Formation of CNHC∧Calkyl and CNHC∧Caryl κ2-Chelate Complexes through Competitive sp3- and sp2-CH Activations: An Experimental and Computational Study

Depending on the chelate ring present, cyclometalated complexes are useful catalysts for various reactions. The reactivity of IrIII and RhIII NHC complexes bearing aliphatic or aromatic N,N'-substituents and thus featuring various metalation sites toward cyclometalation has been investigated. The RhIII complex bearing an N-mesityl-N'-benzyl-NHC does not participate in any cyclometalation, while the IrIII complex reacts under metalation of an ortho-methyl group of the Mes substituent to give complex [3] with a six-membered chelate ring. The RhIII and IrIII complexes bearing an N-o-tolyl,N-benzyl-NHC undergo sp2-CH activation to yield the cyclometalated complexes [4] and [5] featuring a five-membered CNHC∧C chelate ring. Density functional theory (DFT) studies corroborated the experimental findings.

Hydration Reactions Catalyzed by Transition Metal-NHC (NHC = N-Heterocyclic Carbene) Complexes

The catalytic addition of water to unsaturated C-C or C-N π bonds represent one of the most important and environmentally sustainable methods to form C-O bonds for the production of synthetic intermediates, medicinal agents and natural products. The traditional acid-catalyzed hydration of unsaturated compounds typically requires strong acids or toxic mercury salts, which limits practical applications and presents safety and environmental concerns. Today, transition-metal-catalyzed hydration supported by NHC (NHC = N-heterocyclic carbene) ligands has attracted major attention. By rational design of ligands, choice of metals and counterions as well as mechanistic studies and the development of heterogeneous systems, major progress has been achieved for a broad range of hydration processes. In particular, the combination of NHC ligands with gold shows excellent reactivity compared with other catalytic systems; however, other systems based on silver, ruthenium, osmium, platinum, rhodium and nickel have also been discovered. Ancillary NHC ligands provide stabilization of transition metals and ensure high catalytic activity in hydration owing to their unique electronic and steric properties. NHC-Au(I) complexes are particularly favored for hydration of unsaturated hydrocarbons due to soft and carbophilic properties of gold. In this review, we present a comprehensive overview of hydration reactions catalyzed by transition metal-NHC complexes and their applications in catalytic hydration of different classes of π-substrates with a focus on the role of NHC ligands, types of metals and counterions.

Optimizing Catalyst and Reaction Conditions in Gold(I) Catalysis-Ligand Development

This review considers phosphine and N-heterocyclic carbene complexes of gold(I) that are used as (pre)catalysts for a range of reactions in organic synthesis. These are divided according to the structure of the ligand, with the narrative focusing on studies that offer a quantitative comparison between the ligands and readily available or widely used existing systems.

Synthesis, characterization, and antileishmanial activity of neutral N-heterocyclic carbenes gold(I) complexes

A series of five new mononuclear neutral gold(I) complexes containing N-heterocyclic carbenes (NHCs) was synthesized and fully characterized by spectroscopic methods. The X-ray structures of four complexes are presented. These gold(I) complexes together with four other neutral gold(I)-NHC complexes previously described were evaluated in vitro against Leishmania infantum promastigotes and axenic amastigotes. Moreover, their cytotoxicity was assessed on the murine macrophages J774A.1. Except one complex (10), eight gold(I)-NHC-Cl complexes show potent activity against the pathological relevant form of L. infantum amastigote with IC₅₀ in the low micromolar and submicromolar range and five of them exhibit a SI close to 10. The lead-complex 11 displays a very high and selective activity (IC₅₀ = 190 nM, SI = 40.29) and constitutes the best promising gold(I)-based drug of this series.

Novel cis-[(NHC)1(NHC)2(L)Cl]platinum(ii) complexes – synthesis, structures, and anticancer activities

Platinum(ii) complexes bearing two different, cis-oriented, N-heterocyclic carbene ligands are readily accessible for catalysis or as selective, pleiotropic anticancer agents.

Synthesis, crystal structures, and in vitro anticancer properties of new N-heterocyclic carbene (NHC) silver(i)- and gold(i)/(iii)-complexes: a rare example of silver(i)–NHC complex involved in redox transmetallation

A series of cationic, linearly coordinated silver(i)– and gold(i/iii)–NHC complexes of (benz)imidazol-2-ylidene ligands was prepared and successfully characterized.

Efficient synthetic protocols for the preparation of common N-heterocyclic carbene precursors

The one-pot condensation of glyoxal, two equivalents of cyclohexylamine, and paraformaldehyde in the presence of aqueous HBF₄ provided a straightforward access to 1,3-dicyclohexylimidazolium tetrafluoroborate (ICy·HBF₄). 1,3-Dibenzylimidazolium tetrafluoroborate (IBn·HBF₄) was obtained along the same lines. To synthesize 1,3-diarylmidazolium salts, it was necessary to isolate the intermediate N,N'-diarylethylenediimines prior to their cyclization. Although this additional step required more time and reagents, it led to a much more efficient overall process. It also proved very convenient to carry out the synthesis of imidazolinium salts in parallel to their imidazolium counterparts via the reduction of the diimines into diammonium salts. The critical assembly of the C² precarbenic unit was best achieved with paraformaldehyde and chlorotrimethylsilane in the case of imidazolium derivatives, whereas the use of triethyl orthoformate under microwave irradiation was most appropriate for the fast and efficient synthesis of imidazolinium salts. This strategy was applied to the synthesis of six common N-heterocyclic carbene precursors, namely, 1,3-dimesitylimidazolium chloride (IMes·HCl), 1,3-dimesitylimidazolium tetrafluoroborate (IMes·HBF₄), 1,3-dimesitylimidazolinium chloride (SIMes·HCl), 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride (IDip·HCl or IPr·HCl), 1,3-bis(2,6-diisopropylphenyl)imidazolinium chloride (SIDip·HCl or SIPr·HCl), and 1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazolium chloride (IDip*·HCl or IPr*·HCl).

Engineering ligands on the Au center: discovering broadly applicable gold catalysis with high turnover numbers

The research groups of Hammond/Xu and Zhang report the discovery of new ligands for gold catalysis.