Bispentiptycenyl-N-Heterocyclic Carbene (NHC) Gold Complexes: Highly Active Catalysts for the Room Temperature Hydration of Alkynes

Gold Trifluoromethyl Complexes as Efficient Regioselective Catalysts in Alkyne Hydration

Gold(III) complexes containing trifluoromethyl ligands are efficient catalysts in the hydration of alkynes, operating at low catalyst loadings, without additives, using environmentally friendly solvents and at mild conditions (60 °C). Hydration of terminal and internal alkynes provides the corresponding ketones in quantitative yields without special precautions as dry solvents or inert atmospheres. Remarkably, hydration of asymmetric internal alkynes proceeds with moderate to notable regioselectivities, providing mixtures of the two possible isomers with ratios up to 90 : 10.

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.

H2O2-mediated room temperature synthesis of 2-arylacetophenones from arylhydrazines and vinyl azides in water

An environmentally benign, cost-efficient and practical methodology for the room temperature synthesis of 2-arylacetophenones in water has been discovered. The facile and efficient transformation involves the oxidative radical addition of arylhydrazines with α-aryl vinyl azides in the presence of H₂O₂ (as a radical initiator) and PEG-800 (as a phase-transfer catalyst). From the viewpoint of green chemistry and organic synthesis, the present protocol is of great significance because of using cheap, non-toxic and readily available starting materials and reagents as well as amenability to gram-scale synthesis, which provides an attractive strategy to access 2-arylacetophenones.

Acenaphthene-Based N-Heterocyclic Carbene Metal Complexes: Synthesis and Application in Catalysis

N-Heterocyclic carbene (NHC) ligands have become a privileged structural motif in modern homogenous and heterogeneous catalysis. The last two decades have brought a plethora of structurally and electronically diversified carbene ligands, enabling the development of cutting-edge transformations, especially in the area of carbon-carbon bond formation. Although most of these were accomplished with common imidazolylidene and imidazolinylidene ligands, the most challenging ones were only accessible with the acenaphthylene-derived N-heterocyclic carbene ligands bearing a π-extended system. Their superior σ-donor capabilities with simultaneous ease of modification of the rigid backbone enhance the catalytic activity and stability of their transition metal complexes, which makes BIAN-NHC (BIAN—bis(imino)acenaphthene) ligands an attractive tool for the development of challenging reactions. The present review summarizes synthetic efforts towards BIAN-NHC metal complexes bearing acenaphthylene subunits and their applications in modern catalysis, with special emphasis put on recently developed enantioselective processes.

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 of 2-Aryl Acetophenones via Hydrobromination and Oxy-isomerization of (o-Arylethynyl)benzyl Alcohols

Hydrobromination and oxy-isomerization of (o-arylethynyl)benzyl alcohols to yield brominated aryl ketones were achieved with bromotrimethylsilane. The substrate scope suggested that vinyl carbocations, stabilized by the conjugated aryl groups, are the reaction intermediates. 1H-Isochromene was also detected by ¹H NMR, and an isolated 1H-isochromene was converted to the product when retreated with TMSBr. The formation of 1H-isochromene is equivalent to a 6-endo-dig cyclization and contrasts with the corresponding reactions under basic conditions, in which the 5-exo-dig process dominated.

Understanding the reaction mechanism of gold-catalyzed reactions of 2,1-benzisoxazoles with propiolates and ynamides

The detailed reaction mechanisms of gold-catalyzed reactions of 2,1-benzisoxazoles with propiolates and ynamides have been investigated with the aid of density functional theory calculations.

Experimental and computational evidence on gold-catalyzed regioselective hydration of phthalimido-protected propargylamines: an entry to β-amino ketones

The results of our investigations on the Au-catalyzed regioselective hydration reaction of both alkyl- and aryl-substituted N-propargyl phthalimides directed to the selective formation of the corresponding β-phthalimido ketones are described. Experimental data, in particular the observed regioselectivity, have been qualitatively supported by quantum-chemical calculations carried out on model systems in the framework of Density Functional Theory (DFT) followed by quantum theory of atoms in molecules (QTAIMS). Our results suggest that the electronic features of the initial adduct between the propargyl triple bond and the Au(i) catalyst, in particular the character of the gold-triple bond interaction, are essential for the observed regioselectivity. Other effects, such as the presence of the solvent and the formation of a H-bond between the water molecule and the phthalimido moiety, although apparently irrelevant for the regioselectivity, have proven to be kinetically and catalytically rather important.

Gold-Catalyzed Intermolecular Alkyne Hydrofunctionalizations—Mechanistic Insights

An overview of the current state of mechanistic understanding of gold-catalyzed intermolecular alkyne hydrofunctionalization reactions is presented. Moving from the analysis of the main features of the by-now-generally accepted reaction mechanism, studies and evidences pointing out the mechanistic peculiarities of these reactions using different nucleophiles HNu that add to the alkyne triple bond are presented and discussed. The effects of the nature of the employed alkyne substrate and of the gold catalyst (employed ligands, counteranions, gold oxidation state), of additional additives and of the reaction conditions are also considered. Aim of this work is to provide the reader with a detailed mechanistic knowledge of this important reaction class, which will be invaluable for rapidly developing and optimizing synthetic protocols involving a gold-catalyzed alkyne hydrofunctionalization as a reaction step.

Efficient [(NHC)Au(NTf2)]-catalyzed hydrohydrazidation of terminal and internal alkynes

The efficient hydrohydrazidation of terminal (6a–r, 18 examples, 0.1–0.2 mol % [(NHC)Au(NTf₂)], T = 60 °C) and internal alkynes (7a–j, 10 examples, 0.2–0.5 mol % [(NHC)Au(NTf₂)], T = 60–80 °C) utilizing a complex with a sterically demanding bispentiptycenyl-substituted NHC ligand and the benign reaction solvent anisole, is reported.

PhI(OAc)2-mediated oxidative rearrangement of allylic amides: efficient synthesis of oxazoles and β-keto amides

A series of 2,5-disubstituted oxazoles and β-keto amides were synthesized from allylic amides via PhI(OAc)₂-mediated intramolecular cyclization and oxidation with the migration of an aryl group.

New homoleptic gold carbene complexes via Ag–Au transmetalation: synthesis and application of [Au(diNHC)2]3+ cations as 1H-NMR and UV-vis halide sensors

Novel homoleptic gold(iii) complexes with bidentate N-heterocyclic carbene ligands have been successfully synthesised by transmetalation reaction and studied as anion sensors in solution by means of ¹H NMR and UV-vis titration experiments.

Hydroalkoxylation of Terminal and Internal Alkynes Catalyzed by Dinuclear Gold(I) Complexes with Bridging Di(N-Heterocyclic Carbene) Ligands

A series of six dinuclear gold(I) complexes with bridging bidentate N-heterocycic carbene ligands (NHCs) of general formula Au2Br2LX (L = diNHC, X = 1–6) have been studied as catalysts in the intermolecular hydroalkoxylation of terminal and internal alkynes. The best catalytic results have been obtained by using Au2Br2L4, characterized by 2,6-diisopropylphenyl wingtip substituents and a methylene bridging group between the two NHC donors. Complex Au2Br2L4 has been structurally characterized for the first time in this work, showing the presence of intramolecular aurophiclic interaction in the solid state. In the adopted reaction conditions Au2Br2L4 is able to convert challenging substrates such as diphenylacetylene. Comparative catalytic tests by using the mononuclear gold(I) complexes AuIL7 and IPrAuCl have been performed in order to determine the possible presence of cooperative effects in the catalytic process.

Hypervalent iodane mediated reactions of N-acetyl enamines for the synthesis of oxazoles and imidazoles

A hypervalent iodane reagent used for the intramolecular cyclization of N-acetyl enamines and intermolecular cyclocondensation of enamines and nitriles was investigated. The reaction was performed under mild conditions and gave oxazoles and imidazoles, respectively, in moderate to excellent yields. This transformation exhibits good reactivity, selectivity and functional group tolerance. The selectivity of the intra- or intermolecular reaction is dependent on the structure of N-acetyl enamines.