Tantalum olefin metathesis

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. 

Early work in this area has involved the investigation of the olefin metathesis activity of [W( Bu)(CH 2Bu)J supported on silica or other oxide supports [65, 66]. While highly active, these systems do not contain a metallocarbene [40, 67]. For instance, the silica-supported system has been characterized as [ iOW( Bu)(CH 2Bu)J, and therefore it is very likely that the propagating metallocarbene is generated in situ even if it is not clear how it is formed. [ iOMo( Bu)(CH 2Bu)J also displays similar reactivity towards olefins, but is more sensitive to functional groups [67, 68]. For tantalum, despite the presence of a weU-defined metallocarbene in [( iO)Ta(=CH Bu)(CH 2Bu)J, this system is... 

Reactivity characteristic of alkylidene complexes of tantalum is that the a-carbon is susceptible to electrophilic attack, in contrast to the electron-deficient a-carbon of Fischer-type carbene complexes of group 6 transition metals [62]. Based on this unique property of the alkylidene metal-carbon double bond, a range of new types of reactions has been developed. The discovery of the alkylidene complexes of tantalum was a key to understanding the mechanism of olefin metathesis, and they continue to play important roles in C—H bond activation, alkyne polymerization, and ring-opening metathesis polymerization. 

Eq. 7. Although tantalum was not among the metals that were found to be most aetive for metathesis of olefins, the radieally new tantalum alkylidene eomplexes offered the first opportunity to test the proposed meehanism of olefin metathesis with an observable alkylidene complex. 

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. 

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. 

Reaction of [(CH3)3CCH2]3TaCl2 with (CH3)3CCH2Li gives a tantalum-carbene complex (Schrock, 1974). Labeling studies indicate that the reaction proceeds via a-elimination from an intermediate RsTa compound. Carbene complexes have been considered as possible intermediates in olefin metathesis... 

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. 

The course of decomposition of confirmed or presumed metallocyclo-butane intermediates is important, but most results reported deal with stoichiometric rather than catalytic processes. Retention of the 3-carbon skeleton via pathways d or f in Eq. (26) occurs much more frequently than does cleavage to metathesis-related products. For example, thermolysis of phenyl-substituted platinocyclobutanes yields propenylben-zenes and phenyl-cyclopropane, but no styrene or ethylene (77). Similarly, the decomposition of tantalum carbene adducts (8) with olefins... 

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... 

Schrock also showed in 1980 that, importantly, some alkylidene eomplexes of tantalum and tungsten can metathesize olefins if they contain alkoxy ligands, and, in 1990, he reported a whole family of very active tungsten metathesis eatalysts including chiral ones, which led him to disclose, in collaboration with Hoveyda, the first examples of enantioselective metathesis reaetions. 

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