Nickel mechanistic aspects

In this paper, results on the ECH of representative substrates are presented i) in order to illustrate the advantages and the limitations of the ECH at Raney metal electrodes as a method of hydrogenation of organic compounds and ii) in order to discuss some synthetic and mechanistic aspects. The Raney metal electrodes consisted of Raney metal particles embedded in a nickel matrix (1) or dispersed in a lanthanum polyphosphate matrix (2), or of pressed Raney metal alloy of which the outmost layer only has been leached (3). 

O Sullivan describes the fundamental theory, mechanistic aspects and practical issues associated with autocatalytic electroless metal deposition processes. Current approaches for gaining fundamental understanding of this complex process are described, along with results for copper, nickel and various alloys. Emphasis is placed on microelectronic applications that include formation of structures that are smaller than the diffusion layer thickness which influences structure formation. 

Steric constraints dictate that reactions of organohalides catalysed by square planar nickel complexes cannot involve a cw-dialkyl or diaryl Ni(iii) intermediate. The mechanistic aspects of these reactions have been studied using a macrocyclic tetraaza-ligand [209] while quantitative studies on primary alkyl halides used Ni(n)(salen) as catalyst source [210]. One-electron reduction affords Ni(l)(salen) which is involved in the catalytic cycle. Nickel(l) interacts with alkyl halides by an outer sphere single electron transfer process to give alkyl radicals and Ni(ii). The radicals take part in bimolecular reactions of dimerization and disproportionation, react with added species or react with Ni(t) to form the alkylnickel(n)(salen). Alkanes are also fonned by protolysis of the alkylNi(ii). 

Bailar and co-workers have used several complexes as catalysts for reduction of linolenic ester to linoleic or oleic ester without any reduction to the saturated stearic ester. A considerable portion of this work was carried out using a mixed catalyst consisting of tin(II) chloride and a platinum(II) complex. However, catalytic work has also been carried out using palladium and nickel complexes, the former again being used along with tin(II) chloride J91). The experimental details have been recently reviewed (f90) so that this article is concerned with the conclusions and mechanistic aspects rather than with the direct results. 

In terms of kinetics and mechanisms, electroless deposition processes have many similarities. In an attempt to analyze the electroless deposition, several mechanisms such as atomic hydrogen, hydride ion, metal hydroxide, electrochemical, and universal have been proposed.1-3 It is important to note that these mechanisms were developed for cases of nickel and copper electroless deposition, which were the most widely studied metals in this respect. Based on the proposed mechanisms, most of the features of electroless deposition can be explained. However, there are some characteristics of electroless deposition, which cannot be explained using these mechanisms. The major problems arise when attempting to generalize the proposed models explaining the mechanistic aspects. 

The most studied systems for oxidative propane upgrading are vanadium [2], vanadium-antimony [3], vanadium-molybdenum [4], and vanadium-phosphorus [5] based catalysts. Another family of light paraffin oxidation catalysts are molybdenum based systems, e.g. nickel-molybdates [6], cobalt-molybdates [7] and various metal-molybdates [8-9]. Recently, we investigated binary molybdates of the formula AM0O4 where A = Ni, Co, Mg, Mn, and/or Zn and some ternary Ni-Co-molybdates promoted with P, Bi, Fe, Cr, V, Ce, K or Cs [10-11]. A good representative of these systems is the composition Nio.5Coo.5Mo04 which was recently selected for an in depth kinetic study [12] and whose mechanistic aspects are now further illuminated here. 

The mechanism and regioselectivity of hydrocyanation is discussed in several reviews13 and a short recent survey on the current status of knowledge about stereochemistry and asymmetric induction is available9. In particular, synthetic and mechanistic aspects of the nickel-catalyzed hydrocyanatioii of butadiene is examined10 in view of the importance of the DuPont adiponi-trile process. 

To improve understanding of mechanistic aspects of Raney nickel s selectivity and activity, a series of seven variously substituted a, 3-unsaturated methyl esters were hydrogenated either under deuterium (deuteriumated) or under nitrogen over deuterated Raney nickel. For comparison, some of the molecules were deuteriumated over Pd/C, and in some instances the Raney nickel was modified with L-glutamic acid. 

The active species for the PBI complexes is not as well characterized as in the nickel and palladium systems. It is assumed to be a cationic alkyl complex formed by reaction of the dihalo precatalyst with a cocatalyst such as methylaluminoxane (MAO). The resulting active species polymerizes ethylene at unusually high rates to form linear high-density polyethylene. Even at ethylene pressures as low as 1 atm, the polymerization is extremely exothermic and the crystalline polymer product rapidly precipitates from solution. Computational chemistry is proving to be of utility in understanding the mechanistic aspects of this chemistry. - Lower barriers to insertion, relative to the nickel a-diimine complexes, support the higher activity. 

This leads us to propose a theoretically verified, refined catalytic cycle for production of linear and cycHc CiQ-olefin products (cf. Scheme 3). Furthermore, a detailed comparison of crucial mechanistic aspects of the catalytic reaction course for co-oligomerization of butadiene and ethylene and for cyclooligomerization of butadiene promoted by zerovalent bare nickel complexes was undertaken. These contribute (first) to a more detailed understan(fing of mechanistic aspects of the [Ni"]-mediated co-oHgomerization of 1,3-dienes and olefins and (second) to a deeper insight into the catalytic structure reactivity relationships in the transition-metal-assisted co-oHgomerization and oligomerization reactions of 1,3-dienes. 

MECHANISTIC ASPECTS OF O2-ACTIVATION ON NICKEL(H) TETRAHYDROSALEN COMPLEXES... 

Jutand, A. and Mosleh, A. (1997) Nickel- and palladium-catalyzed homocoupling of aryl triflates. Scope, limitation and mechanistic aspects. J. Org. Chem., 62, 261-74. 

In 2011, Nakai et al. found that acyclic compounds also participated in cycloaddition through carbon-carbon bond activation (Scheme 12.22). Reaction of o-arylcarboxybenzonitrile 56 and 4-octyne in the presence of a nickel catalyst, prepared in situ from Ni(cod)2 and PMes, and MAD in toluene at 120 °C for 12 h resulted in the formation of coumarin 58 in 80% yield. Sequential inter- and intramolecular carbon-carbon cleavage in the presence of a nickel catalyst has been used to construct flve-membered oxanickelacycle 57, which reacts with alkynes to furnish cycloadducts [27]. Detailed observations revealed that the catalytic reaction proceeded with the elimination of aryl cyanide. A similar sequence has been utilized for the synthesis of quinolone derivative 59 (Scheme 12.23). These reaction outcomes suggest an unusual mechanistic aspect cleavage of two independent C—CN and C—CO bonds via the formation of a heteronickelacycle intermediate. 

E. -i. Negishi, Selective Carbon-Carbon Bond Formation via Transition Metal Catalysis Is Nickel or Palladium Better Than Copper , in Aspects of Mechanistic and Organometallic Chemistry , ed. J. H. Brewster, Plenum Press, 1978, 285. 

As in COMC (1982) and COMC (1995), catalysis results are only mentioned in those cases where rt-bonded organometallic complexes have been isolated or characterized. However, since the discovery of the a-diimine nickel catalysts in 1996, the interest in the field has been strongly polarized toward the study of the new olefin polymerization and oligomerization catalysts, many of them cr-organonickel compounds themselves. In order to account for this important aspect of the chemistry of (j-bonded organonickel compounds, the different kinds of catalysts and mechanistic studies are discussed in Section 8.02.3.4.4. 

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