Tungsten olefin metathesis

King, A.J. (2010) The Evolution of Molybdenum and Tungsten Olefin Metathesis Catalysts. PhD thesis. Massachusetts Institute of Technology. 

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 ]. 

Pyridine complexes of Pd- and Pt-pincer ligands are also suitable substrates for olefin metathesis [116]. The first-generation catalyst 9 efficiently mediates the RCM of diallylphosphines and diallyl sulfide when the heteroatom is com-plexed by a cationic [C5H5(NO)(PPh3)Re] moiety [117]. This principle has been exploited in the same study for tungsten, rhodium, and platinum complexes. 

Schrock RR (1998) Olefin Metathesis hy Well-Defined Complexes of Molybdenum and Tungsten. 1 1-36... 

A high metathesis activity was also observed when CH3Re03 was chemisorbed on Nb205. There appears to be a correlation between the catalytic activity and the Lewis acidity of the support.267 Catalytic activity of supported tungsten phenoxide in olefin metathesis was also studied.268... 

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... 

Because of the importance of olefin metathesis in the industrial production of olefins and polymers, many different catalysts have been developed. Almost all of these are transition metal-derived, some rare exceptions being EtAlCl2 [758], Me4Sn/Al203 [759], and irradiated silica [760]. The majority of catalytic systems are based on tungsten, molybdenum, and rhenium, but titanium-, tantalum-, ruthenium-, osmium-, and iridium-based catalysts have also proven useful for many applications. 

Both W(CO)5[C(C6Hs)2] and the analogous di-p-tolylmethylene complex have been used in model studies of the olefin metathesis reaction.2 3 In contrast to heteroatom-stabilized carbene complexes such as W(CO)s [C(OCH3)(C6Hs)], pentacarbonyl(diphenylmethylene)tungsten(0) reacts with alkenes to give cyclopropanes and 1,1-diphenylalkenes.2 The compound W(CO)5 [C(C6H5)2] is the best reported catalyst for the metathetical polymerization of 1-methylcyclo-butene.4... 

Both homogeneous and heterogeneous catalysts can be used in olefin metathesis.6 26 28 29 31 In general, tungsten or molybdenum halides or carbonyl complexes... 

In die last 10 years or so an exciting new strategy has emerged for the formation of carbon-carbon double bonds, namely olefin metathesis. This work grew out of the development of Ziegler-Natta catalysts for die polymerizarion of cyclic olefins. It was found that when 2-pentene was treated with a catalyst prepared from tungsten hexachloride and ethylaluminum dichloride, a mixture of 2-pentene, 2-butene, and 3-hexene was produced in minutes at room temperature (rt) ... 

The breakdown of metal-metal bonds without changes in the oxidation state. The processes of this kind have been first discovered by Schrock et al. in the course of investigation of the mechanisms of the catalytical action of low-valent tungsten alkoxocomplexes in the reaction of olefine metathesis [349,1017] ... 

In 1967, Calderon, Chen, and Scott4) reported a homogeneous catalyst system comprised of tungsten hexachloride, ethanol, and ethylaluminum dichloride would disproportionate internal olefins. These authors used the term olefin metathesis to describe the reaction. At room temperature 2-pentene was transformed in one to three minutes into a mixture containing, at equilibrium, 25, 50, and 25 mole per cent of 2-butene, 2-pentene, and 3-hexene, respectively. Double-bond isomerization was not detected and a quantitative reaction selectivity was obtained. Additional reports by Calderon and cowor-... 

The proposed idea that metal alkyhdene complexes are be able to catalyze olefin metathesis was confirmed in 1980 [8] and consolidated in 1986 by Schrock with the development of the first well-characterized, highly active, neutral tungsten (Cl, Fig. 3) [9] and molybdenum (C2) [10] alkylidene complexes. These complexes were able to catalyze both the metathesis of different olefins and the ROMP of functionalized norbomene to polynorbomene with low polydispersities [11]. Moreover, these catalysts were used by Wagener and coworkers to perform the first quantitative ADMET polymerization [12] and copolymerization [13] of 1,5-hexadiene and 1,9-decadiene. However, the low stability of these catalysts in... 

Fig. 3 Olefin metathesis catalysts Schrock tungsten (Cl) and molybdenum (C2) alkylidene complexes, Grubbs first- (C3) and second-generation (C4) catalysts, Hoveyda-Grubbs second-generation catalyst (C5), and Grubbs third-generation catalyst (C6)...

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