Nickel zerovalent

In 1977, Fahey and Mahan described the first C-F oxidative addition of hexafluo-robenzene (C ) to the zerovalent nickel complex [Ni(cod)(PEt3)j] [11], The reaction was very slow and led, after several days at 30-35°C, to the formation of trani-[Ni(CgF3)(F)(PEt3)2] in very poor yield (7%). The use of [NiCPEtj) ] for the same reaction by Perutz and co-workers 20 years later allowed for a better conversion but the reaction rate remained slow (4 weeks were necessary to obtain 48% of the desired product) [12],... 

Although the copper mediated Ullmann reaction is a well known method for biaryl synthesis, drastic conditions in the range of 150-280 °C are required. Zerovalent nickel complexes such as bis(l,5-cyclooctadiene)nickel or tetrakis(triphenylphosphine)nickel have been shown to be acceptable coupling reagents under mild conditions however, the complexes are unstable and not easy to prepare. The method using activated metallic nickel eliminates most of these problems and provides an attractive alternative for carrying out aryl coupling reactions(36,38). 

A3-Pyrrolinones have also been obtained from metal-mediated cyclooligomerization processes in which concomitant hydrolytic or carbonyl insertion occurs. For example, tert-butyl isocyanide is converted in aqueous methanol by zerovalent nickel compounds e.g., Ni(t-BuNC)4, Ni(CO)4, into a di(alkylamino)-A3-pyrrolinone in moderate yield (Scheme 34). The reaction takes a different course in anhydrous methanol in which a di-tert-butylamino)ethylene derivative is formed, albeit in poor yield (Scheme 34).62... 

Structure-Reactivity Relationships in the Cyclo-Oligomerization of 1,3-Butadiene Catalyzed by Zerovalent Nickel Complexes... 

T. Yamamoto, A. Morita, Y. Miyazaki, T. Marayama, H. Wakayama, Z.H. Zhou, Y. Nakamura, T. Kanbara, S. Sasaki, and K. Kubota, Preparation of ir-conjugated poly(thiophene-2,5-diyl), poly(p-phenylene), and related polymers using zerovalent nickel complexes. Linear structure and properties of the TT-conjugated polymers, Macromolecules, 25 1214—1223, 1992. 

The method can be favorably compared to the chemical method developed by Hoberg, who has used stoichiometric amounts of a zerovalent nickel complex, mostly the air-sensitive Ni(COD)2 [115, 116]. Instead, the electrochemical method uses a readily available starting Ni(II) complex. 

Eisch and Im (1977) found another simple example of a technique controlling the stereoisomeric composition of the reaction product. The technique consists of varying the time of contact between the reactants. Scheme 7.2 illustrates the transformation of p-(trimethylsilyl)styrene oxide into P-(trimethylsilyl)styrene under the action of complexes of zerovalent nickel the reaction involves oxidation of the complex-bonded metal. 

The trans isomer remains unchanged, whereas the cis isomer is converted into the trans form. The mixture thus becomes enriched with the molecules having the trans configuration. In a reference experiment, the treatment of pure cXp-CtrimethylsilyOstyrene with the same zerovalent nickel complex results in a 95% conversion into the trans isomer. 

II. Coordination Compounds of Zerovalent Nickel and Molybdenum with Fluorine-Containing Phosphine Ligands... 

Tetrakis(phenyldifluorophosphine)nickel-(0) could also be obtained, using arsenic trifluoride in the presence of catalytic amounts of antimony pentachloride, or zinc fluoride as fluorinating agents. Yields as high as 50% could be obtained, but sizable decomposition on the process of fluorination of the chlorophosphine nickel-(0) complexes in solution could not be entirely suppressed. The marked instability of zerovalent nickel-phosphine complexes in solution in organic solvents, even under strictly anhydrous and anaerobic conditions, has been noted by several workers (16,20), but is still lacking a detailed explanation. A closer examination of the system carbon tetrachloride-tetrakis(trichlorophosphine)nickel-(0) (23) showed the main pa h of the reaction to consist in the formation of hexachloro-ethane with conversion of zerovalent into bivalent nickel, while the coordinated... 

The comparatively simple method of preparation of tetrakis (trifluorophos-phine)nickel-(0) encouraged some scouting experiments on its still unexplored chemistry. Whereas the compound is hydrolytically remarkably stable, it was found to react readily with amines and ammonia with complete aminolysis of the phosphorus-fluorine bonds. Very typical of tetrakis(trifluorophosphine) nickel-(0) and similar fluorophosphine and chlorophosphine complexes of zerovalent nickel is the rapid decomposition with precipitation of elemental nickel by aqueous alkali hydroxide. 

Two reports have appeared on the catalyzed reaction of C02 with epoxides to form alkylene carbonates. One of the processes uses phosphine complexes of zerovalent nickel as the catalyst (157), and appears closely related to the more recent isolation of (PCy3)2Ni(C02) (115). Ethylene oxide reacts in benzene under 500 psi pressure of C02 in a stainless steel autoclave at 100°C to form ethylene carbonate with 95% selectivity, (77), using as the catalysts NiL2, L = PCy3 or PPh3. 

The copper-catalysed, Ullman-type coupling of aryl, heteroaryl and alkenyl halides may be achieved at ambient temperature using copper(I) thiophene-2-carboxylate as catalyst.60 A new semiconducting poly(anthraquinone-l,5-diyl) with nitro groups at the 4- and 8-positions has been prepared by Ullman-type coupling using metallic copper or a zerovalent nickel complex as catalyst.61... 

Benn R, Bussemeier B, Flolle S, Jolly PW, Mynott R, Tkatchenko I, Wilke G (1985) Transition-metal allyls. 6. The stoichiometric reaction of 1,3-dienes with ligand modified zerovalent-nickel systems. J Organomet Chem 279 63-86... 

The hydrocyanation of alkenes [1] has great potential in catalytic carbon-carbon bond-formation because the nitriles obtained can be converted into a variety of products [2]. Although the cyanation of aryl halides [3] and carbon-hetero double bonds (aldehydes, ketones, and imines) [4] is well studied, the hydrocyanation of alkenes has mainly focused on the DuPont adiponitrile process [5]. Adiponitrile is produced from butadiene in a three-step process via hydrocyanation, isomerization, and a second hydrocyanation step, as displayed in Figure 1. This process was developed in the 1970s with a monodentate phosphite-based zerovalent nickel catalyst [6],... 

However most organic halides are less easily reduced than Zn(II). So such a process can only be applied if one is able to catalyse the electroreduction of RX. This can be achieved by electrochemically formed zerovalent nickel complexes. Thus Ni(II)-2,2 -bipyridine (Ni-bpy) compounds can catalyse the electroreduction of alkyl and aryl halides1 at potential more positive than... 

The catalytic cycle proposed for the dimerization of butadiene is shown in Fig. 7.8. As shown by 7.24, two molecules of butadiene coordinate to NiL. A formal oxidative addition, as shown by Eq. 7.8, produces two nickel-carbon bonds and the carbon-carbon bond required for ring formation. The structure of 7.25 with two nickel-carbon bonds (see Fig. 7.8), is a hypothetical one that helps us to understand the carbon-carbon bond formation process. The actual catalytic intermediates that have been observed by spectroscopy have an rf-allyl type of bonding. As shown by reaction 7.9, species 7.25 can reductively eliminate 1,5-cyclooctadiene and the zerovalent nickel complex Ni-L. 

It is appropriate to include here two bis(T)3-allyl)nickel complexes that are formed by reacting zerovalent nickel species with 1,3-dienes and that have been shown to be involved as intermediates in the nickel-catalyzed cyclooligomerization of butadiene, viz., (T)3,i73-C12H18)Ni and (rj3-QH NiPRa. 

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