Tantalum carbene

Scheme 8.12 Preparation of the first bis(carbene) tantalum complexes 46 and 48 from mono(carbene) or mono(carbyne) tantalum compounds as starting materials...

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 chemistry of tantalum is different. The tantalum pentaalkyl complex does not exists, because it transforms easily into a carbene complex by a-elimination. This complex reacts also with silica, leading to a supported tantalum complex.262,263 Their reaction proceeds first by the addition of the silanol OH group across the tantalum-carbon double bond followed by elimination of an alkane (Scheme 7.19). 

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... 

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... 

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... 

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. 

The first non-heteroatom-substituted carbene complex was prepared by Schrock in 1974 [392] (Figure 3.4). Treatment of tris(neopentyl)tantalum dichloride with neopentyllithium led to the formation of neopentane and (2,2-dimethyl-1-propylidene)tris(neopentyl)tantalum. This carbene complex reacts violently with water or oxygen, but can be sublimed (80 °C) and stored indefinitely at room temperature under argon. 

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. 

As already discussed (Section 3.1.1) the elimination of, for instance, neopentane from penta(neopentyl)tantalum corresponds to an a-deprotonation of one alkyl ligand by another, the latter being eliminated as neopentane. Hence in the reverse reaction the carbene carbon atom of the (nucleophilic) carbene complex must formally deprotonate the incoming alkane with simultaneous electrophilic attack of the metal at the newly formed, carbanionic alkyl group (Figure 3.36). 

Inter- and intramolecular (cyclometallation) reactions of this type have been ob-.served, for instance, with titanium [408,505,683-685], hafnium [411], tantalum [426,686,687], tungsten [418,542], and ruthenium complexes [688], Not only carbene complexes but also imido complexes L M=NR of, e.g., zirconium [689,690], vanadium [691], tantalum [692], or tungsten [693] undergo C-H insertion with unactivated alkanes and arenes. Some illustrative examples are sketched in Figure 3.37. No applications in organic synthesis have yet been found for these mechanistically interesting processes. 

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. 

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). 

In the case of ethane, this mechanism cannot occur since the resulting metal-ethyl intermediate does not display any alkyl group in the P-position. Consequently, with tantalum hydride(s), 3, which cleave ethane, another process must take place, involving only one carbon atom at a time. Among various reasonable possibilities, we assume a carbene deinsertion from a tantalum-ethyl species because the reverse step is known in organometallic chemistry (Scheme 3.4) [22]. Note that this reverse step has been postulated as the key step in Fischer-Tropsch synthesis [23]. 

In fact, the C-H bond activation by the zirconium or tantalum hydride on 2,2-dimethylbutane can occur in three different positions (Scheme 3.5) from which only isobutane and isopentane can be obtained via a P-alkyl transfer process the formation of neopentane from these various metal-alkyl structures necessarily requires a one-carbon-atom transfer process like an a-alkyl transfer or carbene deinsertion. This one-carbon-atom process does not preclude the formation of isopentane but neopentane is largely preferred in the case of tantalum hydride. 

Figure 5-48 BP/LACVP vs. Experimental Ta=CR Bond Angles in Tantalum Carbenes R R R Ta=CHR...

Tantalum carbene complexes such as 5 and Ta(=CHCMe3)(S—C6H2-i-Pr3-2,4,6)3 (py) are effective, provided the conditions are such as to allow the coordinated base (THF or py) to give way to monomer (M). In the first example the initially formed tantalacyclobutane complex has been isolated and shown to have a trigonal-bipyramidal structure, and to polymerize NBE at a rate that is independent of [M], In this case the rearrangement of the intermediate tantalacyclobutane complex, to form the tantalum carbene complex, controls the rate of polymerization. In contrast, in the second example the rate is first-order in monomer here the reaction of the tantalum carbene complex with the monomer is the slower step. In both cases the polymer, after termination by reaction with benzaldehyde, is nearly monodisperse93,94. 

Bis(butadiene) complexes, with tantalum, 5, 173 Bis(z-butanethiolato) complexes, with bis-Cp Ti(IV), 4, 601 Bis(calixarene) complexes, as organic molecule hosts, 12, 799 Bis(carbene) complexes with gold(I), 2, 287-288 with manganese, 5, 780, 5, 826 with mercury, 2, 429 with palladium, 8, 230 with silver , 2, 206... 

Di(carbene)gold(I) salts, oxidation, 2, 293—294 Dicarbido clusters, with decarutheniums, 6, 1036 Dicarbollide amides, with tantalum, 5, 184 Dicarbollide thorium complexes, synthesis and characterization, 4, 224—225 Dicarbollyl ligands, in nickel complexes, 8, 185 Dicarbonyl complexes arylation with lead triacetates diastereoselectivity, 9, 389 enantioselectivity, 9, 391 mechanisms, 9, 387 reaction examples, 9, 382 indium-mediated allylation, 9, 675 with iridium, 7, 287 reductive cyclization, 10, 529 in Ru and Os half-sandwiches, 6, 508 with Zr—Hf(II), 4, 700... 

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