Preparation and properties of aryl(dipyridyl)nickel halide complexes

Deactivation Modes in Nickel-Mediated Suzuki-Miyaura Cross-Coupling Reactions Using an NHC-Pyridonate Ligand

The catalytic activity of an NHC-pyridonate-supported nickel(0) complex for Suzuki-Miyaura coupling of aryl halides was evaluated. Product formation was observed in the absence of a basic additive. However, low turnover numbers resulted from competitive catalyst deactivation. The nature of deactivation-dimerization of the nickel(II) aryl intermediate-was elucidated through a combination of NMR monitoring, direct synthesis, and X-ray diffraction. Lewis basic and Lewis acidic additives were evaluated with the goal of improving the stability of the nickel(II) aryl intermediate but failed to enable catalytic turnover. Taken together, these findings highlight both the promise and the pitfalls associated with incorporating secondary-sphere Lewis basic groups for cooperative catalysis.

Synthetic Applications of Nickel-Catalyzed Cross-Coupling and Cyclisation Reactions

Since the first studies about fifty years ago, the direct formation of C-C bonds, catalyzed by nickel complexes, has appeared as an important research topic, and has re-emerged recently as a renewal of nickel chemistry. This account provides a summary of the use of nickel complexes in catalysis, and highlights the evolution of our own research.

Nickel-Catalyzed Electrochemical Cyclization of Alkynyl Aryl Iodide and the Domino Reaction with Aldehydes

An intramolecular carbometallation of a triple bond promoted by electrochemistry and mediated by nickel catalysis is described. This domino process transforms various aryl halides bearing a propargyl chain into substituted heterocycles in one single operation, with high stereoselectivities and in good to high yields. This reaction, characterized by a cyclic voltammetry set of experiments, proceeds following a syn-exo-dig cyclization process. When run at 80 °C, vinylbenzofuranes that are suitable substrates for cycloaddition reactions are obtained.

Exploiting the trifluoroethyl group as a precatalyst ligand in nickel-catalyzed Suzuki-type alkylations

We report herein the exploitment of the partially fluorinated trifluoroethyl as precatalyst ligands in nickel-catalyzed Suzuki-type alkylation and fluoroalkylation coupling reactions. Compared with the [L n NiII(aryl)(X)] precatalysts, the unique characters of bis-trifluoroethyl ligands imparted precatalyst [(bipy)Ni(CH2CF3)2] with bench-top stability, good solubilities in organic media and interesting catalytic activities. Preliminary mechanistic studies reveal that an eliminative extrusion of a vinylidene difluoride (VDF, CH2[double bond, length as m-dash]CF2) mask from [(bipy)Ni(CH2CF3)2] is a critical step for the initiation of a catalytic reaction.

Nickel-Catalyzed Enantioselective Conjunctive Cross-Coupling of 9-BBN Borates

Catalytic enantioselective conjunctive cross-coupling between 9-BBN borate complexes and aryl electrophiles can be accomplished with Ni salts in the presence of a chiral diamine ligand. The reactions furnish chiral 9-BBN derivatives in an enantioselective fashion and these are converted to chiral alcohols and amines, or engaged in other stereospecific C-C bond forming reactions.

Nickel-catalyzed direct formation of the C–S bonds of aryl sulfides from arylsulfonyl chlorides and aryl iodides using Mn as a reducing agent

Unsymmetrical aryl sulfides were synthesized by nickel-catalyzed arylsulfonyl chlorides and aryl iodides via intermediate disulfides using Mn as a reducing agent.

Unsymmetrical N-Aryl-1-(pyridin-2-yl)methanimine Ligands in Organonickel(II) Complexes: More Than a Blend of 2,2'-Bipyridine and N,N-Diaryl-α-diimines?

The new organonickel complexes [(R-PyMA)Ni(Mes)X] [R-PyMA = N-aryl-1-(pyridin-2-yl)methanimine; aryl = phenyl, 2,6-Me₂-, 3,5-Me₂-, 2,4,6-Me₃-, 2,6-ⁱPr₂-, 3,5-(OMe)₂-, 2-NO₂-4-Me-, 4-NO₂-, 2-CF₃-, and 2-CF₃-6-F-phenyl; Mes = 2,4,6-trimethylphenyl; X = F, Cl, Br, or I] were obtained as approximate 1/1 cis and trans isomeric mixtures or pure cis isomers depending on the PyMA ligand and X. The [(R-PyMA)Ni(Mes)X] complexes with X = Br or Cl were directly synthesized from the precursors trans-[(PPh₃)₂Ni(Mes)X], while [(PyMA)Ni(Mes)X] derivatives with X = F or I were obtained from [(PyMA)Ni(Mes)Br] through X exchange reactions. Although density functional theory (DFT) calculations show a preference for the sterically favored cis isomers, both isomers could be observed in many cases; in three cases, even single crystals for X-ray diffraction could be obtained for the trans isomers. Possible intermediates for the isomerization were investigated by DFT calculations. All complexes were studied by multiple spectroscopic means, electrochemistry, and spectroelectrochemistry (for the reduction processes). The long-wavelength metal-to-ligand charge-transfer (MLCT) absorptions vary markedly with the R substituent of the ligand and the cathodic electrochemical potentials to a far smaller degree. Both are almost invariable upon variation of X. All of this is in line with Ni-based and π*-based lowest unoccupied molecular orbitals (LUMOs). In line with the unsymmetric character of the N_Py^N_methanimine ligand, electrochemistry and MLCT transitions seem to not correspond to the same type of π* LUMO, making these PyMA ligands more interesting than the symmetric heteroaromatic polypyridine ligands such as 2,2'-bipyridine (bpy; N_Py^N_Py) and N,N-diaryl-substituted aliphatic α-diimines (N_methanimine^N_methanimine) such as the diaza-1,3-butadienes (DAB). First attempts to use these complexes in Negishi-type cross-coupling reactions were successful.

A modular, air-stable nickel precatalyst

The synthesis and catalytic activity of [(TMEDA)Ni(o-tolyl)Cl], an air-stable, crystalline solid, is described. This complex is an effective precatalyst in a variety of nickel-catalyzed transformations. The lability of TMEDA allows a wide variety of ligands to be used, including mono- and bidentate phosphines, diimines, and N-heterocyclic carbenes. Preliminary mechanistic studies are also reported, which suggest that [(TMEDA)Ni(o-tolyl)Cl] can activate by either a Ni-B or Ni-Ni transmetalation event, depending on the reaction conditions.

Nickel-catalyzed C-H arylation of azoles with haloarenes: scope, mechanism, and applications to the synthesis of bioactive molecules

Novel nickel-based catalytic systems for the C-H arylation of azoles with haloarenes and aryl triflates have been developed. We have established that Ni(OAc)(2)/bipy/LiOtBu serves as a general catalytic system for the coupling with aryl bromides and iodides as aryl electrophiles. For couplings with more challenging electrophiles, such as aryl chlorides and triflates, the Ni(OAc)(2)/dppf (dppf = 1,1'-bis(diphenylphosphino)ferrocene) system was found to be effective. Thiazoles, benzothiazoles, oxazoles, benzoxazoles, and benzimidazoles can be used as the heteroarene coupling partner. Upon further investigation, we discovered a new protocol for the present coupling using Mg(OtBu)(2) as a milder and less expensive alternative to LiOtBu. Attempts to reveal the mechanism of this nickel-catalyzed heterobiaryl coupling are also described. This newly developed methodology has been successfully applied to the syntheses of febuxostat (a xanthine oxidase inhibitor that is effective for the treatment of gout and hyperuricemia), tafamidis (effective for the treatment of TTR amyloid polyneuropathy), and texaline (a natural product having antitubercular activity).

Halide ligand-more than just sigma-donors? A structural and spectroscopic study of homologous organonickel complexes

The isoleptic organonickel complexes [(bpy)Ni(Mes)X] (bpy ) 2,2'-bipyridine; Mes ) 2,4,6-trimethylphenyl; X ) F,Cl, Br, or I, and for comparison X ) OMe and SCN) have been investigated by multiple spectroscopic means.Their structures have been determined in part by single-crystal X-ray diffraction, the full series by extended X-ray absorption fine structure. The long-wavelength charge transfer absorptions (mainly metal-to-ligand charge transfer)obtain contributions from the mesityl coligand but are almost invariable upon variation of X. UV-vis spectroscopy allowed investigation of the solvolysis reaction [(bpy)Ni(Mes)X] + Solv a [(bpy)Ni(Mes)(Solv)]+ + X-, which occurs very fast for X ) I (k ) 0.176(4) M(-1) s(-1)) or Br but very slow for X ) Cl (k ) 5.18(5) x 10(-5) M(-1) cm(-1))or F. Quantum chemical (density functional theory) calculations on the geometry, electronic states, and electronic transitions (time-dependent density functional theory) are very helpful for detailed insight into the role the X coligands play in these complexes. The combination of methods reveals rather strong, highly covalent Ni-X bonds for all halide coligands but marginal pi-donation.

Palladium(0)-Catalyzed Trimerization of Arylisocyanates into 1,3,5-Triarylisocyanurates in the Presence of Diimines: A Nonintuitive Mechanism

We show here that palladium(0) (dibenzylideneacetone) complexes bearing 1,10-phenanthroline constitute efficient catalysts for the cyclotrimerization of aromatic isocyanates. For the first time, the mechanism of this reaction has been investigated experimentally and theoretically with group 10 catalysts. This investigation provides a very consistent picture of the catalytic cycle. Notably, we establish that the reaction does not proceed by stepwise cycloadditions or ring insertions involving metallacyclic intermediates, as might have been anticipated. Rather, in our proposal, the initial steps of the mechanism resemble the chain-growth process operative during the anionic polymerization of isocyanates and feature charge-separated intermediates. These steps are then followed by ring closure on the metal center of the last intermediate formed to yield a seven-membered metallacycle that reductively eliminates the cyclotrimer and re-forms the active species. In addition, we conclusively show that the (known) palladacycles that could be isolated during the experimental investigations are not catalytic intermediates but result from catalyst deactivation. Thus, with Pd(0) diimine catalysts, the actual trimerization mechanism appears to be a blend between the two types of mechanisms proposed thus far for the oligomerization of heterocumulenes with very different catalysts. In conclusion, this work contributes to a better understanding of the reactivity of arylisocyanates in the vicinity of late group 10 metal centers in low oxidation state and sheds some light on the detrimental self-poisoning processes observed during the reductive carbonylation of nitroaromatic substrates catalyzed by related catalysts in non-nucleophilic media.

Electrochemical homocoupling of 2-bromomethylpyridines catalyzed by nickel complexes

2,2'-Bipyridine (bpy) and a series of dimethyl-2,2'-bipyridines were synthesized from 2-bromopyridine and 2-bromomethylpyridines, respectively, using an electrochemical process catalyzed by nickel complexes. The method is simple and efficient, with isolated yields between 58 and 98% according to the structure. We first studied the influence of the presence and the position of the methyl group on the yield, using N,N-dimethylformamide (DMF) or acetonitrile (AN) as the solvent, NiBr(2)bpy as the catalyst, and Zn as the sacrificial anode, in an undivided cell and at ambient temperature. On the basis of a better understanding of the reaction mechanism based on electroanalytical studies, we could improve the dimerization both by substituting the catalyst ligand (bpy) by the reagent itself, i.e., 2-bromomethylpyridine or 2-bromopyridine, and by using Fe instead of Zn as the sacrificial anode.

Nickel-catalyzed electrochemical couplings of vinyl halides: synthetic and stereochemical aspects

Homo- and cross-coupling involving alkenyl halides have been performed efficiently using an electroassisted nickel-complex catalysis. Valuable product such as conjugated dienes, beta,gamma- or gamma,delta-unsaturated esters, ketones, or nitriles, as well as alkenylated aryl compounds are thus prepared with high yields and high stereoselectivity. Partial isomerization is only observed in a few cases, when the alkenyl halide is involved in a late step of the catalytic cycle. This is the case in the preparation of (Z,Z)-1,3-diene.

Nickel-Catalyzed Direct Electrochemical Cross-Coupling between Aryl Halides and Activated Alkyl Halides

The electrochemical reduction of a mixture of aryl halides and activated alkyl halides in DMF in the presence of catalytic amount of NiBr(2)bipy leads to cross-coupling products in good to high yields. The method applies to the synthesis of alpha-aryl ketones, alpha-aryl esters, and allylated compounds from readily available organic halides. Optimization of the process has been obtained by slowly adding the most reactive organic halide (usually the activated alkyl halide) during the electrolysis which is best conducted at 70 degrees C when aryl bromides are involved.