Nickel complexes reduction

Macrolactones 77 and/or 78 can be prepared from the reductive cyclisation of ynals 76 in the presence of NHC-nickel complexes (Scheme 5.21) [21], This maaolactonisation occnrs with different selectivity depending on the ligands attached to the nickel. If carbenes snch as IMes or IPr are nsed, the exocyclic olefin 77 is preferentially obtained, however when phosphine ligands are nsed, the endocyclic adducts 78 are preferentially obtained. 

Scheme 5.21 Macrolactones prepared from the reductive cyclisation of ynals in the presence of NHC-nickel complexes...

Nickel complexes of this group are of interest in biomimetic work. By means of ligand (320) the complete reaction cycle of acteyl CoA synthase could be executed (Scheme 2). Ligand (320) can also be synthesized by a template reaction. Upon reduction of the Ni11 complex (321) with Na/Hg, the ligand backbone is cleaved, resulting in a thermally stable trinuclear Ni11 alkyl thiolato complex (322). 

Bora-2,5-cyclohexadienes have a much greater synthetic potential than is apparent from the examples given so far. This may be exemplified by two recent reactions. Reductive complex formation in the system Co(acac)3/COD/25/Mg/THF affords complex 51 via the organotin route (77) while an earlier synthesis used the cobaltocene route (60). Ni(COD)2 very cleanly forms the (Tj3-l,4,5-cyclooctenyl)nickel complex 52 (29). 

Alkylnickel amido complexes ligated by bipyridine have been prepared that undergo reductive elimination of V-alkyl amines (Equation (54)).207,208 Unlike the phosphine-ligated palladium arylamides, these complexes underwent reductive elimination only after oxidation to nickel(III). Thermally induced reductive elimination of alkylamines from phosphine-ligated nickel complexes appears to occur after consumption of phosphine by arylazides 209... 

Low-valent nickel complexes of bpy are also efficient electrocatalysts in the reductive coupling reaction of aromatic halides.207 Detailed investigations are in agreement with a reaction mechanism involving the oxidative addition (Equation (40)) of the organic halide to a zero valent complex.208-210 Starting from [Nin(bpy)2(X)2]0 with excess bpy, or from [Nin(bpy)3]2 +, results in the [Ni°(bpy)2]° complex (Equations (37) and (38)). However, the reactive complex is the... 

The electrochemistry of cobalt-salen complexes in the presence of alkyl halides has been studied thoroughly.252,263-266 The reaction mechanism is similar to that for the nickel complexes, with the intermediate formation of an alkylcobalt(III) complex. Co -salen reacts with 1,8-diiodo-octane to afford an alkyl-bridged bis[Co" (salen)] complex.267 Electrosynthetic applications of the cobalt-salen catalyst are homo- and heterocoupling reactions with mixtures of alkylchlorides and bromides,268 conversion of benzal chloride to stilbene with the intermediate formation of l,2-dichloro-l,2-diphenylethane,269 reductive coupling of bromoalkanes with an activated alkenes,270 or carboxylation of benzylic and allylic chlorides by C02.271,272 Efficient electroreduc-tive dimerization of benzyl bromide to bibenzyl is catalyzed by the dicobalt complex (15).273 The proposed mechanism involves an intermediate bis[alkylcobalt(III)] complex. 

Cyclohexane-1,2-dione dioxime (nioxime) complexes of cobalt (II) and nickel (II) were concentrated from 10 ml seawater samples onto a hanging mercury drop electrode by controlled adsorption. Cobalt (II) and nickel (II) reduction currents were measured by differential pulse cathodic stripping voltammetry. Detection limits for cobalt and nickel were 6 pM and 0.45 mM, respectively. The results of detailed studies for optimising the analytical parameters, namely nioxime and buffer concentrations, pH, and adsorption potential are discussed. 

Reaction 2.22 a may be followed by various other reactions such as insertions, 13-eliminations or regular reductive eliminations (See Figure 2.24). The reductive elimination reaction is governed by the common rules given in the section on reductive elimination. The reaction shown has been observed for nickel complexes. 

The Pd-PPh3 system (Scheme 3) is characterized by a two-electron reduction step of the cr-aryl-palladium intermediate [37], as also proposed previously for aryl-nickel complexes ligated to PPha [23, 38]. The formation of the biaryl proceeds by reductive elimination from the diarylpalladium and regeneration of Pd°. 

Nucleophilic additions can also be carried out using nickel" complexes which lead to Ni° by cathodic reduction. Ni(0) complexes, notably Ni°bpy, react... 

Alternatively, CO2 can be used as source of CO. Indeed, it is well known that low-valent transition metal complexes can catalyze the chemical or electrochemical reduction of CO2 into CO. This approach was used to generate the mixed nickel complex Ni°bpy(CO)2 by the electrochemical reduction of Nibpy in NMP or DMF in the presence of CO2. The reduced complex can react with alkyl, benzyl, and allylhalides to give the symmetrical ketone along with the regeneration of Nibpy ". A two-step method alternating electroreduction and chemical coupling leading to the ketone has thus been set up (Scheme 9) [126,127]. 

The controlling factor of the reductive elimination on Pd(R)(C=CR )Lm (Eq. 5) may be different from that observed with the nickel complex. However, participation of a similar activation process by coordination of electron-with-drawing RX and R C=CH is conceivable. The Pd(R)(C=CR )Lm-type complex can be isolated, and it has been shown that isolated Pd(R)(C=CR )Lm undergoes the reductive elimination exhibited in Eq. 5 [8]. The reductive elimination seems to be enhanced by addition of Cul. Cul may interact with the Pd complex, and an acceleration effect of Lewis acids on the reductive elimination reaction of NiR2(bpy) has been shown [22]. The X-ray crystallographic structure of an isolated Pd(R)(C=CR )Lm (R=C6H4Me-p R =C6H5) has been determined [8]. 

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). 

Another example is butene dimerization catalyzed by nickel complexes in acidic chloroaluminates 14). This reaction has been performed on a continuous basis on the pilot scale by IFF (Difasol process). Relative to the industrial process involving homogeneous catalysis (Dimersol process), the overall yield in dimers is increased. Similarly, selective hydrogenation of diene can be performed in ionic liquids, because the solubility of dienes is higher than that of monoene, which is higher than that of paraffins. In the case of the Difasol process, a reduction of the volume of the reaction section by a factor of up to 40 can be achieved. This new Difasol technology enables lower dimer (e.g., octenes) production costs 14). 

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