Tungsten—alkylidene complexes

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... 

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). 

Probably the first isolated tungsten alkylidene complex active in metathesis and completely characterised is the one shown in Figure 16.10 reported by Wengrovius and Schrock the analysis included an X-ray structure determination by Churchill and co-workers [18], The alkylidene was transferred from a tantalum complex to yield the hexacoordinate tungsten complex containing two PEt3 ligands. One of these can be removed by the addition of half an equivalent of palladium chloride. The total turnover number of these catalysts with Lewis acids added was 50 in 24 hours. 

Figure 16.11. Highly active tungsten alkylidene complex and tungstenacyclobutane complex...

The ionic compound [Br(Mc3CCH20)2W=CHCMe3]1 [Ga2Br7] displays catalytic activity increased by a factor of ca 3400 compared with the neutral tungsten alkylidene complex. 

Finally, the synthesis of fhe aryloxy(chloro)tungsten alkylidene complex reported by Basset et al. needs to be mentioned (Scheme 5.2) [60]. 

One of the first examples of Lewis-acid-free initiators based on tungsten I), W(0-2,6-i-Pr2-C6H3)2Cl2-(CH-t-Bu)(OR)2, (R = Et, i-Pr), which allowed the polymerization of substituted norbornenes, was reported by J. M. Basset and co-workers. Additionally, Lewis-acid-free W(=C(CH2)4)(OCH2-f-Bu)2Cl2 was reported to effectively polymerize substituted nor-bomenes such as exo-norbom-5-ene-2,3-dicarboxylic anhydride. Finally, the synthesis of the aryloxy— alkyloxy tungsten alkylidene complex reported by Basset et al. needs to be mentioned (Scheme 2). The... 

Vinyltin derivatives (92) have been prepared by the reaction of acyltin compounds with phosphonium ylides or with phosphonate carbanions. The tungsten complex (93) reacts with a wide range of aryl-substituted ylides (94), and the vinyl-substituted ylide (95), to give phosphorus to metal alkylidene transfer and hence provides a route to tungsten alkylidene complexes (96). ... 

Similar to most conventional catalysts, tungsten alkylidene complex 12 (without GaBra) selectively polymerizes the anti isomer of 7-MeNB from a 1 1 mixture of the syn and anti isomers. The resulting poly(fl ti-7-MeNB) contains less than 5% 7-iy -MeNB incorporated. This polymer is atactic and contains 41% trans C=C structures. The hydrogenated polymer (poly-H-(anft-7-MeNB)) is semicrystalline despite its atactic stereochemistry. This is similar to the partial crystallinity of the atactic hydrogenated polynorbornene previously described. The DSC curve for poly-H-(a ti-7-MeNB) displays an endothermic melt transition with a peak maximum at Tm = 176 °C (A// = 26 J/g) (there is an additional minor peak at 137 C). This result shows that partial... 

Tab. 4.20. Carbonyl olefination with tungsten alkylidene complexes 60 and 61.

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