Neopentylidene complex

The first neopentylidene complex was prepared by the reaction shown 1n equation 1 (5). Although the exact details of this reac-... 

Imido alkylidene complexes were first prepared by a reaction analogous to that shown In equation 6. Recently they have been prepared from imido alkyl complexes by well-behaved a-hydrogen abstraction reactions (16) Imido neopentylidene complexes seem to be more stable than oxo neopentylidene complexes, possibly because the oxo ligand is sterically more accessible to Lewis acids, including another tungsten center. 

Osborn s discovery (14) that aluminum halides bimTto oxo ligands in tungsten oxo neopentyl complexes, and that these complexes decompose to give systems which will efficiently metathesize olefins, raised more questions concerning the role of the Lewis acid. A subsequent communication (20) answered some of the questions the aluminum halide removes We oxo ligand and replaces it with two halides to yield neopentylidene complexes (equation 8). 

The exact nature of the alkylidenes formed on various oxide surfaces is still uncertain, as is the nature of the alkylidenes responsible for the often observed metathesis activity. Mo(N)(CH2CMe3)3 also has been employed as a precursor to a surface-bound species believed to be of the type Mo(NH)(CHCMe3)(CH2CMe3) (Osurf) [115]. Although the alkylidene carbon atom could not be observed in solid state NMR spectra, which is typical of surface supported alkylidenes, reaction with acetone to give 2,4,4-trimethylpent-2-ene quantitatively confirmed the presence of the reactive neopentylidene complex. Such species would initiate various metathesis reactions when prepared on partially dehydroxylated silica. 

The surface species is an active catalyst for propene metathesis (780 eq., TOI min = 0-89 molpropene hioImo s" for the catalyst obtained by the benzene impregnation route) and ethyl oleate metathesis (lOOeq. TOI mm =0.11 moloieate hioImo s for the catalyst obtained by the benzene impregnation route) [136]. The high catalytic activity of the surface neopentylidene complex has not been modeled by the silsesquioxane analogue [136]. 

A series of tantalum imido complexes has been synthesized by the addition of imines or nitriles to tantalum neopentylidene complexes as shown in Eq. (76). 

As regards the metathesis polymerisation of cyclic trienes, it has been carried out in an attempt to find alternative routes for preparing soluble and meltable precursors of polyacetylene [149, 150], Hence, several substituted or unsubstituted tricyclic or other polycyclic trienes were subjected to polymerisation in the presence of metathesis catalysts such as WCl6-SnMe4 [151-154] and the tungsten neopentylidene complex [Me(F3C)2CO]2W(=NAr)(=CHCMe3) [155]. A successful solution of the problem is outlined below [125,150] ... 

R. R.Schrock, The Reaction of Niobium and Tantalum Neopentylidene Complexes with the Carbonyl Function, J. Am. Chem. Soc. 98, 5399-5400 (1976). 

Although many cationic secondary carbene complexes are known, deprotonation of these carbene ligands to afford carbyne complexes is not a common synthetic method. Conversion of a Ta-neopentylidene complex to a Ta-neopentylidyne complex by using Ph3P=CH2 as an external base occurs by deprotonation of a cationic Ta-neopentyli-dene intermediate ... 

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. 

The alkoxo-neopentylidene complex, Ta(CHCMe3)(OCMc3)2Cl(PMe3), reacts with ethylene to give t-butylethylene and propylene ... 

Carbene complexes for which metathesis is the preferred synthetic method include the W alkylidene complexes, W(0)(PEt3)2Cl2CHR (R = CH3, Ph, C2H5, H), prepared from the corresponding neopentylidene complex ... 

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