Synthesis and Reactivity of Air-Stable N-Heterocyclic Carbene Gold(I) Bis(trifluoromethanesulfonyl)imidate Complexes

Advances in gold catalyzed synthesis of quinoid heteroaryls

This review explores recent advancements in synthesizing quinoid heteroaryls, namely quinazoline and quinoline, vital in chemistry due to their prevalence in natural products and pharmaceuticals. It emphasizes the rapid, highly efficient, and economically viable synthesis achieved through gold-catalyzed cascade protocols. By investigating methodologies and reaction pathways, the review underscores exceptional yields attainable in the synthesis of quinoid heteroaryls. It offers valuable insights into accessing these complex structures through efficient synthetic routes. Various strategies, including cyclization, heteroarylation, cycloisomerization, cyclo-condensation, intermolecular and intramolecular cascade reactions, are covered, highlighting the versatility of gold-catalyzed approaches. The comprehensive compilation of different synthetic approaches and elucidation of reaction mechanisms contribute to a deeper understanding of the field. This review paves the way for future advancements in synthesizing quinoid heteroaryls and their applications in drug discovery and materials science.

Gold(I)-N-Heterocyclic Carbene Synthons in Organometallic Synthesis

The prominent role of gold-N-heterocyclic carbene (NHC) complexes in numerous research areas such as homogeneous (photo)catalysis, medicinal chemistry and materials science has prompted organometallic chemists to design gold-based synthons that permit access to target complexes through simple synthetic steps under mild conditions. In this review, the main gold-NHC synthons employed in organometallic synthesis are discussed. Mechanistic aspects involved in their synthesis and reactivity as well as applications of gold-NHC synthons as efficient pre-catalysts, antitumor agents and/or photo-emissive materials are presented.

One-Pot Synthesis of Pentasubstituted Pyridines following the Gold(I)-Catalyzed Aza-Enyne Metathesis/6π-Electrocyclization-Aromatization Sequence

The one-pot de novo synthesis of pentasubstituted pyridines was realized following the process of Au(I)-autotandem catalysis and subsequent aromatization. The process involves aza-enyne metathesis with aryl propiolates to yield 1-azabutadienes and their addition/6π-electrocyclization sequence with the other propiolate units. The resultant 1,4-dihydropyridines were aromatized to furnish the pyridines in the presence of atmospheric oxygen. The aryl propiolates were regioselectively incorporated into the ring system to afford 2-arylpyridines as the sole product.

Gold-Catalyzed [4 + 1] Heterocyclization of Hydroxamic Acid and Nonactivated Alkyne: A Protocol to Construct 5-Methyl-1,4,2-dioxazole

A novel gold-catalyzed [4 + 1] heterocyclization of nonactivated alkyne and hydroxamic acid is developed for the regiospecific synthesis of 5-methyl-1,4,2-dioxazole, which is an important structural motif in various bioactive molecules. The current methodology is characterized by high efficiency, simple operation, mild reaction conditions, and good functional group compatibility. Moreover, gram-scale synthesis and synthetic application toward bioactive molecular skeletons have been realized.

N-heterocyclic Carbene Gold Complexes Active in Hydroamination and Hydration of Alkynes

Until the year 2000, gold compounds were considered catalytically inert. Subsequently, it was found that they are able to promote the nucleophilic attack on unsaturated substrates by forming an Au–π-system. The main limitation in the use of these catalytic systems is the ease with which they decompose, which is avoided by stabilization with an ancillary ligand. N-heterocyclic carbenes (NHCs), having interesting s-donor capacities, are able to stabilize the gold complexes (Au (I/III) NHC), favoring the exploration of their catalytic activity. This review reports the state of the art (years 2007–2022) in the nucleophilic addition of amines (hydroamination) and water (hydration) to the terminal and internal alkynes catalyzed by N-heterocyclic carbene gold (I/III) complexes. These reactions are particularly interesting both because they are environmentally sustainable and because they lead to the production of important intermediates in the chemical and pharmaceutical industry. In fact, they have an atom economy of 100%, and lead to the formation of imines and enamines, as well as the formation of ketones and enols, all important scaffolds in the synthesis of bioactive molecules, drugs, heterocycles, polymers, and bulk and fine chemicals.

Redox-Neutral and Atom-Economic Route to β-Carbolines via Gold-Catalyzed [4 + 2] Cycloaddition of Indolylynamides and Cyanamides

Gold(I)-catalyzed [4 + 2] cycloaddition of indolylynamides and cyanamides (aminonitriles) is an efficient redox-neutral and atom-economic route to diversely substituted 1,3-diamino-β-carbolines. The protocol operates under mild conditions (Ph₃PAuNTf₂ 5 mol %, DCE, 60 °C) with a good tolerance to functional groups (23 examples and yields up to 98%). The obtained β-carboline systems represent a versatile synthetic platform with modifiable substituents for successive functionalizations. Control experiments indicate the crucial role of both the nature of reactants and the identity of employed catalysts in the developed cycloaddition.

Room Temperature Allenation of Terminal Alkynes with Aldehydes

A gold-catalyzed room temperature allenation of terminal alkynes (ATA) with aldehydes affording 1,3-disubstituted allenes with diverse functional groups has been developed by identifying a gold(I) catalyst and an amine. The practicality of this reaction has been demonstrated by a ten gram-scale synthesis and the synthetic potentials have been demonstrated via various transformations and formal total synthesis of (-)-centrolobine. Mechanistic studies revealed that the gold catalyst, the aldehyde effect, the fluoroalkyl hydroxyl solvent (TFE or HFIP) and the structure of amine are vital in this room temperature ATA reaction.

Gold N-Heterocyclic Carbene Catalysts for the Hydrofluorination of Alkynes Using Hydrofluoric Acid: Reaction Scope, Mechanistic Studies and the Tracking of Elusive Intermediates

An efficient and chemoselective methodology deploying gold-N-heterocyclic carbene (NHC) complexes as catalysts in the hydrofluorination of terminal alkynes using aqueous HF has been developed. Mechanistic studies shed light on an in situ generated catalyst, formed by the reaction of Brønsted basic gold pre-catalysts with HF in water, which exhibits the highest reactivity and chemoselectivity. The catalytic system has a wide alkyl substituted-substrate scope, and stoichiometric as well as catalytic reactions with tailor-designed gold pre-catalysts enable the identification of various gold species involved along the catalytic cycle. Computational studies aid in understanding the chemoselectivity observed through examination of key mechanistic steps for phosphine- and NHC-coordinated gold species bearing the triflate counterion and the elusive key complex bearing a bifluoride counterion.

Straightforward Synthesis of Indenes by Gold-Catalyzed Intramolecular Hydroalkylation of Ynamides

An original and straightforward entry to polysubstituted indenes from readily available ynamides is reported. Upon reaction with a N-heterocyclic carbene-gold complex under mild conditions, activated keteniminium ions are generated whose unique electrophilicity triggers a [1,5]-hydride shift and a subsequent cyclization. The presence of an endocyclic enamide in the densely functionalized resulting indenes was shown to be especially useful and versatile, offering a range of opportunities for their further postfunctionalization.

Synthesis of γ,δ-Unsaturated Esters and Amides via Au(I)-Catalyzed Reactions of Aryl Ynol Ethers or Ynamides with Allylic Alcohols

Polarized alkynes such as ynol ethers and ynamides have attracted much attention due to their inherent unique reactivity. Herein, we report Au(I)-catalyzed hydroalkoxylation/Claisen rearrangement cascade reactions of aryl ynol ethers and ynamides with allylic alcohols. At the first stage (hydroalkoxylation) of this cascade reaction, attack of allylic alcohols to aryl ynol ethers or ynamides occurs at the α-position of the polarized alkynes in a completely regioselective manner. Claisen rearrangement of the resulting adducts subsequently takes place to give γ,δ-unsaturated esters or amides, respectively. The [Au(IPr)NTf2] catalyst is most effective for this reaction, and the reaction proceeds under mild conditions (in the case of aryl ynol ether: in THF, 60 °C; in the case of ynamides: in toluene, 80 °C) in an atom-economical way.

Au(I) Catalyzed Synthesis of Densely Substituted Pyrazolines and Dihydropyridines via Sequential Aza-Enyne Metathesis/6π-Electrocyclization

A gold(I) autotandem catalysis protocol is reported for the de novo synthesis of densely substituted pyrazolines and dihydropyridines from the corresponding imine derivatives in a highly regioselective fashion via a one-pot aza-enyne metathesis/6π-electrocyclization sequence. The substituents on the nitrogen atom of the imine perfectly control the reaction pathways from the pivotal 1-azabutadiene intermediate; thus, carbazates were converted into pyrazolines via 6π-electrocyclization of α,β-unsaturated hydrazones, while aryl imines provided dihydropyridines via 6π-electrocyclization of 3-azahexatrienes.

Studies on gold(I) and gold(III) alcohol functionalised NHC complexes

Five pairs of novel chiral alcohol functionalised gold(i) and gold(iii) NHC complexes derived from chiral amino alcohols, were synthesized and characterised (NMR, IR, HRMS). Single crystal X-ray diffraction data of gold(i) and gold(iii) complexes are reported and discussed. The chiral imidazolium preligands were readily synthesized via the oxalamides, subsequent reduction and final orthoformate condensation. An improved method was used for generation of gold(i) NHC complexes (up to 92%) and further oxidation afforded the corresponding gold(iii) NHC complexes (up to 99%). All the Au(i) and Au(iii) NHC complexes proved far more catalytically active in a 1,6-enyne alkoxycyclization test reaction than our previously tested N,N- and P,N-ligated Au(iii) complexes. Comparative gold(i) and gold(iii) catalytic studies demonstrated different catalytic ability, depending on the NHC ligand flexibility and bulkiness. Excellent yields (92-99%) of target alkoxycyclization product were obtained with both gold(i) and gold(iii) complexes with the bulky N1-Mes-N2-ethanol based NHC ligand.

Gold(I)-NHC-catalysed synthesis of benzofurans via migratory cyclization of 2-alkynylaryl ethers

Homogeneous cationic gold(i) catalysis emerged as a preferred avenue for the activation of alkenes and alkynes towards reactions with weak nucleophiles, especially in cyclization reactions. Here we report an intramolecular carboalkoxylation reaction of electron-rich benzyl ethers of 2-ethynylaryl phenols catalysed by a digold(i)-NHC complex. The reaction proceeds efficiently with low catalyst loading and the resulting 2,3-disubstituted benzofurans form in moderate to good yields. Based on the results of a cross-over experiment, spectroscopic data, and DFT calculations, we propose a mechanism that accounts for the observed chemo- and regioselectivity.

Dinuclear gold(i) complexes: from bonding to applications

The last two decades have seen a veritable explosion in the use of gold(i) complexes bearing N-heterocyclic carbene (NHC) and phosphine (PR₃) ligands.

Gold-Catalyzed Cascade Reactions of 4 H-Furo[3,2- b]indoles with Allenamides: Synthesis of Indolin-3-one Derivatives

Merging the ability of cationic gold(I) catalysts to activate unsaturated π-systems with the electrophiles-driven ring-opening reactions of furans, we describe a new approach to synthesize 2-spiroindolin-3-ones from 4 H-furo[3,2- b]indoles. The reaction occurs through a cascade sequence involving addition of a gold-activated allene to the furan moiety of the starting furoindole followed by a ring-opening/ring-closing event affording 2-spirocyclopentane-1,2-dihydro-3 H-indolin-3-ones in moderate to good yields.

Chiral AuI - and AuIII -Isothiourea Complexes: Synthesis, Characterization and Application

During an investigation into the potential union of Lewis basic isothiourea organocatalysis and gold catalysis, the formation of gold-isothiourea complexes was observed. These novel gold complexes were formed in high yield and were found to be air- and moisture stable. A series of neutral and cationic chiral gold(I) and gold(III) complexes bearing enantiopure isothiourea ligands was therefore synthesized and fully characterized. The steric and electronic properties of the isothiourea ligands was assessed through calculation of their percent buried volume and the synthesis and analysis of novel iridium(I)-isothiourea carbonyl complexes. The novel gold(I)- and gold(III)-isothiourea complexes have been applied in preliminary catalytic and biological studies, and display promising preliminary levels of catalytic activity and potency towards cancerous cell lines and clinically relevant enzymes.

1,3-Chlorine Shift to a Vinyl Cation: A Combined Experimental and Theoretical Investigation of the E-Selective Gold(I)-Catalyzed Dimerization of Chloroacetylenes

Metal-catalyzed dimerization reactions of terminal acetylenes are well known in the literature. However, only a few examples of the dimerization of halogen-substituted acetylenes are described. The products of the latter metal-catalyzed dimerization are the branched head-to-tail enynes. The formation of the corresponding linear head-to-head enynes has not been reported yet. Herein, we demonstrate by means of quantum chemical methods and experiments that the head-to-head dimerization of chloroarylacetylenes can be achieved via mono gold catalysis. Under the optimized conditions, a clean and complete conversion of the starting materials is observed and the dimeric products are obtained up to 75% NMR yield. A mechanistic investigation of the dimerization reaction reveals that the branched head-to-tail vinyl cation is energetically more stable than the corresponding linear head-to-head cation. However, the latter can rearrange by an unusual 1,3-chlorine shift, resulting in the highly stereoselective formation of the trans product, which corresponds to the gold complex of the head-to-head E-enyne. The activation barrier for this rearrangement is extremely low (ca. 2 kcal/mol). As the mono gold-catalyzed dimerization can be conducted in a preparative scale, this simple synthesis of trans-1,2-dichloroenynes makes the gold(I)-catalyzed head-to-head dimerization of chloroarylacetylenes an attractive method en route to more complex conjugated enyne systems and their congeners.

Gold(i)-catalyzed nucleophilic cyclization of β-monosubstituted o-(alkynyl)styrenes: a combined experimental and computational study

The diastereospecific 5-endo cyclization of β-monosubstituted o-(alkynyl)styrenes to 1-functionalized-1H-indenes is reported. The regiospecific generation of a cyclopropyl gold carbene intermediate is also supported by DFT calculations.

Taming the Cationic Beast: Novel Developments in the Synthesis and Application of Weakly Coordinating Anions

This Review gives a comprehensive overview of the most topical weakly coordinating anions (WCAs) and contains information on WCA design, stability, and applications. As an update to the 2004 review, developments in common classes of WCA are included. Methods for the incorporation of WCAs into a given system are discussed and advice given on how to best choose a method for the introduction of a particular WCA. A series of starting materials for a large number of WCA precursors and references are tabulated as a useful resource when looking for procedures to prepare WCAs. Furthermore, a collection of scales that allow the performance of a WCA, or its underlying Lewis acid, to be judged is collated with some advice on how to use them. The examples chosen to illustrate WCA developments are taken from a broad selection of topics where WCAs play a role. In addition a section focusing on transition metal and catalysis applications as well as supporting electrolytes is also included.

Migratory Insertion of Carbenes into Au(III)-C Bonds

Migratory insertion of carbon-based species into transition-metal-carbon bonds is a mechanistic manifold of vast significance: it underlies the Fischer-Tropsch process, Mizoroki-Heck reaction, Ziegler-Natta and analogous late-transition-metal-catalyzed olefin polymerizations, and a number of carbonylative methods for the synthesis of ketones and esters, among others. Although this type of reactivity is well-precedented for most transition metals, gold constitutes a notable exception, with virtually no well-characterized examples known to date. Yet, the complementary reactivity of gold to numerous other transition metals would offer new synthetic opportunities for migratory insertion of carbon-based species into gold-carbon bonds. Here we report the discovery of well-defined Au(III) complexes that participate in rapid migratory insertion of carbenes derived from silyl- or carbonyl-stabilized diazoalkanes into Au-C bonds at temperatures ≥ -40 °C. Through a combined theoretical and experimental approach, key kinetic, thermodynamic, and structural details of this reaction manifold were elucidated. This study paves the way for homogeneous gold-catalyzed processes incorporating carbene migratory insertion steps.

Gold(i)-catalyzed diastereoselective synthesis of 1-α-oxybenzyl-1H-indenes

The gold(i)-catalyzed oxycyclization of β-aryl monosubstituted o-(alkynyl)styrenes gives rise to 1-α-methoxy or 1-α-hydroxybenzyl-1H-indenes in a diastereospecific way.

Unveiling the mechanisms and secrets of chemoselectivities in Au(i)-catalyzed diazo-based couplings with aryl unsaturated aliphatic alcohols

The mechanisms and origins of the unsaturated-aliphatic-alcohols-modulated chemoselectivity of Au-catalyzed carbene transfers from diazo compounds are disclosed by DFT studies.

Integration of multiple active sites on large-pore mesoporous silica for enantioselective tandem reactions

Facile construction of a multifunctional heterogeneous catalyst through the assembly of Au/carbene and chiral ruthenium/diamine dual complexes in large-pore mesoporous silica was developed. This enables an efficient one-pot hydration-asymmetric transfer hydrogenation enantioselective tandem reaction of haloalkynes, affording chiral halohydrins with up to 99% enantioselectivity. Combined multifunctionalities, such as substrate-promoted silanol-functionality, BF₄^- anion-bonding gold/carbene and covalent-bonding chiral ruthenium/diamine active centers, contributed cooperatively to the catalytic performance.