Neutral and Cationic Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Complexes: Reactivity in Selected Olefin Metathesis Reactions and Immobilization on Silica

Abstract

The synthesis and single-crystal X-ray structures of the novel molybdenum imido alkylidene N-heterocyclic carbene complexes [Mo(N-2,6-Me2C6H3)(IMesH2)(CHCMe2Ph)(OTf)2] (3), [Mo(N-2,6-Me2C6H3)(IMes)(CHCMe2Ph)(OTf)2] (4), [Mo(N-2,6-Me2C6H3)(IMesH2)(CHCMe2Ph)(OTf){OCH(CF3)2}] (5), [Mo(N-2,6-Me2C6H3)(CH3CN)(IMesH2)(CHCMe2Ph)(OTf)](+)BArF(-) (6), [Mo(N-2,6-Cl2C6H3)(IMesH2)(CHCMe3)(OTf)2] (7) and [Mo(N-2,6-Cl2C6H3)(IMes)(CHCMe3)(OTf)2] (8) are reported (IMesH2=1,3-dimesitylimidazolidin-2-ylidene, IMes=1,3-dimesitylimidazolin-2-ylidene, BArF(-)=tetrakis-[3,5-bis(trifluoromethyl)phenyl] borate, OTf=CF3SO3(-)). Also, silica-immobilized versions I1 and I2 were prepared. Catalysts 3-8, I1 and I2 were used in homo-, cross-, and ring-closing metathesis (RCM) reactions and in the cyclopolymerization of α,ω-diynes. In the RCM of α,ω-dienes, in the homometathesis of 1-alkenes, and in the ethenolysis of cyclooctene, turnover numbers (TONs) up to 100,000, 210,000 and 30,000, respectively, were achieved. With I1 and I2, virtually Mo-free products were obtained (<3 ppm Mo). With 1,6-hepta- and 1,7-octadiynes, catalysts 3, 4, and 5 allowed for the regioselective cyclopolymerization of 4,4-bis(ethoxycarbonyl)-1,6-heptadiyne, 4,4-bis(hydroxymethyl)-1,6-heptadiyne, 4,4-bis[(3,5-diethoxybenzoyloxy)methyl]-1,6-heptadiyne, 4,4,5,5-tetrakis(ethoxycarbonyl)-1,7-octadiyne, and 1,6-heptadiyne-4-carboxylic acid, underlining the high functional-group tolerance of these novel Group 6 metal alkylidenes.

Origin and Use of Hydroxyl Group Tolerance in Cationic Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Catalysts

“…We already have shown that cationic molybdenum imido alkylidene NHC complexes are highly active catalysts in various olefin metathesis reactions . Here we prepared a wide range of complexes with the general formula [Mo( N R′)(CHCMe 2 R)(NHC)(X)][B(Ar F ) 4 ] (Figure , X=anionic ligand).…”

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

“…Both catalysts tolerate ethers, thioethers, and esters under biphasic reaction conditions. 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.…”

High Oxidation State Molybdenum N‐Heterocyclic Carbene Alkylidyne Complexes: Synthesis, Mechanistic Studies, and Reactivity

“…Its formation is proposed to be a result of the smaller buried volume for Mo‐1 compared Mo‐4 , resulting in easier access of substrates to the metal core. Thus, cation formation is believed to occur simultaneously with substrate (or CD 3 CN) coordination, similar to the formation of cationic species in Mo and W imido/oxo‐alkylidene NHC complexes –. With this mechanistic understanding at hand, the slower reactions rates of Mo‐1 – Mo‐4 compared to complex I are comprehensible, because tetracoordinated species I requires no dissociation of the NHC or an alkoxide ligand.…”