Role of the paramagnetic nickel(I) complex in the catalytic dimerization of norbornadiene

In a joint investigation of the kinetic laws of the catalytic cyclodimerization of norbornadiene in the presence of bis-³-allyl complexes of nickel by gas chromatography and ESR, a definite correspondance in the change in the concentrations of norbornadiene, its dimers, and the paramagnetic nickel(I) complex was detected. The absence of an ESR signal in model systems, as well as the substantial differences in the relative amounts of Ni(I) when bis-₃ -allyl nickel complexes of different structure were used, suggests that paramagnetic Ni(I) does not participate as one of the intermediates in the catalytic process.M. V. Lomonosov Moscow Institute of Fine Chemical Technology. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 27, No. 4, pp. 513–516, July–August, 1991. Original article submitted October 16, 1989.     

Redoxreaktionen an nickelorganokomplexen durch nitrosoverbindungen und aliphatische aldehyde: Möglichkeiten des einsatzes der spintrapmethode

ESR investigations of the reaction between (bipy)Ni(C₂H₅)₂ and aromatic nitroso compounds (RNO) show the formation of paramagnetic, unstable complexes containing the radical RNO^. which is followed by elimination of nitroxide radicals C₂H₅(R)NO^..(bipy)Ni(COD) is oxidized by RNO to give nickel(I) species and several trapped radicals derived from COD. In the presence of aldehydes no paramagnetic nickel species, but ethyl radicals and spin adducts of the aldehydes can be observed. The mechanism of the reaction is discussed.     

An electroanalytical investigation of the olefin isomerization reaction promoted by electrogenerated cationic nickel(I) complexes

Summary Evidence for the ability of the electrogenerated cationic nickel(I) complex [Ni(PPh₃)₄]⁺ to promote the isomerization of allylbenzene is reported. However, the corresponding triethylphosphitenickel(I) complex displays no catalytic activity, apparently due to the poor leavinggroup character exhibited by the phosphite. The involvement of a -allylnickel hydride in the isomerization reaction is inferred from a comparison of the results obtained with those for the same reaction promoted by nickel hydride.     

Problems of the stereoselectivity in the norbornadiene [2+2]-cyclodimerization reactions catalyzed by hydride nickel(i) complexes. Theoretical aspects

Cyclodimerization of norbornadiene (NBD) yielding pentacyclic products of exo-trans(cis)endo-structure in the presence of the model catalytically active complex Ni(H)(η⁴-NBD) has been studied using the DFT/PBE method. The rate-limiting reaction step is the reductive elimination of the metallacycle, the decomposition of the latter yields the norbornadiene dimer.     

Paramagnetic complexes of nickel(I) stabilized by norbornadiene

Nickel(I) compounds whose concentration was 10^–4–10^–6 of the total concentration of nickel added to the system were identified by EPR in the reaction of 2,5-norbornadiene with nickel homoligand allyl complexes Niall₂ (all is C₃H₅, 1-CH₃C₃H₄, or 2-CH₃C₃H₄). The Ni(I) complexes were stable at room temperature under oxygen-free conditions. It was shown that the paramagnetic complexes were in equilibrium with diamagnetic forms. The temperature dependence of the concentration of the paramagnetic species was determined. The structure of the paramagnetic nickel(I) complexes and the possible routes of their formation are discussed on the basis of the obtained data.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 4, pp. 490–493, July–August, 1990.     

Propylene Dimerization by（η^5—C9H7）2 Nickel（Ⅱ）Catalytic System

The catalytic performance of Ni(η^5-Ind)2 complex in the dimerization of propylene was studied in combimation with an organoaluminum co-catalyst,eventually in the presence of a phosphine ligand.The effects of the type of aluminum co-catalyst and its relative amount,the nature of phosphine ligand and P/Ni ratio as well as the reaction temperature were examined.The results indicated that the nickel precatalyst exhibited high productivity for the propylene dimerization together with organoaluminum.It was likely to strongly modify the reactivity in the catalytic sytem when using phosphine ligand as additives,especially at the reaction temperature below 0℃.The catalytic system based on Ni(η^5-Ind)2 complex displaed an extremely high productivity(TOF up to 16900h^-1)and a good regioselectivity to 2,3-dimethylbutenes (2,3-DMB) in dimers(66.4%)under proper reaction parameters.     

Selective oxidation of ethylbenzene by dioxygen. The effect of chelate center on catalysis by bicyclic nickel complexes

The effect of the nature of the chelate center in Ni^II complexes on their catalytic activity in the selective oxidation of ethylbenzene by dioxygen to α-phenylethyl hydroperoxide in the presence of nickel bis(acetylacetonate) (chelate center Ni(O,O)₂) and nickel bis(enaminoacetonate) (chelate center Ni(O,NH)₂) was studied. The efficiency of selective oxidation of ethylbenzene increases substantially in the presence of the chelate with the Ni(O,NH)₂ active center as a catalyst, which is mainly due to the transformation of the catalyst into more active species during the oxidation process. The mechanism of transformation of nickel bis(enaminoacetonate) under the action of dioxygen was suggested. The sequence of formation of the reaction products at different stages of the catalytic process was determined. The activity of the nickel complex with the Ni(O,NH)₂ chelate center and the products of its transformation in the elementary stages of chain oxidation of ethylbenzene is discussed. Translated fromIzvestiya Akedemii Nauk. Seriya Khimicheskaya, No. 1, pp. 55–60, January, 1999.     

Formation of the paramagletic Ni(I) π-allyl complex in the Ni(allyl)2 - (2,6-diisopropylphenyl)diazabutadiene system

It has been shown that the reaction of Ni(allyl)₂ with (2,6-diisopropylphenyl)diazabutadiene gives the imino-amide Ni complex (1). The imino-amide moiety of this complex undergoes some complicated rearrangements resulting in spontaneous formation of a paramagnetic π-allyl Ni(I) complex. Nickel complexes formed in the system have been studied with ESR, FTIR, 2D NMR, and mass spectrometry. The structure of complex 1 was studied with X-ray diffraction.     

Nickel(I) complex as the final product of the sequence of spontaneous transformations in the system Ni(allyl)2-(2,6-diisopropylphenyl)diazabutadiene

A reaction of Ni(Allyl)₂ with bis(2,6-diisopropylphenyl)diazabutadiene gave an imino amide allyl nickel(II) complex (I). Complicated rearrangements of the imino amide ligand in the coordination sphere of complex I spontaneously yielded a paramagnetic Ni(I) ??-allyl complex as a final reaction product. The nickel complexes produced in this system were studied by EPR, IR, and 2D NMR spectroscopy and mass spectrometry. Structure I was examined by X-ray diffraction.     

Chemical synthesis with metal Atoms. Cyclodimerization of norbornadiene via nickela-cyclopentane intermediates.

The catalytic cyclodimerization of norbornadiene by reaction with nickel atoms has been re-investigated. Nickela-cyclopentane derivatives are formed in the presence of α,α′-dipyridyl, α,α′-dipyridyl-exo-trans-endo-3-nickela-pentacyclo[9.2.1.1^5,8 0^2,10.0^4,9]-pentadeca-6,12-diene being the major component. By contrast, the catalytic dimerization leads predominantly to the exo-trans-exo isomer of pentacyclo[8.2.1.1^4,7.0^2,9.0^3,8] tetradeca-5,11-diene. A norbornadiene cyclotrimer of exo-trans-exo-trans-exo structure is subsequently formed.     

Metallacyclic mechanism of the addition polymerization of norbornene involving Ni(I) and Ni(III) complexes

The catalytic system Ni(COD)₂/BF₃·OEt₂ is highly active in the addition polymerization of nor-bornene (NB). Its activity, which is up to 1930 (kg NB) (mol Ni)⁻¹ h⁻¹, is higher than the activity of the other known nickel complex catalysts. Another advantage of this system over the latter is that it contains a smaller proportion of a Lewis acid (5 molar parts or below) and no conventional stabilizing organoelement ligands. The activity of this system in NB polymerization has been investigated by Fourier-transform IR spectroscopy. According to EPR data, NB polymerization is accompanied by the formation of low-spin complexes of trivalent nickel, which result from the oxidative addition of the monomer to univalent nickel complexes. A metallacyclic mechanism involving Ni(I) and Ni(III) complexes is suggested for NB polymerization.     

Catalytic syntheses of polycyclic compounds based on norbornadiene-2,5 in the presence of nickel complexes: II. cyclic dimerization of norbornadiene-2,5. Kinetics and mechanism of the process

Kinetics of the cyclodimerization of norbornadiene-2,5 (NBD) is studied in the presence of a catalytic system based on bis(η³-allyl)nickel. The forms of the rate laws characterized by different reaction orders with respect to NBD are determined. The influence of temperature and solvent nature on the process is studied. The thermodynamic parameters are determined. The structure of the products is shown to be determined by the structure of intermediates. The mechanism of the process, consisting of the following main steps, is proposed: (1) the formation of Ni(NBD)₂, which is the true catalyst; (2) the reversible addition of NBD to the indicated complex, resulting in the formation of Ni(NBD)₃ and Ni(NBD)₄ η-complexes and accompanied by a change in ligand coordination; (3) the oxidative addition of coordinated NBD molecules to a nickel atom that gives five and six-membered metallacyclic intermediates; and (4) the reductive elimination of nickel from them to form cyclic dimers. The conditions for the selective formation of individual isomers are proposed.     

Redox properties of the nickel(II),(I),(0)-triphenylphosphine system in acetonitrile

The cathodic behaviour of electrochemically generated nickel(II) has been investigated in acetonitrile in the presence of triphenylphosphine at a platinum electrode. An appropriate combination of voltammetric, spectrophotometric and NMR findings has allowed us to establish that Ni(II) is present in solution as [Ni(PPh₃)₂ (CH₃CN)₄^2+.]. For the reduction of this species an EECE mechanism is proposed which is consistent with the data. It undergoes an irreversible two-electron reduction giging the Ni(0) complex [Ni(PPh₃)₄] which reacts quickly with the depolarizer. In this last homogeneous redox reaction the not previously reported Ni(I) complex [Ni(PPh₃)₄⁺] is obtained. The degree of reversibility of the redox processes involved has been discussed taking into account the structure, the coordination number and the nature of the ligands in both the redox partners.     

Study of the nature and mechanism of the formation of paramagnetic species in nickel-based Brookhart-type catalytic systems

The catalytic systems NiBr₂(DPP-DAB) + 20 MAO and NiBr₂(DPP-BIAN) + 20 MAO (DPP-BIAN = bis(2,6-diisopropylphenyl)-bis(imino)-acenaphtene, MAO = methylaluminoxane), as well as a number of model systems, are studied under conditions of their activation and functioning. There are paramagnetic nickel complexes and radical-anion aluminum complexes in the systems under real conditions of activation and functioning. The highest activity is observed when the Ni(I) signal intensity in the ESR spectrum is maximal. A mechanism of paramagnetic species formation is proposed.     

Well-Defined Ni(0) and Ni(II) Complexes of Bicyclic (Alkyl)(Amino)Carbene (MeBICAAC): Catalytic Activity and Mechanistic Insights in Negishi Cross-Coupling Reaction

Negishi cross-coupling reaction of organozinc compounds as nucleophiles with aryl halides has drawn immense focus for C−C bond formation reactions. In comparison to the well-established library of Pd complexes, the C−C cross-coupling of this particular approach is largely primitive with nickel-complexes. Herein, we describe the syntheses of Ni(II) complexes, [(^MeBICAAC)₂NiX₂] (X=Cl ( 1 ), Br ( 2 ), and I ( 3 )) by employing the bicyclic (alkyl)(amino)carbene (^MeBICAAC) ligand. The reduction of complexes 1 – 3 using KC₈ afforded the two coordinate low valent, Ni(0) complex, [(^MeBICAAC)₂Ni(0)] ( 4 ). Complexes 1 – 4 have been characterized by spectroscopic techniques and their solid-state structures were also confirmed by X-ray crystallography. Furthermore, complexes 1 – 4 have been applied in a direct and convenient method to catalyze the Negishi cross-coupling reaction of various aryl halides with 2,6-difluorophenylzinc bromide or phenylzinc bromide as the coupling partner in the presence of 3 mol % catalyst. Comparatively, among all-pristine complexes, 1 exhibit high catalytic potential to afford value-added C−C coupled products without the use of any additive. The UV-vis studies and HRMS measurements of controlled stochiometric reactions vindicate the involvement of Ni(I)−NI(III) cycle featured with a penta-coordinated Ni(III)-aryl species as the key intermediate for 1 whereas Ni(0)/Ni(II) species are potentially involved in the catalytic cycle of 4 .     

Catalytic cycloisomerization of enynes by using a nickel-zinc-acid system

Catalytic cycloisomerization of enynes has been accomplished in the presence of an Ni0-PPh3-Zn-carboxylic acid or -ZnCl2 system. A nickel(I)-hydride complex, thought to be generated by reduction of the protonated nickel(II) complex with Zn, is proposed as the catalytic species. This cycloisomerization shows reactivity behavior that is different from that of a conventional metal-catalyzed reaction. In particular, in the reaction with (E)-enynes, the catalytic system has a selectivity that favors the formation of the 1,3-diene over the 1,4-diene. In addition, this catalytic system has been applied to the domino cyclization of dienynes for the construction of tricyclic compounds.     

Cr/Ni-catalyzed vinylation of aldehydes: a mechanistic study on the catalytic roles of nickel and chromium

The roles of nickel and chromium catalysts in the coupling reaction of vinyl halides and aldehydes, the so-called Nozaki-Hiyama-Kishi (NHK) reaction, have been studied by UV/Vis spectroscopy, electrochemical, and spectroelectrochemical methods. Electrochemical studies revealed that nickel plays the central role in activating the vinyl halide by reductive cleavage, to form a rapidly decomposing vinyl-Ni species. The latter can, however, be stabilized in the presence of the Cr complex. The redox behavior of the Ni complexes in the presence of vinyl halide demonstrated that the vinyl halide activation results from interaction with a one-electron reduced nickel species [formally Ni(I) ], not necessarily with a Ni(0) species. It was furthermore shown by UV/Vis spectroscopy and spectroelectrochemical methods that low-valent nickel [Ni(0) ] results from the interaction of the Ni(II) catalyst with CrCl(2) .     

Synthesis and Reactivity of Paramagnetic Nickel Polypyridyl Complexes Relevant to C(sp2)–C(sp3)Coupling Reactions

A number of new transition metal catalyzed methods for the formation of C(sp²)–C(sp³) bonds have recently been described. These reactions often utilize bidentate polypyridyl‐ligated Ni catalysts, and paramagnetic Ni^I halide or aryl species are proposed in the catalytic cycles. However, there is little knowledge about complexes of this type. Here, we report the synthesis of paramagnetic bidentate polypyridyl‐ligated Ni halide and aryl complexes through elementary reactions proposed in catalytic cycles for C(sp²)–C(sp³) bond formation. We investigate the ability of these complexes to undergo organometallic reactions that are relevant to C(sp²)–C(sp³) coupling through stoichiometric studies and also explore their catalytic activity.     

Quantum-chemical study on the mechanism of catalytic dimerization of norbornadiene in the presence of hydride nickel(I) complex

[2+2]-Cyclodimerization of norbornadiene to pentacyclic products with exo-trans(cis)-exo structure in the presence of model catalytically active η⁴-norbornadiene nickel hydride complex was simulated at the DFT/PBE level of theory. According to the calculations, the rate-determining step in the cyclodimerization process is reductive elimination of the metallacycle whose decomposition yields norbornadiene dimer. The formation of cis-dimer is unfavorable for both kinetic and thermodynamic reasons.