Nickelacycle formation

The nickel(0)-promoted co-cyclization oi unsaturated organic molecules has been extended to useful catalytic reactions, where nickelacycle formation similar to eq (28) and (33) has been postulated (eq (34) (36)) [43 -45]. 

The advantage of using a sacrificial anode has been clearly pointed out. Magnesium was found to be the most convenient, the oxidation of which produces Mg ions which can enter the catalytic cycle to cleave the nickela-cycle intermediate and liberate Ni for further catalytic cycles (Scheme 7). Such a mechanism has been substantiated on the basis of the formation of the nickelacycle and its characterization by cyclic voltammetry. In the absence of Mg (reactions conducted in a divided cell in the presence of ammonium ions) the nickelacycle does not transform and the reaction stops when all the starting nickel compound has been reacted. Upon addition of MgBr2 to an electrochemically prepared solution of the nickelacycle, Ni(II) is recovered [114]. 

The formation of the dimerization products can be envisioned as resulting from reductive elimination from nickelacycle 7. Besides 2, which directly results from this process, 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. 

The nickel-catalysed 2 + 2 + 2-cycloaddition of a 3-methyl-2-pyridyl aldimines with alkynes produced 1,2-dihydropyridine adducts in good yields. A key step in this transformation is the formation of aza-nickelacycle intermediates. The iron-catalysed 2 + 2 + 2-cycloadditions of alkyne nitriles with alkynes, in the presence of pyridyl bisimine ligands (95), formed substituted pyridines in good yields. The nickel-catalysed 2 + 2 + 2-cycloadditions of diynes and cyanamides have been investigated. The reactions have been shown to be regioselective, and cycloadducts are produced in good to excellent yields. ... 

The intermediate nickelacycles decompose by P-hydride elimination to form mixtures of alkenes. For example, tranj-dimethylsuccinic anhydride reacts with Ni(CO)j(PPh3)2 to form a 10 10 1 mixture of trans-and ds-2-butene and 1-butene (Scheme 4). The formation of these alkenes indicates that an intermediate nickel hydride, such as the one shown in Scheme 4, is involved because control experiments demonstrate that no isomerization of the alkenes takes place under the reaction conditions. ... 

This isomerization reaction leads to the buildup of a new stereogenic center. Therefore, addition of a chiral diphosphine may lead to the formation of an optically active product. Addition of the chiral bidentate diphosphine (5, -2,3-bis(diphenylphosphino)butane (5,S)-chiraphos) to a CHjCIj solution of the six-membered ring nickelacycle derived from glutaric anhydride leads to a mixture of diastereomers with a 16% diastereomeric excess favoring the R configuration (Scheme 5). However, on standing at 24 °C, the mixture of diastereomers equilibrates favoring the thermodynamically more stable S nickelacycle isomer with 52% diastereomeric excess. 

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). ... 

Oxanickelacycles of type la have also been synthesized by an alternative procedure based on inserting of carbon dioxide into the Ni-C bond of certain reactive nickelacycles. Thus, reaction of complex 13 with carbon dioxide leads to die formation of metallacycle 14 resulting from an insertion into the Ni-C(aryl) bond of 13 (Eq. 7). Complex 13 is... 

Although reactions in the presence of PCyj as the ligand lead to the formation of nickelacycles, treatment of acrylic acid or acrylamide with... 

The formation of nickelacycles from a,3 unsaturated carboxylic acids or amides can be rationalized as shown in Scheme 8. Thus, formation of nickelacycles from carboxylic acids or amides proceed by oxidative addition of the polar followed by insertion of the alkene into the... 

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). 

Methylation of nickelacycle 30, obtained in the reaction between Ni(COD)(py)2 and 2-cyclopentencarboxylic acid, unexpectedly leads to the formation of cw-2-methylcyclopentanecarboxylic acid (Scheme 29) 33(0 pjijs product probably arises by methylation of isomeric nickelacycle 56, formed by 3-hydride elimination and insertion with the opposite regiochemistry. When the same reaction is carried out with a large excess of iodomethane, d.y-3-methylcyclopentanecarboxylic acid is obtained. Methylation reactions of related azanickelacycles have also been reported. ... 

C, a 1 1.5 ratio of the same two products is obtained. The formation of alkyne 74 may be explained by an elimination reaction of the alkenyl Ni(II) complex. Alternatively, this alkyne may be the result of the reaction of nickelacycle 66 with the alkyne to form an alkynyl Ni(II) complex, which may undergo reductive elimination to form 74. Alkenyl derivative 75 probably arises from insertion of the alkyne into the Ni-C... 

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. 

Scheme 5.5 shows two possible ways through which a nickelacycle carboxylate may, in principle, further react, once formed. Formation of p-Iactone, which may... 

Scheme 2.23) [36]. Formation of cyclopentene byproducts is attributed to P-hydride elimination from a nickelacycle followed by reductive elimination. Maleate and fumarate also reacted similarly. A comparison of the stereochemistry of thermal and catalyzed reactions was performed by studying the results of a deuterium-labeling experiment. All attempted reactions of bicyclo[3.1.0]hexane and bicyclo[4.1.0]heptane were unsuccessful. 

Fig. 7 Formation of amido-amidate nickelacycle form allyloxycarbonylaminoamides [69] X is a functional group, a polymer chain or another peptide...

Finally, Houk and Louie reported a novel cycloaddition of diynes and aromatic tropone catalyzed by IPr-Ni or SIPr-Ni leading to fused tricyclic products. The detail of the mechanism showed that the major formation of [5-6-7]-fused heterocycles could be rationalized by an 8it insertion of tropone onto a nickelacycle obtained by oxidative coupling of the diyne (Scheme 10.9). 

---