Tantalum complexes olefin

A key question remains how is the olefin formed in the overall process Molecular tantalum complexes are known to undergo facile a- and transfer processes, leading to tantalumalkylidene and tantalum tt-olefin complexes, respectively (mechanism 9, Scheme 29) [98]. Moreover, olefin polymerization with tantalum complexes belongs to the rare case in which the Green-Rooney mechanism seems to operate (Eq. 10, Scheme 29) [102]. Finally, intramolecular H-transfer between perhydrocarbyl ligands has been exemplified (Eq. 11, Scheme 29) [103,104]. 

It is particularly interesting, that some titanium and tantalum carbene complexes olefinate derivatives of carboxylic acids. These reagents are, moreover, much less basic than phosphorus ylides, and thus enable the olefination of strongly C-H acidic carbonyl compounds. 

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... 

Alkylidene tantalum complex C affords with low yield a-olefins ranging from C40 to C2oo- Its importance lies in its mechanistic implications vide infra). 

Tantalum complex D not only converts ethylene to 1-butene but selectively transforms all the a-olefins into a mixture of t,t or h,t dimers, depending on the size of the substituent (Table IX). 

The Wittig-type olefination of carbonyl compounds is one of the characteristic reactions of carbene complexes. High-valent carbene complexes of early transition metals show ylide-like reactivity to vards carbonyl compounds. In 1976, Schrock first demonstrated that niobium and tantalum neopentylidene complexes 1 and 2, the typical nucleophilic Schrock-type carbene complexes, olefinate various carbonyl compounds including carboxylic acid derivatives [4]. 

Interestingly, co-ordination of isocyanide ligand to the endo-hydride olefin tantalum complexes gives adducts containing organic groups [equation (6)]. ... 

The cycloolefin-tantalum complex (r] -C5Me5)Ta(cyclooctene)Cl 48 is capable of dimerization of various olefins into a mixture of the head-to-tail and head-to-head dimers 49 and 50 (Scheme 21) [27]. Some representative results are summarized in Table 10. It was shown that the dimerization process proceeds through series of reversible metallacycle formations such as tantalacy-clopentanes and tantalacyclobutanes, and jS-hydrogen eliminations. The dimerization ends in reductive elimination of the organyl moiety from the intermediate alkylhydridotantalum species. 

The reactivity of a remarkable electronically unsaturated tantalum methyli-dene complex, [p-MeCgH4C(NSiMe3)2]2Ta( = CH2)CH3, has been investigated. Electrophilic addition and olefination reactions of the Ta = CH2 functionality were reported. The alkylidene complex participates in group-transfer reactions not observed in sterically similar but electronically saturated analogs. Reactions with substrates containing unsaturated C-X (X = C, N, O) bonds yield [Ta] = X compounds and vinylated organic products. Scheme 117 shows the reaction with pyridine N-oxide, which leads to formation of a tantalum 0x0 complex. ... 

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... 

Some notable reversals in the order of olefin reactivities were observed in contrast to the tantalum carbenes. Further, in comparing the reactivities of (CO)5W=CPh2 and (CO)5W=CHPh, Casey (70) noted additional striking contrasts The monophenyl carbene complex reacted rapidly at... 

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). 

With the exception of(fj -C5Me5)Ta(HC=CH)Cl2 (71), the corresponding alkyne complexes of tantalum do not react with olefins 71 cyclizes with ethylene at 80°C to give the tantalacyclopentene 72 (Scheme 25). ... 

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... 

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. 

Scheme 19 intramolecular hydroaminoalkylation of unactivated olefins with secondary amines catalyzed by mono- and bis(amidate) tantalum ami do complexes... 

Recently, other pathways where the C—C bond is postulated to form via metalla-cyclopentanes (Scheme 3) have been established for the catalytic dimerization of ethylene and higher a-olefins by zirconium , tantalum, and nickel complexes. At least for the tantalum systems, the mechanisms of conversion of the metallacyclopentane to the product olefin differ significantly with the specific catalyst. With some variations, the metal-... 

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