Tantalum complexes carbene

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. 

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 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. 

Scheme 2.2 Possible mechanism for generation of tantalum carbene complexes.

Note that there is a preferred direction of reaction with the original complex. The reaction is prevented from developing into a chain reaction by the propensity of the [Ta]=CH2 complex to undergo the homologation reaction (McLain 1977 Schrock 1980 Rocklage 1981) see Abbenhuis (1994) for another example of a tantalum carbene complex which will undergo only stoichiometric metathesis. However, the complexes 2 and 3 (Table 2.2) are good initiators of ROMP as also is the tantalacyclobutane complex derived from 2 by the addition of norbomene. The complexes 1 and 2 are both effective for the metathesis of ds-pent-2-ene. 

The tantalum carbene complex 27 reacts with norbomene at —30°C in ether, eqn. (19), to give the d tantalacyclobutane complex 28, which initiates the ROMP of norbomene at 60°C (Wallace 1986, 1987). 

The kinetics of polymerization are remarkably similar to those for the reaction initiated by the analogous titanium complexes 25 and 26 first-order in initiator, zero-order in monomer for the major part of the reaction, and activation energy of 105 kJ mol Propagation via tantalacyclobutane and tantalum carbene complexes is clearly implied. 

The reactions of ethene with titanium and tantalum carbene complexes are described in Sections 2.2.1 and 2.3.2, respectively. 

Again to confirm the presence of tantalum carbene on silica surface C-eruiched tantalum-carbene complex, [Ta(=C H Bu)(CH2 Bu)3] was grafted on silica (5oo). The CP NMR showed a peak at 246 ppm which corresponds to (=C H Bu) along with other peaks confirming the presence of tantalum carbene on the supported complex [39]. 

Mashima, K. Kaidzu, M. Tanaka, Y. Nakayama, Y Nakamura, A. Hamilton, J. G Rooney, J. J. Control of stereoselectivity in the ring-opening metathesis polymerization of norbornene by the auxiliary ligands butadiene and o-xylylene in well-defined pentamethylcyclopentadiene tantalum carbene complexes. Organometallics 1998,17,4183 195. 

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-carbene complexes, originally reported by Schrock, opened new paths for advancement in metal-carbene chemistry. Serendipitously, a Ta yhde , (Me3CCH2)3Ta"-CHCMe3, 2, was obtained (along with neopentane) from Ta(CH2CMe3)3Cl2 upon treatment with LiCH2CMe3 (Scheme 5). ... 

Scheme 4.52. Carbonyl olefination utilizing the tantalum carbene complex 1.

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