Neopentylidene

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 Reaction of Tantalum Neopentylidene Hydride Complexes with Ethylene... 

Other complexes have been reported to activate alkanes, including thermally generated tungsten neopentylidene analogs (Equation (3)).11... 

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. 

As with all previous complexes, tris(neopentyl)neopentylidene tantalum(V) reacts... 

To follow the mode of grafting of an organometallic species onto Si02, the tris (neopentyl)neopentylidene tantalum complex has been used to show the reactivity of its groups with silanol groups of silica (dehydroxylated at 300 to 700 °C)... 

Upon reaction with ethylene, neither supposed [(=SiO)2Ti(=CHCMe3)j nor [(=SiO)2Zr(=CHCMe3)] species produced neohexene, the expected metathetical exchange product Instead, oligomerization of ethylene was observed, in agreement with results reported for neopentylidenes of group 5 [50]. 

For all these well-defined catalysts, the initiation step corresponds to the crossmetathesis of the olefin and the neopentylidene ligand. When the olefin is propene. 

The tris-neopentyl Mo(VI) nitride, Mo(-CH2- Bu)3(=N) [134], reacts with surface silanols of silica to yield the tris-neopentyl derivative intermediate [(=SiO)Mo (-CH2- Bu)3(=NH)] followed by reductive elimination of neopentane, as indicated by labeling studies from labeled starting organometallic complex, to yield the final imido neopentylideneneopentyl monosiloxy complex [(=SiO)Mo(=CH- Bu)(-CH2 - Bu)(=NH)] [135]. The surface-bound neopentylidene Mo(VI) complex is an active olefin metathesis catalyst [135]. Improved synthesis of the same surface complex with higher catalytic activity by benzene impregnation rather than dichlorometh-ane on silica dehydroxylated at 700 °C has been reported [136],... 

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]. 

N-Neopentylidene-r-butylamine iV-oxide adds to isocyanides to give 4-imino-l,2-oxazetidines in reasonable yields provided the reaction is catalyzed by Lewis acids (Scheme 112) <80AG(E)45>. 

X-Ray structure, M=M and N=N metathesis Stable with respect to neopentylidene formation X-Ray structures for related compounds see Scheme 2... 

Neopentylidenes, with titantium(IV), 4, 364 Neoplasia, Ti(IV) complex treatments, 4, 664 Nephrosteranic acid, via Ti(IV) complexes, 4, 406 Neutral alkyl-bridged compounds, transition metal-Group 13 alkyl combinations, 3, 271... 

Further developments have been made possible by the introduction of third-generation catalysts. These are the commercially available, highly active Schrock-type Mo-based catalyst 19 and W catalyst 20, coordinated by a bulky neopentylidene group and alkoxides [2], They are well-defined, single-component catalysts. The W complex 20 is very active, but the Mo complex 19 is more versatile. These catalysts have some... 

Basuli, F., Bailey, B.C., Huffman, J.C. and Mindiola, D.J., (2005) Intramolecular C-H activation reactions derived from a terminal titanium neopentylidene functionality. Redox-controlled 1,2-addition and a-hydrogen abstraction reactions. Organometallics 24, 3321. 

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). 

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-... 

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). 

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