Niobium allyl

Yasuda and coworkers51 reported a route to niobocene-allyl compounds by hydrometa-lation of conjugated dienes with niobium hydrido-olefin complexes, NbH(C5H5)2(olefin),... 

The coordinated ethylene is readily expelled at 25 °C by the attack of a conjugated diene and the hydride is transferred to the sterically less crowded diene terminus. Thus the niobium hydrido-olefin complexes serve as convenient reagents for the preparation of 1,2- or 1,3-dialkyl-substituted allylniobium compounds starting from butadiene, (E,E) and (/i,Z)-2,4-hcxadicnc, (E)- and (Z)-l,3-pentadiene (equation 1), 3-methyl-l,3-pentadiene and isoprene (equation 2). All the allyl niobium compounds synthesized were isolated as air-sensitive pale-yellow crystals by crystallization from hexane. 

NaCl and subsequent heating (68) or by treating with acetonitrile (69) failed to alter the spectrum, but the rate of the reaction of Nb(7r-allyl)4 with NaCl-treated alumina was markedly reduced. An alternative explanation for the spectrum (Figure 13) could be that all the niobium atoms are attached to the alumina surface by multiple oxide bridges, a conclusion which was also reached from the ESR spectrum of the VC14/ alumina reaction product (60). This is not confirmed by the stoichiometric results of the reaction of Zr( benzyl )4 with alumina previously dried at 750° (Table II). 

Finally the ESR spectrum of Nb(7r-allyl)4/alumina was unaffected by the addition of ethylene gas to the ESR sample tube. It is assumed that polyethylene is produced in this process since polymer can be isolated from larger scale reactions under similar conditions. The accepted mechanism for the ethylene growth reaction postulates a steady-state concentration of a a-bonded transition metal-hydrocarbon species which would be expected to modify the ESR spectrum of the supported complex. A possible explanation for the failure to detect a change in the ESR spectrum may be that only a small number of the niobium sites are active for polymerization. Although further experiments are needed to verify this proposition, it is consistent with IR data and radiochemical studies of similar catalyst systems (41, 42, 43). 

Cyclopentadienyl carbonyl complexes, with niobium, 5, 64 Cyclopentadienyl chromium bis(Ti-allyl) complexes, with chromium, 5, 305-306 Cyclopentadienyl chromium carbonyls... 

Table 8 gives the data on the electron structure of valence shells of atoms of the corresponding transition metals [86]. Taking into account the fact that the total number of electrons provided by three allyl (crotyl) ligands to the molecular orbitals of the complex is nine, it may be concluded that the complexes of Cr (chromium), Nb (niobium) and Ti (titanium) should be paramagnetic and those of Rh (rhodium) should be diamagnetic. In this case the total... 

Niobium carbide is used as a protective coating for niobium metal.[10] It can be prepared by the reaction of elemental niobium metal with methane or by reacting NbCIs with CCI4 and H2. It also has been obtained from the arene complex Nb(CH3-C(,H,[Pg.385]

ALKENES Allyl dimethyldithiocarbamate. Bis(t -cyclopentadienyl)niobium trihydride. Cyanogen bromide. Di-n-butylcopperlithium. a,o-Dichloromethyl methyl ether. 2,3-Dimethyl-2-butylborane. N,N-Dimethyl dichlorophosphoramide. Diphenyl diselenide. Di-n-propylcopperlithium. Ferric chloride. Grignard reagents. Iodine. Lithium phenylethynolate. Lithium 2,2,6,6-tetramethylpiperidide. Methyl iodide. o-Nitro-phenyl selenocyanate. Propargyl bromide. rra s-l-Propenyllithium. Selenium. Tetrakis(triphenylphosphine)palladium. Titanium(IH) chloride. Titanium trichloride-Lithium aluminum hydride. p-Toluenesulfonylhydrazine. Triphenylphosphine. Vinyl-copper reagents. Vinyllithium. Zinc. 

Hotha and Tripathi [45] reported a practical method based on microwave irradiation for Perrier reaction of per-O-acetylated glycals 49a,b with primary, secondary, allylic, benzylic, and monosaccharide alcohols they used a catalytic amount of niobium pentachloride to give the corresponding 2,3-unsaturated O-glycosides 51 and 53 with a stereoselectivity in high yields and with short reaction times (Scheme 12.24). 

The following metal compounds are used for the preparation of the catalysts oxides, metal carbonyls, halides, alkyl and allyl complexes, as well as molybdenum, tungsten, and rhenium sulfides. Oxides of iridium, osmium, ruthenium, rhodium, niobium, tantalum, lanthanum, tellurium, and tin are effective promoters, although their catalytic activity is considerably lower. Oxides of aluminum, silicon, titanium, manganese, zirconium as well as silicates and phosphates of these elements are utilized as supports. Also, mixtures of oxides are used. The best supports are those of alumina oxide and silica. 

An early example was the failure to obtain penta-tertiary alkoxides of niobium owing to the preferential formation of the oxo compounds Nb20(0R)g and NbO(OR)3 in contrast to tantalum which formed thermally stable Ta(OBu )5. This behaviour was explained in terms of the greater tendency of niobium to form the metal-oxo double bond. In the reactions of W(NMe2)e with ROH the hexa-alkoxides W(OR>6 (R = Me, Et, Pr , allyl) were formed but not the tertiary butoxide which gave WO(OBu )4 instead. " The tendency of MoO(OR)4 compounds to form Mo02(OR>2 by ether elimination has also been noted (Eq. 5.4). ... 

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