Tantalum complexes alkylidene

The decisive difference between, e.g., [Cp(CO)2Fe]" and H4Ta- is the smaller amount of orbital overlap of the former with the carbene 2 p orbital, resulting in less efficient transfer of electron density from the metal to C . Although Fischer-type carbene complexes are formally low valent, backbonding to the carbene is less effective than in tantalum alkylidene complexes. 

A study has been made of the reactivity of high-oxidation state tantalum-alkylidene complexes stabilized by the aryldiamine ligand [C6H3(CH2NMe2)2-2, 6] , and an unusual rearrangement involving the unique activation of an aryl C—H bond by the alkylidenetantalum moiety has been observed.531... 

In decomposition reactions of dimethyl-metal complexes of palladium(II) and nickel (II) one finds the formation of only traces of methane [49] which may also attributed to an a-elimination process. In regard to the valence state, note that, formally, the alkylidene ligand is considered as a neutral ligand and therefore, in the tantalum-alkylidene complex in Fig. 4.29, tantalum is trivalent. The electronic structure of the alkylidene is of course reminiscent of the corresponding oxide CpTa(Cl)20, which we would definitely call pentavalent. All that matters is that there should be a sufficient number of electrons for the multiple bonds which we draw. 

Table 1. Olefinic Products from the Reaction of Tantalum-Alkylidene Complexes with Olehns...

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. ... 

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. 

Schrock-type carbenes are nucleophilic alkylidene complexes formed by coordination of strong donor ligands such as alkyl or cyclopentadienyl with no 7T-acceptor ligand to metals in high oxidation states. The nucleophilic carbene complexes show Wittig s ylide-type reactivity and it has been discussed whether the structures may be considered as ylides. A tantalum Schrock-type carbene complex was synthesized by deprotonation of a metal alkyl group [38] (Scheme 7). 

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. ... 

Alkylidene complexes are generally considered to be reactive intermediates but the actual surface organometallic species have never been fully characterized. However, the synthesis of silica-supported tantalum(V) carbene complexes and their characterization have been reported.332... 

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 chemistry of alkylidene and alkylidyne complexes of early transition metals was developed by Schrock and co-workers and these complexes turned out to be of crucial importance to alkene and alkyne metathesis. Initially their research focused on tantalum complexes of the type CpTaCEIE, which after a-elimination (Figure 16.6) led to alkylidene complexes Cp(R)Cl2Ta=CHR [11]. 

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. 

Alkylidene complexes can be prepared by ligand displacement with phosphorus ylides [513,514] or nucleophilic tantalum carbene complexes [409,515]. This methodology has, however, not found widespread use. Representative examples are given in Figure 3.21. 

Rupprecht, G.A., Messerle, L.W., Fellmann, J.D. and Schrock, R.R. (1980) Multiple metal-carbon bonds. 15. Octahedral alkylidene complexes of niobium and tantalum by ligand-promoted a abstraction. J. Am. Chem. Soc., 102, 6236. 

Despite dramatically different ancillary ligand sets, two distinct niobium and tantalum alkylidene systems provide isolable metallacyclobutanes upon reaction with ethylene. In one case, the tantalum aryldiamine pincer complex 148 reacts with ethylene to provide the cr-trimethylsilyltantalacyclobutane complex 149 (Equation 66) <19940M3259>. In a more comprehensive study, alkadiene-supported half-sandwich alkylidene complexes of both tantalum and niobium (the former isolable, the latter generated in situ) undergo [2+2] cycloaddition with a range of acyclic and cyclic alkenes, albeit in modest isolated yield (Equation 67). 

Alkylidene complexes are of two types. The ones in which the metal is in a low oxidation state, like the chromium complex shown in Fig. 2.4, are often referred to as Fischer carbenes. The other type of alkylidene complexes has the metal ion in a high oxidation state. The tantalum complex is one such example. For both the types of alkylidene complexes direct experimental evidence of the presence of double bonds between the metal and the carbon atom comes from X-ray measurements. Alkylidene complexes are also formed by a-hydride elimination. An interaction between the metal and the a-hydrogen atom of the alkyl group that only weakens the C-H bond but does not break it completely is called an agostic interaction (see Fig. 2.5). An important reaction of alkylidene complexes with alkenes is the formation of a metallocycle. 

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