Cationic Silica‐Supported N‐Heterocyclic Carbene Tungsten Oxo Alkylidene Sites: Highly Active and Stable Catalysts for Olefin Metathesis

Abstract

Designing supported alkene metathesis catalysts with high activity and stability is still a challenge, despite significant advances in the last years. Described herein is the combination of strong σ-donating N-heterocyclic carbene ligands with weak σ-donating surface silanolates and cationic tungsten sites leading to highly active and stable alkene metathesis catalysts. These well-defined silica-supported catalysts, [(≡SiO)W(=O)(=CHCMe2 Ph)(IMes)(OTf)] and [(≡SiO)W(=O)(=CHCMe2 Ph)(IMes)(+) ][B(Ar(F) )4 (-) ] [IMes=1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene, B(Ar(F) )4 =B(3,5-(CF3 )2 C6 H3 )4 ] catalyze alkene metathesis, and the cationic species display unprecedented activity for a broad range of substrates, especially for terminal olefins with turnover numbers above 1.2 million for propene.

Molybdenum and Tungsten Imido Alkylidene N‐Heterocyclic Carbene Catalysts Bearing Cationic Ligands for Use in Biphasic Olefin Metathesis

“…Finally, a comparison with the reactivity of a silica‐supported version, [Mo{ N ‐2,6‐(Me 2 )C 6 H 3 }(CHCMe 2 Ph)(2‐{silica‐ O ‐methylene}‐imidazolin‐2‐ylidene)](OTf) 2 ( I 2 ) revealed superior activity of both 3 and 5 . Compared to a silica‐based version of a tungsten oxo alkylidene NHC catalyst, [(≡SiO)W(=O)(=CHCMe 2 Ph)(IMes)][B(Ar F ) 4 ], again similar activity was observed, at least for diallyl ether and diallyl diphenylsilane.…”

Dendritic polarizing agents for DNP SENS

“…Furthermore, to validate the hypothesis that PyPolB-D2[G3] prevents contact of the radical with the surface sites, we studied a highly reactive single-site cationic alkene metathesis heterogeneous catalyst ( W 13 C @SiO 2 ), 47 for which conventional binitroxide radicals do not yield to any DNP effect. This is illustrated in Fig.…”

“…We also illustrate the advantage of dendritic PAs by characterizing a reactive material, a recently developed cationic heterogeneous alkene metathesis catalyst. 47 Notably, for this particular catalyst, the currently favored “conventional” biradical, TEKPol, showed no DNP enhancement.…”

Bridging the Gap between Industrial and Well‐Defined Supported Catalysts

“…[519][520][521][522][523][524][525][526] Supporteda lkylidynes were also shownt o be active metathesis catalystp recursors. Preliminaryr eports appeared in early2 000s, [541,542] andt he applicationo fm odern characterization techniques eventually allowedf or precise controlofthe structures of thesesurface speciesand gave rise to theg enerationo fr emarkablya ctive well-defined silica-supportedMo- [543][544][545][546][547][548][549] andW-imido-alkylidenes [550][551][552][553][554][555] andmore recently W-oxo-alkylidenes [556][557][558][559][560] (Section 7.3.2.4).I nt he 1970sa nd 1980st he groups of Yermakov [561][562][563][564] andI wasawa [565,566] were thef irst to useo rganometallicp recursors (Mo(C 3 H 5 ) 4 and W(C 4 H 7 ) 4 )t op repare well-defined supported Mo and Woxides relevant to industrial systems and discriminate the activities of different oxidation states.T hese studies suggested that neither fully oxidized Mo VI /W VI nor highly reduced Mo II /W II species were responsible for the metathesis activity and that active sites likely arose from the oxidation state + IV.Recent advances in describing the redox chemistry of well-defined supported W-oxo sites in relation to alkene metathesis are discussed in Section 7.3.2.5. [441] Supportedt ungstenh ydridesr epresent as pecial classo fi nsitug enerated olefin m...…”

“…[482,486] Indeed, replacing the aryloxide by an isoelectronic arylthiolate ligand (Scheme 43 d) led to as ignificant improvement in the catalytic activity towards terminal olefins. [560] 7.3.2.5. [560] 7.3.2.5.…”