Alkyne tantalum complex

An intermolecular cyclotrimerization of an acetylenic nitrile was reported to proceed via the same alkyne-tantalum complex. The resultant pyridine derivative was obtained in 73% yield (Scheme 56).210... 

In 1982, Cotton et al prepared the first phosphinocarbene tantalum complex 99 from the reaction of Ta2Cl6(SMe2)3 with bis(diphenylphosphi-no)acetylene. The most remarkable feature of this reaction is the formal dimerization of the alkyne moiety.91... 

The chemistry of alkylidene and alkylidyne complexes of early transition metals was developed by Schrock and co-workers and these complexes turned out to be of crucial importance to alkene and alkyne metathesis. Initially their research focused on tantalum complexes of the type CpTaCEIE, which after a-elimination (Figure 16.6) led to alkylidene complexes Cp(R)Cl2Ta=CHR [11]. 

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

Scheme 17. Formation of supracyclopentadienyl derivatives from the alkylidyne moiety in the dimeric tantalum complex and alkynes (13). Drawings of molecules are schematic.

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. 

Scheme 51 Formation of niobium-alkyne and tantalum-alkyne complexes.

Tantalum(v) chloride can be used in the generation of 72-alkyne complexes (general formula 118) which have been shown to react with aldehydes to afford, after basic aqueous workup, allylic alcohols 119 (Scheme 50).180... 

C-M bond addition, for C-C bond formation, 10, 403-491 iridium additions, 10, 456 nickel additions, 10, 463 niobium additions, 10, 427 osmium additions, 10, 445 palladium additions, 10, 468 rhodium additions, 10, 455 ruthenium additions, 10, 444 Sc and Y additions, 10, 405 tantalum additions, 10, 429 titanium additions, 10, 421 vanadium additions, 10, 426 zirconium additions, 10, 424 Carbon-oxygen bond formation via alkyne hydration, 10, 678 for aryl and alkenyl ethers, 10, 650 via cobalt-mediated propargylic etherification, 10, 665 Cu-mediated, with borons, 9, 219 cycloetherification, 10, 673 etherification, 10, 669, 10, 685 via hydro- and alkylative alkoxylation, 10, 683 via inter- andd intramolecular hydroalkoxylation, 10, 672 via metal vinylidenes, 10, 676 via SnI and S Z processes, 10, 684 via transition metal rc-arene complexes, 10, 685 via transition metal-mediated etherification, overview,... 

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

Trisubstituted allylic alcohols. A low-valent tantalum prepared by reduction of TaCIs with Zn in DME/benzene adds to alkynes to form a complex that reacts with aldehydes to form (E)-allylic alcohols. The regioselectivity is determined by the bulkiness of the groups on the alkyne and of the R group of the aldehyde. 

A mononuclear tantalum-benzyne complex (121) has been prepared by thermolysis of 120 [Eq. (20)].14 An X-ray crystal structure was reported for 121. Bond lengths for the benzyne unit are given in Table III. Complex 121 exhibits a rich insertion chemistry similar to that of Ti, Zr, and Ru benzyne complexes. Insertion reactions of 121 with ethylene, 2-butyne, acetonitrile, and carbon dioxide give 122, 123, 124, and 125, respectively (Scheme 15). Diphenylacetylene does not couple with 121, presumably because of steric constraints. Reagents with acidic protons such as methanol or terminal alkynes cleave the Ta—C bond to give butyl isocyanide and carbon monoxide, but... 

Treatment of tantalum-alkyne complexes, prepared in situ from TaCls, Zn and RC=CR (R = n-C5Hn), with the lithio-imine Li N=CR Me (R = n-CgHn), followed by aqueous NaOH, gives primary ( )-allylic amines E-RCH=CR-CR Me(NH2). Treatment of these complexes with the terminal alkyne R"C=CH (R" = n-CeHis), followed by aqueous NaOH, yields tetrasubstituted benzene derivatives (99a-d). ... 

With the exception of(fj -C5Me5)Ta(HC=CH)Cl2 (71), the corresponding alkyne complexes of tantalum do not react with olefins 71 cyclizes with ethylene at 80°C to give the tantalacyclopentene 72 (Scheme 25). ... 

Reactivity characteristic of alkylidene complexes of tantalum is that the a-carbon is susceptible to electrophilic attack, in contrast to the electron-deficient a-carbon of Fischer-type carbene complexes of group 6 transition metals [62]. Based on this unique property of the alkylidene metal-carbon double bond, a range of new types of reactions has been developed. The discovery of the alkylidene complexes of tantalum was a key to understanding the mechanism of olefin metathesis, and they continue to play important roles in C—H bond activation, alkyne polymerization, and ring-opening metathesis polymerization. 

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