Neopentylidene hydride complexes

The Reaction of Tantalum Neopentylidene Hydride Complexes with Ethylene... 

S. J. Holmes, D. N. Clark, H. W. Turner, and R. R. Schrock, o-Hydride Elimination from Methylene and Neopentylidene Ligands. Preparation and Protonation of Tungsten(IV) Methylidyne and Neopentylidyne Complexes, J. Am. Chem. Soc. 104, 6322-6329 (1982). 

A conceptual process for ethylene dimerization in the presence of tantalum or niobium based catalysts has been developed by MIT researchers [6-8]. The technology is based on a metal hydride-based homogeneous catalyst that selectively dimerizes ethylene to butene-1. The particular catalyst is neopentylidene complex of tantalum or niobium. The preparation of the homogeneous catalyst is rather a complex process the tantalum complex is prepared by reacting tri-neopentyl tantalum dichloride, Ta(CH2CMe3)3Cl2 and neopentyl lithium LiCH2CMe3 in octane solvent to yield thermally stable neopentylidene tantalum catalyst in quantitative yield. 

Metathesis is the best route to these complexes. The preparation of W alkylidehes by a-hydride abstraction has had only limited success , and the neopentylidene substrate is prepared by an unusual heterometallic exchange reaction ... 

Supported Ta-Neopentylidene Complexes A silica-supported Ta-alkylidene [(=SiO) Ta(=CHCMe3)(CH2CMe3)p j] catalyst has been synthesized and well characterized [76, 77]. This Ta-alkylidene catalyst possesses a neopentyl ligand and an alkylidene group, and was found to be active for propane metathesis (TON=33). Since the precursor is not a hydride and a proposed carbene hydride is known to be the intermediate, this implies that Ta-neopentyl and neopentylidene organometaUic species are initially transformed into this key species (Scheme 2.10) [78-83]. Different initiation steps have been suggested. One involves the initial addition of the alkane into the alkylidene moiety to form multiple Ta-alkyl species, which can then decompose via an a-H abstraction to produce a Ta-alkylidene (Scheme 2.10a) [84, 85]. 

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