Nickel catalytic system

The NHC-nickel catalytic system is also useful in the synthesis of pyridones 48. The [2h-2h-2] cycloaddition of diynes 44 and isocyanates 47 affords a wide range of pyridones 48 in excellent yields in presence of [Ni(COD)2]/SIPr catalytic system (Scheme 5.14) [18]. 

Reactions of substituted a-ketoalkynes (RC=CCOR ) with 6-amino-l,3-dimethyluracil and a water-soluble nickel catalytic system [Ni(CN)2-CO-KCN-NaOH] afforded 2,4-dioxopyrido[2,3- pyrimidine derivatives 532 under very mild conditions (room temperature and atmospheric pressure). The mechanism involved a nucleophilic attack by Ni(0), formed in situ, onto the triple bond of the substrate. The reaction terminates within 30 min, giving 98% of 532, while in the absence of this catalytic system the reaction took a longer time (lOh) to reach a maximum yield of 30% <2001J(P1)2341>. A regioselective interaction of 6-aminouracil derivatives with GF3COCH2COR in boiling AcOH afforded the cyclized 5-trifluoromethylpyrido[2,3-, pyrimidines 533 <200381531 >. 

The carbonylation of alcohols to give the next higher carboxylic acid is catalyzed by nickel. Catalytic systems are based on Ni(CO)4, NiX2, or metallic nickel in the presence of a halogen (86,87). In all of these cases, though, the active species can be derived from nickel(O) species, for example... 

In order to compare the new nickel catalytic system with the classical copper catalysis used for the arylation of alcohols (ref. 16), several experiments were performed with various copper catalysts (eqn. 5) (Table 5). 

Influence of the Precursor Composition and Reaction Conditions on Raney-Nickel Catalytic System... 

Much recent attention has been paid to the development of new ligand systems providing enhanced reactivity. As for nickel, catalytic systems of interest can be obtained with bulky NHC ligands. The family of PEPPSI (pyridine-enhanced precatalyst, preparation, stabilization, and initiation) precatalysts is well suited for crowded or poorly reactive electrophiles such as aromatic chlorides 67 (Scheme 5.12) [53]. As illustrated with the formation of 69 from biaryhnagnesium bromide 68, in the presence of PEPPSI-IPr (70) (2 mol%), reactions take place with high yields at room temperature in THF. It has been proposed that the pyridine attaches to and detaches from the Pd(0) complex in solution and therefore slows down the rate of palladium black formation and deactivation of the catalyst... 

Meanwhile, in 2013, Yamaguchi, Itami, and coworkers reported a C-H alkenylation of 1,3-azoles with enol derivatives (C-H/C-O type alkenylation) or alkenyl esters (decarbonylative C—H alkenylation) as electrophiles and applied this strategy to the formal synthesis of siphonazole B (Scheme 16.29b) [63]. Oxazole 137 was coupled with enol derivative 144 under their nickel catalytic system using the dcype ligand to form alkenyl oxazole 145 in 72% yield by C-H alkenylation at the C2 position of oxazoles. Since oxazole 145 was a known intermediate from the previous synthesis of siphonazole B [64], a convergent formal synthesis of siphonazole B has therefore been achieved. 

Earlier catalysts were based on cobalt, iron, and nickel. However, recent catalytic systems involve rhodium compounds promoted by methyl iodide and lithium iodide (48,49). Higher mol wt alkyl esters do not show any particular abiUty to undergo carbonylation to anhydrides. 

To give an idea of the wide rai e of catalytic systems that have been investigated where chemisorption data were essential to interpret the results, some of the author s papers will be discussed. Measurements were reported on the surface areas of a very wide range of metals that catalyze the hydrogenation of ethane. In the earliest paper, on nickel, the specific catalytic activity of a supported metal was accurately measured for the first time it was shown also that the reaction rate was direcdy proportional to the nickel surface area. Studies on the same reaction... 

The selective, Ni-catalyzed, biphasic dimerization of 1-butene to linear octenes has been studied in the author s group. A catalytic system well loiown for its ability to form linear dimers from 1-butene in conventional organic solvents - namely the square-planar Ni-complex (q-4-cycloocten-l-yl](l,l,l,5,5,5,-hexafluoro-2,4-pen-tanedionato-0,0 )nickel [(H-COD)Ni(hfacac)] [103] - was therefore used in chloroaluminate ionic liquids. 

The reaction takes place at low temperature (40-60 °C), without any solvent, in two (or more, up to four) well-mixed reactors in series. The pressure is sufficient to maintain the reactants in the liquid phase (no gas phase). Mixing and heat removal are ensured by an external circulation loop. The two components of the catalytic system are injected separately into this reaction loop with precise flow control. The residence time could be between 5 and 10 hours. At the output of the reaction section, the effluent containing the catalyst is chemically neutralized and the catalyst residue is separated from the products by aqueous washing. The catalyst components are not recycled. Unconverted olefin and inert hydrocarbons are separated from the octenes by distillation columns. The catalytic system is sensitive to impurities that can coordinate strongly to the nickel metal center or can react with the alkylaluminium derivative (polyunsaturated hydrocarbons and polar compounds such as water). 

To be applied industrially, performances must be superior to those of existing catalytic systems (activity, regioselectivity, and recyclability). The use of ionic liquid biphasic technology for nickel-catalyzed olefin dimerization proved to be successful. 

IH of alkynylamines has been performed with a variety of catalytic systems based on palladium [274-281], cobalt, rhodium, iridium, ruthenium, platinum, copper, silver, zinc, cadmium, mercury [279-281], nickel [279-282], gold [279-281, 283], and molybdenum [284] derivatives. 

A few further general examples of zinc catalytic activity or reactivity include the following. Other zinc-containing systems include a zinc phenoxide/nickel(0) catalytic system that can be used to carry out the chemo- and regioselective cyclotrimerization of monoynes.934 Zinc homoenolates have been used as novel nucleophiles in acylation and addition reactions and shown to have general utility.935,936 Iron/zinc species have been used in the oxidation of hydrocarbons, and the selectivity and conditions examined.362 There are implications for the mechanism of metal-catalyzed iodosylbenzene reactions with olefins from the observation that zinc triflate and a dizinc complex catalyze these reactions.937... 

Catalytic systems (platinum, palladium and nickel) have been investigated as have been non-metal lie reagents. Our results in these areas are also presented. 

Nickel-bpy and nickel-pyridine catalytic systems have been applied to numerous electroreductive reactions,202 such as synthesis of ketones by heterocoupling of acyl and benzyl halides,210,213 addition of aryl bromides to activated alkenes,212,214 synthesis of conjugated dienes, unsaturated esters, ketones, and nitriles by homo- and cross-coupling involving alkenyl halides,215 reductive polymerization of aromatic and heteroaromatic dibromides,216-221 or cleavage of the C-0 bond in allyl ethers.222... 

In the literature there are many reports of the formation of active catalyst for the 1 1 codimerization or synthesis of 1,4-hexadiene employing a large variety of Co or Fe salts, in conjunction with different kinds of ligands and organometallic cocatalysts. There must have been many structures, all of which are active for the codimerization reaction to one degree or another. The scope of the catalyst compositions claimed to be active as the codimerization catalysts is shown in Table XV (69-82). As with the nickel catalyst system discussed earlier, the preferred Co or Fe catalyst system requires the presence of phosphine ligands and an alkylaluminum cocatalyst. The catalytic property can be optimized by structural control of these two components. 

It is about 20 years since the combination of transition-metal catalysis and electroreduction was shown to be applicable to the coupling of organic molecules. This was followed by a number of fundamental investigations and basic syntheses using various nickel, cobalt, or pdladium compounds which can easily be reduced in situ electrochemically to low-valent reactive intermediates. The last decade has been less characterized by reports on new catalytic systems than by the development of new synthetic applications. The aim of this review is to show that the electrochemical processes described here offer valuable advantages in organic synthesis. 

Two catalytic cycles having the rr-aryl-nickel intermediate in common should be taken into account for the nickel-bpy system, as illustrated in Scheme 2. In the first cycle (left-hand side), similar to the previous one with dppe as ligand, Ni X which is formed in the reductive elimination step disproportion-ates into Ni° and Ni followed by a further reduction of Ni" [35]. An alternative mechanism has also been proposed in which the product results from the... 

The product of the electrochemical reaction was extracted with cyclohexane. The yields observed in the reactions of PhBr and PhCH2Br were 35 and 75%, respectively. In the reaction of PhCH2Br, no toluene was formed, indicating that the process was highly selective and that the reduction of the halogenated substrate was avoided. It was further verified that, at the end of the electrolysis, the catalytic system completely regained its reversibility. The nickel(II) catalyst remained totally in the ionic liquid after the extraction of products, and the catalyst system was reusable. 

The transition group compound (catalyst) and the metal alkyl compound (activator) form an organometallic complex through alkylation of the transition metal by the activator which is the active center of polymerization (Cat). With these catalysts not only can ethylene be polymerized but also a-olefins (propylene, 1-butylene, styrene) and dienes. In these cases the polymerization can be regio- and stereoselective so that tactic polymers are obtained. The possibilities of combination between catalyst and activator are limited because the catalytic systems are specific to a certain substrate. This means that a given combination is mostly useful only for a certain monomer. Thus conjugated dienes can be polymerized by catalyst systems containing cobalt or nickel, whereas those systems... 

To prevent systematic mistakes in the dilution series of the ligand standard solutions, leading to relative shifts in the [L]-control maps, we carried out independent control catalyses on the 250-ml scale. For the ([L]o/[Ni]o) ratio we selected inflection points in the varying product distribution of the [L]-control maps. In Fig. 3.2-2 is exemplified the [L]-control map of the catalytic system nickel/phenyl-diphenoxi-phos-phine/butadiene. ... 

In the system nickel/L/butadiene, secondary amines can shift the cyclodimerization of butadiene to the acyclic products (7a) and (75) Its cocatalyst functfon can be visualized by the corresponding [L]-control map (Scheme 3.3-2). In the three-component system nickel/morpholine/butadiene the open-chain products are formed for log ([morpholine]o/[Ni]o) > -1. Both octatrienes (7a) and (75) are formed at the constant ratio of 1.8 over the entire range of the examined amine/nickel scale. However, the efficiency of the catalytic system is low. After a turnover of 30% butadiene, the catalytic activity ends because of the formation of stop complexes of the nickel amide type. 

Although the nickel-containing systems have been extensively studied also by electrochemical methods [1] due to their practical importance, for example, in electrochemical power sources (Ni—Fe, Ni—Cd, Fi—NiF2 batteries), in corrosion-resistant alloys (tableware, coins, industrial instruments) as well as due to their interesting (magnetic, spectral, catalytic) properties most of the standard potentials of electrode... 

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