Tungsten imido complexes, olefin metathesis

In the case of other Group 6 metals, the polymerization of olefins has attracted little attention. Some molybdenum(VI) and tungsten(VI) complexes containing bulky imido- and alkoxo-ligands have been mainly used for metathesis reactions and the ring-opening metathesis polymerization (ROMP) of norbornene or related olefins [266-268]. Tris(butadiene) complexes of molybdenum ) and tungsten(O) are air-stable and sublimable above 100°C [269,270]. At elevated temperature, they showed catalytic activity for the polymerization of ethylene [271]. 

Schrock, R. R., Hoveyda, A. H. Molybdenum and tungsten imido alkylidene complexes as efficient olefin-metathesis catalysts. Angew. Chem., Int. Ed. Engl. 2003, 42, 4592-4633. 

Schrock, R. R. Recent advances in olefin metathesis by molybdenum and tungsten imido alkylidene complexes. J. Mol. Catal. A Chemical... 

Asymmetric syntheses of natural products cyclization reactions using molybdenum and tungsten imido alkylidene complexes as efficient olefin-metathesis catalysts 03AG(E)4592. 

The development of well-defined ROMP initiators has expanded the scope of the metathesis reaction, permitting the controlled polymerisation of highly functionalised monomers. Schrock s well-defined olefin metathesis initiators became available in the mid 1980s and they were based on alkoxy imido complexes of tungsten and molybdenum of general... 

Oxo-alkyhdene complexes (e.g., W(0)(CHR)X2, where X is a chloride, alkoxide, etc.) are likely to be the catalysts in at least some of the classical olefin metathesis systems [65]. However, few tungsten oxo-alkylidene complexes have been isolated and characterized [66]. Of note, several have now been prepared that contain one or more terphenoxide ligands [41, 42]. Recent theoretical studies have focused on the subtle differences between imido alkylidene complexes and oxo-alkylidene complexes in olefin metathesis reactions [7f]. 

In the preparative section 3.2 devoted to metal-carbene complexes, it is shown how the a-elimination reaction from high oxidation state early-transition-metal-alkyl complexes is one of the general methods of synthesis of Schrock s Ta and Nb alkylidene complexes. The other direction, formation of an alkylidene from an alkylidyne complex, can also be a valuable route to metal alkylidenes. For instance, Schrock s arylamino-tungsten-carbynes can be isomerized to imido-tungsten-carbene by using a catalytic amount of NEts as a base. These compounds are precursors of olefin metathesis catalysts by substitution of the two Cl ligands by bulky alkoxides (dimethoxyethane then decoordinates for steric reasons), and this route was extended to Mo complexes ... 

R.R. Schrock, A.H. Hoveyda - Molybdenum and Tungsten Imido Alkylidene Complexes as Efficient Olefin-Metathesis Catalysts,... 

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