Nickel complexes vinylidenes

Evidence for the formation of intermediates 113 and 114 has not been obtained yet. Once, disilacyclopentene 109 was isolated and its stmcture analyzed by spectroscopic and elemental analyses. It was treated with dimethylphe-nylsilane again to afford 117, in 1% yield. The formation of 117 was explained by a series of intermediates shown in Scheme 14. Insertion of a nickel species into one of the two trimethylsilyl-carbon bonds in 109 gave nickel complex 115. This, it was proposed, is followed by a trimethylsilyl shift from the sp -hybridized carbon to a nickel atom to give the reactive vinylidene carbene-nickel complex 116. The reaction of this nickel species with (dimethylphenyl)silane affords product 117. 

One-electron oxidation of the vinylidene complex transforms it from an Fe=C axially symmetric Fe(ll) carbene to an Fe(lll) complex where the vinylidene carbon bridges between iron and a pyrrole nitrogen. Cobalt and nickel porphyrin carbene complexes adopt this latter structure, with the carbene fragment formally inserted into the metal-nitrogen bond. The difference between the two types of metalloporphyrin carbene, and the conversion of one type to the other by oxidation in the case of iron, has been considered in a theoretical study. The comparison is especially interesting for the iron(ll) and cobalt(lll) carbene complexes Fe(Por)CR2 and Co(Por)(CR2) which both contain metal centers yet adopt... 

To date, no computational evidence has been presented to support these reaction mechanisms. Moreover, monomeric nickel vinylidene complexes have not been characterized outside of matrix isolation [37]. Given the unique properties of disilacyclobutenes, the significance of this work in the context of metal vinyli-dene-mediated catalysis remains to be established. 

The conventional vinylidene complex could be isolated when the hydroxyl group was protected as the tetrahydropyranyl ether derivative reaction of this with acid immediately gave the cyclic carbene complex, even under mild conditions. The reaction is related to the formation of similar nickel(II)- and platinum(IV)-carbene complexes from the [Pg.71]

Similarly, treatment of the nickel-ruthenium complex 62 with a variety of donors (CO, CNCy, NCfBu, PPh3) affords the doubly bridged complexes 78-81 (Scheme 7), wherein the donor binds to ruthenium. However, upon treatment with PhC=CH, only one carbonyl adopts a bridging mode, the second bridge being provided by a r2 vinylidene (63), derived from a 1,2-hydride shift within the PhC=CH ligand. 

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,... 

Methfune Is reported t-o react with photoexclted nickel atoms in Ar matrices to give MeNiH. A FTIR matrix Isolation study of the reactions of atomic and diatomic nickel in solid argon indicates that at very low concentrations of the metal, the nickel atom forms a ir-con lex with the triple bond of the acetylene.Photorearrangement of this complex gives nickel vinylidene, NiCCH2>... 

The formal cyclooligomerization of vinylidene units via copper or nickel car-benoids represents another route to [4]radialenes. To this end, a l,l-dibromo(or chloro)-alkene can be converted into a bis(l-halogenovinyl)cuprate at low temperatures, which, on warming, furnishes the [4]radialene, often accompanied by the corresponding dimerization product, the [3]cumulene. This method is so far the only route to octaphenyl[4]radialene (see Scheme 4.10, Section 4.2.1) and other octaaryl[4]radialenes [10, 37]. With l,l-dibromo-2-methylprop-l-ene (32) as the starting material, a mixture of [4]radialene 71 and [Sjradialene 84 as the major products is formed [76] (Scheme 4.17). While this method is not the preferred route to the former radialene, it represents the only known way for the preparation of 84. Treatment of dibromoalkene 32 with a Ni(0) complex [37] or activated nickel [10] does not improve the yield of radialene 71. 

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