Tantalum ylide complexes

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 is the first element of subgroup Ya for which both pentaalkvl and ylidic compounds could be completely characterized and fully investigated. Not only was it possible to prove the existence of Ta(CH3)6 and some of its complexes (S3), but the tantalum ylide, trisneopentyl neopentylidene tantalum, could also be isolated (82a). This ylide is the product of a reaction between trisneopentyltantalum dichloridc and neo-... 

Note added in proof Very recent work by Schrock has extended this tantalum ylide chemistry very considerably and some cyclopentadienyl tantalum methylenes and benzylidenes could be isolated and fully characterized (82b). Among the new compounds the complex (C5H4CH3)2Ta(CH3)(CH2) is particularly noteworthy. Its crystal structure has been determined and the temperature dependence of its nmr spectra has been carefully investigated (17b). The ylidic carbon is in a trigonal planar configuration ... 

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]. 

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). 

Schrock discovered that the r-butylalkylidene complex of tantalum (and in lower yield, niobium) was a structural analog to the phosphorus ylide. The complex proved to be a reagent for r-butylalkene formation. Of the Wittig-type reactions tried, most notable was the ability of the complex to react with esters and amides to form the corresponding r-butylalkenes in good yields. 

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