Nickel-catalysed amination of aryl chlorides using a dihydroimidazoline carbene ligand

The influential IPr: 25 years after its discovery

N-Heterocyclic carbenes (NHCs) have emerged as a privileged ligand family in organometallic chemistry, widely recognized for their unique steric and electronic properties. Among them, the 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene (IPr) ligand has become a cornerstone of NHC chemistry for its remarkable versatility, stability, and broad use. Since its discovery by the Nolan group in 1999, IPr has played a pivotal role in advancing catalytic transformations and facilitating the utilization of NHC ligands in various domains. This article highlights major contributions where IPr has helped shape modern organometallic chemistry, with a focus on its influence in transition metal catalysis and ligand design. Twenty five years after its discovery, the IPr ligand continues to be a benchmark ligand, inspiring and driving innovation.

Phenylboronic Ester-Activated Aryl Iodide-Selective Buchwald-Hartwig-Type Amination toward Bioactivity Assay

In this study, a phenylboronic ester-activated aryl iodide-selective Buchwald-Hartwig-type amination was developed. When the reaction of aryl iodides and aryl/aliphatic amines using Ni(acac)₂ is carried out in the presence of phenylboronic ester, the Buchwald-Hartwig-type amination proceeds smoothly to afford the corresponding amines in high yields. This reaction does not proceed in the absence of phenylboronic ester. A wide variety of aryl iodides can be applied in the presence of aryl chlorides and bromides, which remain intact during the reaction. The mechanistic studies of this reaction suggest that the phenylboronic ester acts as an activator for the amines to form the ″ate complex″. Chemical kinetics studies show that the reaction of aryl iodides, base, and Ni(acac)₂ follows first-order kinetics, while that of amines and phenylboronic ester follows zero-order kinetics. The bioactivity screening of the corresponding products showed that some amination products exhibit antifungal activity.

Selective amination of aryl chlorides catalysed by Ni(PMe3)4

C,N-coupling reactions of aryl chlorides and aryl amines catalyzed by a nickel catalyst are reported.

Nickel-Catalyzed Amination of Aryl Chlorides and Sulfamates in 2-Methyl-THF

The nickel-catalyzed amination of aryl O-sulfamates and chlorides using the green solvent 2-methyl-THF is reported. This methodology employs the commercially available and air-stable precatalyst NiCl2(DME), is broad in scope, and provides access to aryl amines in synthetically useful yields. The utility of this methodology is underscored by examples of gram-scale couplings conducted with catalyst loadings as low as 1 mol % nickel. Moreover, the nickel-catalyzed amination described is tolerant of heterocycles and should prove useful in the synthesis of pharmaceutical candidates and other heteroatom-containing compounds.

Development of an air-stable nickel precatalyst for the amination of aryl chlorides, sulfamates, mesylates, and triflates

A new air-stable nickel precatalyst for C-N cross-coupling is reported. The developed catalyst system displays a greatly improved substrate scope for C-N bond formation to include both a wide range of aryl and heteroaryl electrophiles and aryl, heteroaryl, and alkylamines. The catalyst system is also compatible with a weak base, allowing the amination of substrates containing base-sensitive functional groups.

Nickel-mediated N-arylation with arylboronic acids: an avenue to Chan-Lam coupling

An efficient use of NiCl(2)·6H(2)O, for the cross-coupling of arylboronic acids with various N-nucleophiles, has been demonstrated. The method is practical and offers an alternative to the corresponding Cu-mediated Chan-Lam process for the construction of the C-N bond.

Nickel-catalyzed amination of aryl sulfamates and carbamates using an air-stable precatalyst

A facile nickel-catalyzed method to achieve the amination of synthetically useful aryl sulfamates and carbamates is reported. Contrary to most Ni-catalyzed amination reactions, this user-friendly approach relies on an air-stable Ni(II) precatalyst, which, when employed with a mild reducing agent, efficiently delivers aminated products in good to excellent yields. The scope of the method is broad with respect to both coupling partners and includes heterocyclic substrates.

Reductions of challenging organic substrates by a nickel complex of a noninnocent crown carbene ligand

The first crown-tetracarbene complex of Ni(II) has been prepared, and its crystal structure determined. The complex can be reduced by Na/Hg, with an uptake of two electrons. The reduced complex reductively cleaves arenesulfonamides, including those derived from secondary aliphatic amines, and effects Birch reduction of anthracenes as well as reductive cleavage of stilbene oxides. Computational studies show that the orbital that receives electrons upon reduction of the complex 2 is predominantly based on the crown carbene ligand and also that the HOMO of the parent complex 2 is based on the ligand.

Half-sandwich NHC-nickel(II) complexes as pre-catalysts for the fast Suzuki coupling of aryl halides: a comparative study

Cationic half-sandwich nickel complexes of general formula [Ni(NHC)(NCMe)(eta(5)-C5R5)](PF6) [NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) a, 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) b; R = H, Me] were prepared from the reaction of their neutral homologues [Ni(NHC)Cl(eta(5)-C5R5)] with 1 equiv. of KPF6 in acetonitrile at room temperature. The new cationic complexes [Ni(IPr)(NCMe)(eta(5)-C5Me5)](PF6) 3a, [Ni(IMes)(NCMe)(eta(5)-C5Me5)](PF6) 3b and [Ni(IMes)(NCMe)(eta(5)-C5H5)](PF6) 4b were obtained in high yield and were fully characterized by 1H and 13C NMR spectroscopy, IR spectroscopy, elemental analyses, and in the case of 3a by a single-crystal X-ray diffraction study. The neutral analogue of 3a, [Ni(IPr)Cl(eta(5)-C5Me5)] 1a was also structurally characterized. Their geometries were compared and no significant structural differences were observed. Nevertheless solution NMR spectroscopy established that the acetonitrile ligand of the cationic species is labile in solution. This results in the absence of any rotational significant barrier about the nickel-carbene carbon bond at ambient temperature in solution in the sterically congested cationic complexes 3a and 3b, in contrast to their neutral analogues 1a and [Ni(IMes)Cl(eta(5)-C5Me5)] 1b. The neutral and the cationic complexes catalyzed the cross-coupling of phenylboronic acid with aryl halides in the absence of co-catalysts or reductants. Surprisingly, the neutral or cationic nature of the complexes proved to have almost no influence on the reaction yields and rates. However, complexes bearing the bulky electron-rich pentamethylcyclopentadienyl ligand were much more active than those bearing the cyclopentadienyl ligand, and TOF of up to 190 h(-1), a high rate for nickel(II) complexes under similar conditions, were observed with these species.

An Efficient Palladium-Catalysed Amination of Aryl Chlorides in Presence of 1,3-bis-(2,6-Diisopropylphenyl)Imidazolinium Chloride

1,3-bis-(2,6-diisopropylphenyl)imidazolinium chloride (SIPr·HCl), a precursor for an N-heterocyclic carbene, was examined as a pro-ligand in C–N coupling reactions. Thus SIPr·HCl associated with a palladium catalyst was found to be efficient for the amination of aryl chlorides under mild conditions.

A new aspect of nickel-catalyzed Grignard cross-coupling reactions: selective synthesis, structure, and catalytic behavior of a T-shape three-coordinate nickel(i) chloride bearing a bulky NHC ligand

A novel NHC-nickel(i) chloride was isolated by the reactions of Ni(IPr)₂ with chloroarenes. High catalyst activity of the Ni(i) complex was first demonstrated in Grignard cross-coupling reactions of aryl halides.

N-heterocyclic carbenes: a new concept in organometallic catalysis

N-Heterocyclic carbenes have become universal ligands in organometallic and inorganic coordination chemistry. They not only bind to any transition metal, be it in low or high oxidation states, but also to main group elements such as beryllium, sulfur, and iodine. Because of their specific coordination chemistry, N-heterocyclic carbenes both stabilize and activate metal centers in quite different key catalytic steps of organic syntheses, for example, C-H activation, C-C, C-H, C-O, and C-N bond formation. There is now ample evidence that in the new generation of organometallic catalysts the established ligand class of organophosphanes will be supplemented and, in part, replaced by N-heterocyclic carbenes. Over the past few years, this chemistry has been the field of vivid scientific competition, and yielded previously unexpected successes in key areas of homogeneous catalysis. From the work in numerous academic laboratories and in industry, a revolutionary turning point in oraganometallic catalysis is emerging.