Tungsten olefin complexes

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]. 

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... 

Similarly, neither zirconium, tantalum, molybdenum, nor tungsten carbene complexes have been applied extensively by organic chemists for carbonyl olefination [609,727-729], probably because of the difficulty of their preparation and the high price of some of these compounds. These reagents can, however, have appealing chemo- and stereo-selectivity (Table 3.11). 

The only direct evidence for the presence of metal-carbene-olefin intermediates in catalytic metathesis systems comes from a study of the interaction of the tungsten cyclopentylidene complex 27 with cycloalkenes such as cycloheptene 28 in CD2CI2. When these are mixed at —96 °C and the temperature raised to between —53 and —28 °C, no polymerization occurs but the 13C NMR spectrum contains additional resonances which may be assigned to the metal-carbene-olefin complex 29. The line intensities show that the equilibrium 7 moves to the right as the temperature is lowered120. 

Cyclopropanation. These carbencs are particularly useful for cyclopropanation of electron-poor olefins. The reaction occurs under milder conditions and at a faster rate with molybdenum carbencs than with chromium- or tungsten-derived complexes. This cyclopropanation has been used to trap a molybdenum vinylearbene generated by intra-... 

Many stable tungstacyclobutane complexes are known, but few will initiate the metathesis of internal olefins or ROMP of cyclic olefins. Yet many will undergo exchange reactions with ethene or terminal olefins by a mechanism which must involve dissociation to a tungsten carbene complex. A great deal can therefore be learnt about the olefin metathesis mechanism from a study of such reactions. The following is a short summary. 

Another example of a stoichiometric carbonyl-olefination reaction facilitated by a reagent which may be assumed to generate an intermediate tungsten carbene complex is shown in eqns. (7) and (8). The ketone can be added as soon as methane evolution has subsided (Kaufftnann 1986). 

The factors that direct a metallacyclobutane intermediate along one of these pathways are only partially understood. For example, in the case shown in Fig. 4.10, both metathesis and cyclopropanation are mediated by the same tungsten-carbene complex when the reaction is performed in a non-coordinating solvent [31]. These conditions are consistent with reaction via a metallacyclobutane because they allow the olefin to coordinate to the metal center, a prerequisite for [2 -I- 2] cycloaddition. In contrast, only cyclopropanation occurs when the reaction is performed in a coordinating solvent, presumably because formation of a metal-solvent adduct prevents olefin coordination and leaves only a direct carbene transfer mechanism available. 

Fig. 4.10. A tungsten-carbene complex that mediates both olefin cyclopropanation and metathesis.

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