Tantalum metathesis catalyst

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... 

Schrock and co-workers note that the chain mechanism is almost certainly correct, but major questions remain unanswered. They are conducting studies with alkyhdene complexes of niobium, tantalum, and tungsten, directed towards understanding in detail how and why metathesis catalysts work. From preliminary results they predict that the olefin co-ordinates to the metal before a metallocyclobutane complex can be formed, that rearrangement of metallocyclobutane is slow relative to the rate of metathesis, and that important chain-termination steps are rearrangement of metallocyclobutane intermediates and bimolecular decomposition of methylene complexes. For these systems, co-catalysts such as the alkyl-aluminium chlorides are not necessary the initial alkyl group on the metal... 

The source of the one-carbon metal fragment was not addressed in the original Chauvin papers. However, groundbreaking work by Schrock showed that alkylidene complexes could be synthesized by treating tantalum precursors with alkyllithium reagents (Eq. 4.5) [3,57]. This tantalum alkylidene complex also does not catalyze olefin metathesis, but the synthesis and isolation of the first alkylidene complex was an important milestone in the development of well-defined olefin metathesis catalysts. 

Synthesis and characterization of several catalyst precursors of [Ta] and [W] on different supports like silica [44], alumina [45], or silica-alumina [70] have been discussed in the previous section. These catalyst precursors were tested mainly in propane metathesis under similar condition in batch reactor at 150°C for 120 h. Catalytic performances revealed that the [Si02-Al203.5oo-W-H] and [AI2O3.500-W-H] have similar and better activity (TON) than the corresponding silica-supported tantalum-based catalyst precursors (Table 2). It was also observed that the product selectivity for tungsten hydrides is narrower than for tantalum hydrides [71]. 

Cross metathesis between two different alkanes represent one of the most difficult challenges in organic chemistry [53]. In 2001, Basset et al. first demonstrated the possibilities of sigma bond metathesis between two different alkanes [55]. In 2004, this same group has reported the cross metathesis between ethane and toluene [81] and methane and propane [82]. Silica-supported tantalum hydride catalyst [(=SiO)2TaH] [(=SiO)2TaH3] was employed for cross-metathesis reaction between toluene and ethane at 250°C. Under static condition, it produced mainly ethyl benzene and xylenes as major product along with propane and methane (Scheme 24). 

Tantalum complexes are excellent models in understanding how effective well-defined W and Mo catalysts might be designed that mimic metathesis catalysts based on W and Mo [6]. The first example of a stable M=CHR complex is the Ta(CH-t-Bu)(CH2-t-Bu)3 complex. In this complex, four bulk covalently bound ligands stabilize even an electronically unsaturated specie toward bimolecular decomposition. This compound is sensitive to oxygen, water, and a variety of functionalities, among them are ketones and aldehydes, with which it reacts to yield polymeric (t-BuCH2)3Ta=0 and the expected olefin [26]. It was established that d alkylidene species were responsible for olefin metathesis and drew attention to... 

Basset and his group have observed that propane and propene metathesis give similar Cn+i/Cn+2 ratios of cross-metathesis products on silica-supported tantalum-neopentylidene catalyst at 150°C. The olefin-metathesis activity of these Schrock-type supported complexes results from the presence of the silyloxy ligand (vide infra) - Organometallic complexes are bound to silica or alumina by reaction of soluble complexes and involve die formation of one or several bonds between the central metal and the oxygen atom of the oxide support. 

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. 

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. 

Upon discovery of this mechanism, new catalysts have been developed, now presenting alkylidene ligands in the metal coordination sphere, such as [(=SiO) Ta(=CH Bu)Np2 and [(=SiO)Mo(=NAr)(=CH Bu)Np] [43, 88]. Table 11.4 presents results obtained with several catalysts prepared by SOMC. Although [(=SiO) Ta(CH3)3Cp (=SiOSi=)] is not active in alkane metathesis (the tantalum site would not be as electrophilic as required) [18], results obtained with [(=SiO)Mo(=NAr) (=CH Bu)Np] show that ancillary ligands are not always detrimental to catalytic activity this species is as good a catalyst as tantalum hydrides. Tungsten hydrides supported on alumina or siHca-alumina are the best systems reported so far for alkane metathesis. The major difference among Ta, Mo and W catalysts is the selectivity to methane, which is 0.1% for Mo and less than 3% for W-based catalysts supported on alumina, whereas it is at least 9.5% for tantalum catalysts. This... 

Le Roux, E. (2004) New Generation of Tantalum and Tungsten Based Catalysts Supported on Oxides for Alkane Metathesis. PhD thesis. University Claude Bemard-Lyon I (Lyon). 

Alkoxide ligands play an important spectator role in the chemistry of metal-carbon multiple bonds. Schrock and coworkers have shown that niobium and tantalum alkylidene complexes are active toward the alkene metathesis reaction. One of the terminating steps involves a j8-hydrogen abstraction from either the intermediate metallacycle or the alkylidene ligand. In each case the -hydrogen elimination is followed by reductive elimination. The net effect is a [1,2] H-atom shift, as shown in equations (73) and (74), and a breakdown in the catalytic cycle. Replacing Cl by OR ligands suppresses these side reactions and improves the efficiency of the alkylidene catalysts. ... 

Richard Schrock investigated the properties of some of the first catalysts for olefin metathesis. His work included catalysts prepared from tantalum, titanium, and molybdenum. The catalysts predominandy in use today, however, are ruthenium catalysts developed by Robert Grubbs. His so-called first generation and second generation catalysts are shown here. 

Scheme 26 Cross metathesis of methane and propane in the presence of silica-supp

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