Tungsten catalysts alkylidenation

On the basis of the fact that tungsten(VI) alkylidene complexes will metathesize olefins one might predict that acetylenes should be metathesized by tungsten(VI) alkylidyne complexes (29). Acetylene metathesis is not unknown, but the catalysts are inefficient and poorly understood (30, 31). 

The report by Basset and co-workers on the metathesis of sulphur-containing alkenes using a tungsten alkylidene complex, mentioned previously for the acyclic cross-metathesis reaction (see Sect. 2.2), also contained early examples of ring-opening cross-metathesis of functionalised alkenes [20]. Allyl methyl sulphide was reacted with norbornene in the presence of the tungsten catalyst 5, to yield the desired ring-opened diene 35 (Eq. 29). 

Around 1990, Richard Schrock developed versatile molybdenum and tungsten catalysts for olefin metathesis that tolerate a wide range of functional groups in the alkylidene fragments of the olefins. The Schrock catalyst shown in Figure 8-10a is now commercially available. The Schrock catalysts tend to be air- and moisture-sensitive, which limits their use in commercial processes. 

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. 

Only two reports have appeared on the formation and properties of linear unsaturated polycarbostannanes and this is a fruitful area for continued study. Such materials have been synthesized by acylic diene metathesis (ACDMET) of bis(4-pentenyl)dibutyltin either with Schrock s Mo-alkylidene, Mo(=CHMe2Ph)( A-2,6-C6H3 - i -Pr2)(OCMe(CF3 )2)2, or with the aryloxo-tungsten catalyst W(0)Cl2(0-2,6-C6H3-Br2)2, Equation 3.8.8). ... 

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

Molybdenum and Tungsten Imide Alkylidene Complexes as Efficient Olefin-Metathesis Catalysts Schrock. R.R. Hovevda. A.H. Angew. Chem. Int. Ed. 2003, 42,4592. 

Since adamantyfimido alkylidene complexes of molybdenum have been the catalysts of choice in some metathesis reactions, and since tungsten arylimido alkylidene complexes are often more selective for forming Z products than the analogous molybdenum complexes, a route to tungsten-based adamantyl and t-butyfimido catalysts was sought. 

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]. 

J. R. Missert, and W. J. Youngs, Tungsten-oxo alkylidene complexes as olefin metathesis catalysts and the crystal structure of W(0)(CHCMe3)(PEt3)Cl2, J. Amer. Chem. Soc. 102 4515 (1980). 

Olefin metathesis is the transition-metal-catalyzed inter- or intramolecular exchange of alkylidene units of alkenes. The metathesis of propene is the most simple example in the presence of a suitable catalyst, an equilibrium mixture of ethene, 2-butene, and unreacted propene is obtained (Eq. 1). This example illustrates one of the most important features of olefin metathesis its reversibility. The metathesis of propene was the first technical process exploiting the olefin metathesis reaction. It is known as the Phillips triolefin process and was run from 1966 till 1972 for the production of 2-butene (feedstock propene) and from 1985 for the production of propene (feedstock ethene and 2-butene, which is nowadays obtained by dimerization of ethene). Typical catalysts are oxides of tungsten, molybdenum or rhenium supported on silica or alumina [ 1 ]. 

These limitations were overcome with the introduction of the well-defined, single-component tungsten and molybdenum (14) alkylidenes in 1990. (Fig. 8.4).7 Schrock s discoveiy revolutionized the metathesis field and vastly increased die utility of this reaction. The Schrock alkylidenes are particularly reactive species, have no side reactions, and are quite effective as polymerization catalysts for both ROMP and ADMET. Due to the oxophilicity of molybdenum, these alkylidenes are moisture and air sensitive, so all reactions using these catalysts must be performed under anaerobic conditions, requiring Schlenk and/or glovebox techniques. 

When alkenes are treated with certain catalysts (most often tungsten, molybdenum, or rhenium complexes), they are converted to other alkenes in a reaction in which the alkylidene groups (R RX=) have become interchanged by a process schematically illustrated by the equation ... 

In retrospect it is not surprising that the niobium and tantalum alkylldene complexes we prepared are not good metathesis catalysts since these metals are not found in the "classical" olefin metathesis systems (2). Therefore, we set out to prepare some tungsten alkylidene complexes. The first successful reaction is that shown in equation 6 (L = PMe3 or PEt3) (11). These oxo... 

Scheme 2. Two tungsten alkylidene catalysts that contain a metal-aryl bond... 

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