Nickel Reagents

RM= 1° RMgBr, RCH=CHAl(t-Bu)2, RCsCZnd, ArZnCl Chem Phann BuU 31 1108, 4346 (1983) 

MgBr2 (R= 1° alkyl, aryl) RMgX (R = Me, Ph) Me3SiCH2MgCl RMgX (R = n-Bu, Ph) 

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Nickel carbonyl is an extremely toxic substance, but a number of other nickel reagents with generally similar reactivity can be used in its place. The Ni(0) complex of 1,5-cyclooctadiene, Ni(COD)2, can effect coupling of allylic, alkenyl, and aryl halides. 

The Raney nickel reagent is prepared by addition of 9.5 g. of sodium hydroxide pellets over 8-10 minutes to a stirred suspension of 7.8 g of nickel-aluminum alloy (50% Ni, 50% A1 powder, purchased from Merck-Schuchardt) in 120 ml. of distilled water contained in a 250-ml. beaker. Fifteen minutes after the addition is completed, the beaker is immersed into a 70° water bath for 20 minutes. The water is decanted, and the catalyst is washed sequentially with two 20-ml. portions of distilled water and two 20-ml. portions of methanol. 

By contrast, formamidine acetate is not hygroscopic and no particular care need be taken to protect it from atmospheric moisture. Furthermore, formamidine acetate can be used directly without prior treatment with base in syntheses requiring free formamidine.8,7-10 Finally, this preparation of formamidine is by far the simplest and most convenient yet reported it obviates the necessity of using either toxic (hydrogen cyanide) or cumbersome (Raney nickel) reagents, and the method can be adapted to the preparation of N,N -disubstituted formamidines by substitution of primary amines for ammonia.11... 

Although these synthetic routes are effective, some problems were observed in the last reaction step. Spectroscopic and elemental analysis indicated that the nickellation of the pincer moiety was incomplete, giving an average of 80 to 90% of metallated pincer sites per dendrimer. This observation was rationalized by partial hydrolysis of the reactive lithiated species prior to the introduction of the nickel reagent, causing incomplete metalla-tion of the ultimate dendrimer species [37,38]. 

K-Allylnickel ImBiles (2, 291), Walter and Wilke reported briefly that Ni(COD)2 is more reactive than nickel carbonyl in the reaction of allylic halides to form 71-allyl-nickel halides. The reaction proceeds below 0° and in quantitative yields. The method has not been used extensively since nickel carbonyl is commercially available. However, Semmelhack recommends use of Ni(COD)2 for the preparation of thermally sensitive a-allyinickel halides such as it-(2-carbocthoxyallyl)nickel bromide," which was prepared from ethyl 2-bromomethylacrylate with this nickel reagent in 76% yield. 

In consecutive screening experiments for polymerization activity [vide infra) it was found that the triphenylphosphane donor was not easily removed. Therefore, methyl nickel precatalysts were synthesized starting from (tmeda)NiMe2 [31] as the nickel reagent, resulting in the formation of several compounds of which (N,O)2Ni and an oxygen-bridged dimer (N,O)Ni-Me 2 were identified by NMR and FAB-mass spectroscopy. However, this mixture of Ni hydroxy imine com-... 

This zerovalent nickel reagent can be generated in situ by reduction of bis(triphenylphospine)nickel(II) chloride with zinc in the presence of triphenyl-phosphine in DMF. 

The coupling of various haloquinolines and -isoquinolines has been effected with palladium " or nickel reagents - a couple of examples are shown below. Even chlorine, at the quinoline 2-position, will take part in such processes. A nice... 

The coupling of various haloquinolines and -isoquinolines has been effected with palladium or nickel reagents. ... 

Caubere and coworkers [43] developed another specific method, based on the unique stoichiometric nickel reagent. Reagent NiCRA (complex reducing agent) was prepared by reaction of sodium hydride (6 eq.) and tert-amyX alcohol (2 eq.) followed by addition of anhydrous nickel(II) acetate (1 eq.) and 2,2 -bipyridine (bpy, 2 eq. or other... 

For selective cross-coupling reactions between haloarenes and stoichiometric amounts of allyl nickel reagents, see (a) Corey, E.). and Semmelhack, M.E. 

Under conditions of high dilution to prevent intermolecular reactions, two allylic halide groups in the same molecule can be coupled to give a cyclic product. Macrocyclic lactones have been made in this way. Coupling occurs only at the primary centres. Note that the nickel reagents do not attack ester groups. Neither do they react with acid chlorides, ethers, nitriles, olefins or alkyl, aryl or vinyl chlorides (in contrast to bromides or iodides). Aldehydes and cyclic ketones are attacked, however, above 40°C affording homoallylic alcohols. 

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