Tantalum 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. 

Niobium and tantalum alkylidenes have also been shown to react with nitriles to form imido groups as follows 90... 

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... 

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). 

Analogously to the titanacycle catalyst, similar trisubstituted tantallacyclo-butanes were prepared, but with the use of tantalum alkylidene for the reaction with norbornene [100-102] ... 

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...

An interesting C-metalation of a coordinated pyrrol ligand in a rhenium complex has been described [19a]. Methylated aromatic hydrocarbons have been activated by (ZrCU) [19b], A novel mode of electrophilic activation of an aliphatic C-H bond, assisted by porphyrin complexes ofZr(IV) and achieved by the use of hydrides of lithium, sodium or potassium, has been reported [19c]. It has been shown that the ligands 2,3,5,6-tetraphenylphenoxide and 3,5-dimethyl-2,6-diphenylphenoxide undergo intramolecular activation by tantalum alkylidene groups at rates 20 and 100 times slower than that of the simple 2,6-diphenylphenoxide ligand [19d]. 

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. ... 

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. 

Addition of other nucleophiles or Lewis bases to neopentyltantalum complexes also generated alkylidene complexes, as shown in Scheme 13.2. Addition of slightly less than one molar equivalent of bis(neopentyl)zinc to tantalum pentachloride generated bis(neopentyl) tantalum trichloride, and addition of thalium cyclopentadienide to this intermediate generated the Cp-ligated tantalum alkylidene. Addition of dative ligands, such as THE, and monodentate or bidentate phosphines to the bis(neopentyl)tantalum complex generated the octahedral alkylidenes in Scheme 13.2. ... 

In the case of alkyl leaving groups, mechanistic studies show that a-bond metathesis pathways are active, proceeding via four-centre, four-electron transition states. In the case of unsaturated functional groups, the reaction involves addition of the CH bond across the r-component, so that the substrate hydrogen atom remains (labelling studies) within the product molecule. The cyclometallation of 2,6-dimethylphenoxide by a tantalum alkylidene ligand has also been demonstrated. ... 

Recent studies have focussed on the generation of metaUation-resistant aryloxide ligation. The strategy adopted has involved the introduction of meto-substituents onto the 2,6-diphenylphenoxide nucleus. The meta-substituents restrict rotation of the orthophenyl ring into a position for CH-bond activation. In the case of 3,5-di-tert-butyl-2,6-diphenylphenoxide, cyclometallation by tantalum alkylidene functional groups is... 

Schrock and co-workers reported a series of tantalum alkylidenes with the general formula Ta(=CH-R)X3(solv) (R = -Bu, etc., X = 0-2,6-/-Pr2-C6H3, 0-2,6-Me2-C6H3, S-2,4,6-/-Pr3-C6H2 solv = py, THF). The complexes were used for ROM polymerizations of norbornene and additionally tantalacyclobutane intermediates were isolated and characterized [117]. Several other Ta based initiators were synthesized and characterized... 

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