Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Nickelacycle, seven-membered

Generally, cyclohexyne is an unstable molecule because of its ring strain. However, it can be stabilized by coordination to transition metals.35 The reduction of 1,2-dibromocyclohexene by sodium/mercury in the presence of a nickel-bromide complex afforded the Ni-alkyne complex 66 as a thermally stable and isolable compound (Scheme 22).36 Complex 66 smoothly reacted with C02 under atmospheric pressure to give nickelacycle 67 in good yield. Dimethyl acetylenedicarboxylate was inserted into the vinyl-nickel bond in 67 to give the seven-membered oxanickelacycle 68. [Pg.546]

Cyclobutanones can act as l-oxobutane-l,4-diyl units they undergo intermolecu-lar alkyne (RC=CR) insertion to give cyclohexanones (68), catalysed by Ni(cod)2, apparently via a seven-membered nickelacycle.197... [Pg.25]

Analogous to the cycloadditions described above, the first step of these ring expansion reactions is believed to involve the initial oxidative coupling between the carbonyl and the alkyne to afford a nickelapentenacycle (12, Scheme 2) [37,38]. Subsequent (3-carbon elimination relieves ring strain and affords a seven-membered nickelacycle 13a that reductively eliminates the... [Pg.168]

If instead the reaction is carried out with the basic ligand tri-n-propylphosphine and the ethene pressure is raised to 25 bar, n-pen-tenoic acid anilides are found in yields up to 770 %, corresponding to 7,7 catalytic cycles. The formation of the anilides can be explained by a common five-membered intermediate in which initial C-C-coup-ling between the isocyanate and ethene is assumed. If PPh3 is used as ligand, a rapid elimination of acrylanilide occurs. In the presence of PPrP, however, the insertion of a second ethene molecule into the Ni-C-bond predominates and a seven-membered nickelacycle is formed, the precursor of the isomeric pentenoic acid anilides. [Pg.65]

The reaction between 1,3-dienes and carbon dioxide leads to the formation of carboxylato (ri -allyl)nickel(II) complexes, which may equilibrate with seven-membered ring nickelacycles in which the metal coordinates the allyl in a Ti -fashion (Eq. 5). ... [Pg.9]

Xie and Qiu have reported the first example of the [2+2+2] carboannulation of arynes, activated alkenes, and arynes. Whereas Pd catalysts promote the two-component benzyne-alkene-benzyne cyclization, under Ni catalysis the three-component reaction is favored leading to 1,2-dihydronaphthalenes 108 from readily available materials. In this case, the catalytic cycle is likely initiated by oxidative coupling of aryne and alkene on Ni to form a nickelacycle that undergoes subsequent insertion of the alkyne into the Ni-C(aryl) bond to give a seven-membered intermediate (Scheme 12.54) [95]. [Pg.329]

It is very likely that the catalysis is initiated by a Ni(0) species which is generated from the reduction of Ni(II) by the lithiocarborane salt [21], Oxidative addition of the Ccage-I bond on Ni(0), followed by the elimination of Lil, affords the Ni-carboryne intermediate F. Sequential insertion of alkynes into Ni-C age and Ni-C jy, bonds produces the seven-membered nickelacycle B. Reductive elimination yields the cycloaddition product 2 and regenerates the Ni(0) species (Scheme 7.7). This work represents the first example of metal-catalyzed reaction of carboryne with unsaturated molecules. [Pg.86]

These results could be explained by an initial oxidative coupling between the carbonyl and the alkyne to afford a nickelapentenacycle that would undergo subsequent p-carbon elimination to produce a seven-membered nickelacycle. In the absence of IPr ligand, p-hydride elimination competed with the p-carbon one. With unsymmetric alkynes, the smaller substituent was mainly introduced a to the carbonyl group. [Pg.429]


See other pages where Nickelacycle, seven-membered is mentioned: [Pg.181]    [Pg.159]    [Pg.351]    [Pg.63]    [Pg.6]    [Pg.11]    [Pg.104]    [Pg.147]    [Pg.259]    [Pg.58]    [Pg.59]    [Pg.63]    [Pg.64]    [Pg.66]    [Pg.451]   
See also in sourсe #XX -- [ Pg.451 ]




SEARCH



Nickelacycle

Seven-membered

© 2024 chempedia.info