Nickel complexes nickelacycles

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. 

In relation to the above mechanisms, direct observation of oxidative cyclizations of nickel-enal complexes such as 191 with Ni(COD)2/R3P (1 equiv.) (R = Cy or Ph) to nickelacycles 192 was reported (Scheme 89).432... 

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. 

The reaction of (methyl acrylate)bis(triphenylphosphane)nickel (34) with 3,3-dimethylcyclo-propene provides the orange, crystalline nickelacycle, methyl 6,6-dimethyl-2-nickela-2-(triphenylphosphane)bicyclo[3.1.0]hexane-3-e c/o-carboxylate (35), in 65% yield.The same complex 35 can also be prepared in 74% yield from the reaction of the cyclopropene with (cyclododeca-l,5,9-triene)nickel and triphenylphosphane. Treatment of 35 with 1,2-bis(dimethylphosphino)ethane affords derivative 36 in 88% yield as yellow-orange crystals. 

Nickelacycles analogous to those involved as intermediates in the previous reactions are isolated by using nickel(0) complexes with biden-tate N-donor ligands. Thus, succinic anhydride reacts readily at 15 °C with Ni(COD)(bpy) in THF solution to form a red-brick six-membered nickelacycle 2, which undergoes decarbonylation to afford a five-mem-bered ring metallacycle 3 (Eq. 2). The expelled CO is trapped by the... 

Maleic anhydride acts as a r) -ligand with nickel(O) complexes and does not yield any nickelacycle. On the other hand, itaconic anhydride furnishes nickelacycle 8 in low yield, along with the Ti -complex 9, isolated as the major product. Complex 8 is also prepared by an alternative route based on the oxidative cycloaddition of carbon dioxide and allene. ... 

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). ... 

The reaction also proceeds with P.y-unsaturated carboxylic acids and nickel(O) complexes to yield six-membered ring nickelacycles 28, which after coordination with a bidentate ligand afford the ring-contracted metallacycles 29 (Eq. 2-Cyclopentencarboxylic acid reacts by... 

Analogously, reaction of a P,y-unsaturated primary carboxamide with a nickel(O) complex affords nickelacycle 36 (Eq. As before,... 

The analogous sulfur nickelacycles are prepared by an entirely different method. Thus, the oxidative addition of ethylene sulfide to a nickel(O) complex, formed in situ from 41 by reductive elimination of butane, gives thianickelacycle 42 (Scheme 11). Similarly, the oxidative addition of the four-membered heterocycle thietane with Ni(COD)bpy leads to five-membered ring complex 43. ... 

Insertion of carbon dioxide is also possible. For example, seven-mem-bered ring nickelacycle 11, derived from 1,3-butadiene, undergoes further carboxylation in the presence of pyridine to form a nickel(II) dicarboxylate complex 48, which yields a diester after treatment with methanol under acidic conditions (Eq. 20). ° Similar results are obtained in the carboxylation of (r) -diene)iron(0) complexes. ... 

Under milder conditions, and with the right choice of ligands, nickelacycles 4,288 can be isolated that may correspond to intermediates in the decarbonylation-decarboxylation reaction, and are related to carboxylation products (see Scheme 4.93). The alkyl carbon-nickel bond in these complexes can be intercepted by both alkyl halides (Scheme 4.100)," and organometallic species (Scheme 4.101). ... 

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. 

Miyashita, A. and Gmbbs, R.H. (1981) Reactions of nickel-carbene complexes generated from nickelacycle complexes. Tetrahedron Letters, 22, 1255—1256. 

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