) were the first high oxidation state Wa lkylidene complexes to be prepared that metathesize both terminal and internal olefins (in the presence of at race amount of AlCl 3 ) and produce an ew alkylidene that could be observed as ac onsequence of olefin metathesis. [4] However,t he vast majority of well-defined Mo and Wc atalysts that have been developed over the last three decades contain an imido ligand that stabilizes the catalyst toward bimolecular decomposition and can be sterically and electronically modulated. Welldefined and catalytically active oxo alkylidenes based on W were first reported in 1996, [5] but it was not until 2012 that ap hosphine-free complex was developed. [6] Thec orresponding supported systems have shown unprecedented activity and stability, [7] especially in the form of cationic compounds. [8] Molecular oxo complexes contain bulky phenoxy ligands that prevent bimolecular decomposition reactions at the expense of poor metathesis activity.However, upon grafting on silica, one of the large phenoxides is replaced by ar elatively small surface siloxy ligand, thereby yielding highly active and stable supported catalysts with activities that are several orders of magnitude greater than the molecular precursor,demonstrating the beneficial effect of site isolation on silica surfaces. Scheme 1. General structures of well-defined metathesis catalysts based on a) Mo and Wimido, and b) Woxo alky...

Molecular and Silica‐Supported Mo and W d0 Imido‐Methoxybenzylidene Complexes: Structure and Metathesis Activity

“…It should be also noted that while we cannot identify the impurities mentioned, we have found that treatment of the olefin substrate with activated alumina and Selexsorb CD helps to remove them to a significant extent and considerably improves the performance of all the catalysts and the reproducibility of catalytic tests, [36] especially at low loading. [37] 1-Nonene. The supported catalysts were also tested in self-metathesis of 1-nonene as a prototypical terminal olefin.…”

Enhanced Olefin Metathesis Performance of Tungsten and Niobium Incorporated Bimetallic Silicates: Evidence of Synergistic Effects

“…Metathesis involving light alkenes is gaining importance as an atom economical route to augment the supply of certain olefinic feedstocks based on demand . Catalysts investigated widely for olefin metathesis include WO 3 , MoO 3 , and Re 2 O 7 supported on either SiO 2 or Al 2 O 3 . Because of its robustness towards poisons and relatively low cost, the WO 3 /SiO 2 catalyst has been widely used in industry to produce propene from ethylene and butenes.…”

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

“…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). [24–33] Yet, these metathesis catalysts display pronounced reactivity differences between terminal versus internal olefins, that depend on the metal and the σ‐donation ability of the X and E ligands. For example, increased activity in the metathesis of internal olefins is observed for weaker σ‐donating X‐ligands in supported metathesis catalysts bearing a donating imido ligand [26,28,31] .…”