Nickelacycle initiators

Scheme 7 Fomation of chain-end functionalized nickelacycle initiators...

Zero-valent metal complexes cannot be used directly for the synthesis of polypeptide hybrid block copolymers. However, N° -allyloxycarbonyl-amino acid allyl amides can be used as universal precursors for the amido-amidate nickelacycle initiators (Scheme 15.8). As shown in Scheme 15.8, the N°"-allyloxycarbonyl-amino acid derivatives may undergo tandem oxidative additions to nickel(O) giving the nickelacycle initiators. This method was initially employed for the synthesis of block copolypeptides and was then expanded by Deming and coworkers to a variety of hybrid structures. 

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... 

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. 

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]. 

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. 

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. 

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