Niobium alkenes

The pinacol-type coupling of aliphatic aldehydes, in the presence of niobium(IH) salts, occurs, with a high anti diastereoselectivity (equation 131)512. In the case of aromatic aldehydes and ketones the alkene product is sometimes formed513. In both cases the cyclic acetals may also be formed. 

Bis(cyclooctene)—iridium(I) complexes, preparation, 7, 316 Bis(cyclopentadienyl) alkenes, with tantalum, 5, 157 Bis(cyclopentadienyl) alkyne niobium complexes, characteristics, 5, 81... 

Cyclopentadienones, in molybdenum carbonyls, 5, 451 Cyclopentadienyl alkenes, with niobium, 5, 76 Cyclopentadienyl-alkoxo complexes, with Ti(IV), 4, 495 Cyclopentadienyl alkyl complexes, with niobium, 5, 66 Cyclopentadienyl-amido complexes, with Ti(IV), via dehalosilylation, 4, 448... 

Heterometal alkoxide precursors, for ceramics, 12, 60-61 Heterometal chalcogenides, synthesis, 12, 62 Heterometal cubanes, as metal-organic precursor, 12, 39 Heterometallic alkenes, with platinum, 8, 639 Heterometallic alkynes, with platinum, models, 8, 650 Heterometallic clusters as heterogeneous catalyst precursors, 12, 767 in homogeneous catalysis, 12, 761 with Ni—M and Ni-C cr-bonded complexes, 8, 115 Heterometallic complexes with arene chromium carbonyls, 5, 259 bridged chromium isonitriles, 5, 274 with cyclopentadienyl hydride niobium moieties, 5, 72 with ruthenium—osmium, overview, 6, 1045—1116 with tungsten carbonyls, 5, 702 Heterometallic dimers, palladium complexes, 8, 210 Heterometallic iron-containing compounds cluster compounds, 6, 331 dinuclear compounds, 6, 319 overview, 6, 319-352... 

Mono(cyclopentadienyl) complexes alkenes and tantalum, 5, 157 alkyne and niobium, 5, 80 calcium, strontium, barium, 2, 133... 

Despite dramatically different ancillary ligand sets, two distinct niobium and tantalum alkylidene systems provide isolable metallacyclobutanes upon reaction with ethylene. In one case, the tantalum aryldiamine pincer complex 148 reacts with ethylene to provide the cr-trimethylsilyltantalacyclobutane complex 149 (Equation 66) <19940M3259>. In a more comprehensive study, alkadiene-supported half-sandwich alkylidene complexes of both tantalum and niobium (the former isolable, the latter generated in situ) undergo [2+2] cycloaddition with a range of acyclic and cyclic alkenes, albeit in modest isolated yield (Equation 67). 

The reactivity of halogenated alkenes in the electrophilic hydrofluorination reaction decreases with an increasing number of halogen atoms. Thus polyhaloalkenes are rather resistant to the electrophilic addition of hydrogen fluoride.Therefore, the hydrofluorination needs catalysis, with boron trifluoride(Table 6), tantalum(V) fluoride (Table 5), antimony(V) chloride, niobium(V) fluoride,titanium(IV)chloride or molybdenum(V) chloride. ... 

The number of niobium and tantalum coordination compounds in oxidation states lower than II is limited, and they often are organometallic derivatives involving carbonyl, cyclopentadienyl, or alkenic ligands. 

It is noteworthy that the transition metals that serve as hydroaluminating catalysts are also active in establishing the equilibrium between aluminum alkyls and their decomposition products, aluminum, hydrogen and alkene (equation 12). Accordingly, these metals, in addition to hafnium, niobium, vanadium, scandium and lanthanum, have found use as activators for the direct synthesis of aluminum alkyls (equation 12, to the left). Probably most of these metal salts will also be capable of accelerating the hydroalumination reaction. 

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. 

Alkyl ligands in niobium and tantalum complexes are susceptible to attack by electrophiles see Electrophilic Reaction). Hydrogenation see Hydrogenation) of niobium or tantalum M-R bonds to provide the metal hydrides is an important reaction of synthetic utility. Insertion reactions of unsaturated reagents into Nb- or Ta-C bonds are common. The unsaturated reagents include alkenes, alkynes, CO, NO, RN=C=NR, CNR, and others. 

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