Nickelacycle carbonylation

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... 

A nickelacycle 10, obtained from cyclopropabenzene 9 and Ni(cod)(PMc3)2, upon carbonyl-ative decomplexation gives a pentacyclic cyclopentanone 11 with two cyclopropane subunits. ... 

The carbonylation of nickelacycles proceeds readily at room temperature to give the corresponding cyclic anhydride by reductive elimination. For example, nickelacycle 44, prepared by oxidative addition of the corresponding cyclic anhydride and decarbonylation, regenerates the starting material after treatment with carbon monoxide (Scheme 12). Acid hydrolysis of the reaction mixtures allows the formation of dicar-boxylic acids, as illustrated for nickelacycles 30,45, and 46 (Scheme 12). 

Although the carbonylation reactions of nickelacycles prepared from cyclic anhydrides is of no synthetic interest, the corresponding reactions of metallacycles synthesized from unsaturated carboxylic acids or alke-... 

In contrast with the results obtained with simple allqfl halides, benzyl bromide leads to the formation of 77 and the ketone 78 in variable ratios (Scheme 26). A similar result has been reported in the reactions between the oxidative addition product of Ni(COD)bpy or Ni(COD)TMEDA with cw-4-cyclohexen-l,2-dicarboxylic anhydride and alkyl iodidesWith allyl bromide as the electrophile, ketone 79 is the only product isolated. However, when the reaction is performed with isolated nickelacycle 66 in the absence of Ni(CO)2Me2Phen, allylated alanine 80 is formed exclusively (60% yield) (Scheme 26). These results show that the carbonyl nickel complex is not inert because with certain reagents it transfers CO to the nickeMactone 66. Alternatively, the formation of ketones in these reactions could be explained by alkylation of the primary oxidative addition product or by carbonylation of allyl or benzyl bromide to give acyl bromides which react with 66 to give the observed products. However, this last reaction pathway seems unlikely because acetyl or benzoyl chloride do not react with in situ generated nickelacycle 66. 

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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