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Tantalum catalysts

W.L. Gladfelter, University of Minnesota Can you polymerize propylene with your tantalum catalyst ... [Pg.367]

Upon discovery of this mechanism, new catalysts have been developed, now presenting alkylidene ligands in the metal coordination sphere, such as [(=SiO) Ta(=CH Bu)Np2 and [(=SiO)Mo(=NAr)(=CH Bu)Np] [43, 88]. Table 11.4 presents results obtained with several catalysts prepared by SOMC. Although [(=SiO) Ta(CH3)3Cp (=SiOSi=)] is not active in alkane metathesis (the tantalum site would not be as electrophilic as required) [18], results obtained with [(=SiO)Mo(=NAr) (=CH Bu)Np] show that ancillary ligands are not always detrimental to catalytic activity this species is as good a catalyst as tantalum hydrides. Tungsten hydrides supported on alumina or siHca-alumina are the best systems reported so far for alkane metathesis. The major difference among Ta, Mo and W catalysts is the selectivity to methane, which is 0.1% for Mo and less than 3% for W-based catalysts supported on alumina, whereas it is at least 9.5% for tantalum catalysts. This... [Pg.432]

The reaction has been done with a palladium catalyst, a titanium catalyst, a tantalum catalyst,and with a gold catalyst. An intramolecular addition of amines to an alkyne unit in the presence of a palladium catalyst generated heterocyclic or cyclic amine compounds.The titanium catalyzed addition of primary... [Pg.1048]

While the molecular tantalum catalyst Ta(OCH2CH3)5 exhibited very poor activity for epoxidation under Sharpless conditions, the surface-supported analogue [a mixture of 70% =SiOTa(OCH2CH3)4 and 30% (=SiO)2Ta(OCH2CH3)3] was shown to have activity comparable with that of the molecular Ti catalyst. Furthermore, excellent enantiomeric ee values (up to 94%, compared with 96 % for Ti[OCH(CH3)2]4 under the same conditions) were obtained. An inversion of the the major enantiomer obtained was observed for both the molecular and supported tantalum catalysts, i. e., the association of tetraisopropyltitanium and (+)-diisopropyltartrate produces (/f)-epoxide whereas the Ti catalyst with (+)-diisopropyltartrate produces the (S)-epoxide. The putative active species, =SiOTa(OCH2CH3)2[(+)-(DET)] (Structure 18) has also been synthesized and tested (eq. (3) [23 a]) Further improvements of catalyst activity have been obtained by modification of the support and refinement of the synthesis of the supported tantalum alkoxide precursor. [Pg.671]

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. [Pg.517]

Recently, in order to further improve catalytic activity, two new catalyst precursors were developed [(=SiO-)WMe5] (27) [23] and [(=SiO-)TaMe4] (13) [36]. Such polymethyl complexes possess no p-H and can easily generate in situ the corresponding surface M-methylidene species (M = W [23,72], Ta [36]) which has been done with little success in the past [44,73]. The improvement in activity is marginal in the case of 13 compared to that of earlier reported tantalum catalyst precursors. However, the catalyst precursor 27 showed notable improvement than previously reported catalyst precursors with TON of 127 (Table 2, Entry 14). [Pg.173]

Scheme 26 Cross metathesis of methane and propane in the presence of silica-supp Scheme 26 Cross metathesis of methane and propane in the presence of silica-supp<nted tantalum catalyst...
Using a procedure related to that for the preparation of tantalum catalysts, supported W-polyhydride species were prepared to evaluate their catalytic activity in alkane metathesis. Hydrogenolysis of the grafted, alkylidyne d°-complex W(=fBu)(CH2tBu)3 [52] led to W-polyhydride complexes (Scheme 2.8) [53]. [Pg.42]

R.R. Schrock - Rapid Selective Dimerization of Ethylene to 1-Butene by a Tantalum Catalyst and a New Mechanism for Ethylene Oligomerization,/. Am. Chem. Soc. 101, 5099, 1979 S.J. McLain, J. Sancho, R.R. Schrock - Metallacyclopentane to Metallacyclobutane Ring Contraction, J. Am. Chem. Soc. 101,5451,1979 ... [Pg.558]


See other pages where Tantalum catalysts is mentioned: [Pg.255]    [Pg.60]    [Pg.211]    [Pg.249]    [Pg.205]    [Pg.46]    [Pg.424]    [Pg.91]    [Pg.107]   
See also in sourсe #XX -- [ Pg.84 ]

See also in sourсe #XX -- [ Pg.155 , Pg.156 , Pg.157 , Pg.158 , Pg.159 , Pg.160 ]




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