Gold-Catalyzed Reactions via Cyclopropyl Gold Carbene-like Intermediates

Three-Component Gold(I)-Catalyzed Alkoxyvinylation

Acetylene has been underexploited despite being a highly valuable feedstock for chemical synthesis. We have developed the first true gold(I)-catalyzed intermolecular three-component reaction between acetylene, alkenes, and alcohols to afford β-vinyl hemiaminal scaffolds from N-vinyl amides. Unusual biscyclopropyl and 3-vinyl N-heterocyclic scaffolds were obtained through the incorporation of a second N-vinyl unit or tethered alkene into the starting material.

Enantioselective C-H Functionalization Reactions under Gold Catalysis

Transition metal-catalyzed enantioselective functionalization of ubiquitous C-H bonds has proven to be promising field as it offers the construction of chiral molecular complexity in a step- and atom-economical manner. In recent years, gold has emerged as an attractive contender for catalyzing such reactions. The unique reactivities and selectivities offered by gold catalysts have been exploited to access numerous asymmetric transformations based on gold-catalyzed C-H functionalization processes. Herein, this review critically highlights the major advances and discoveries made in the enantioselective C-H functionalization under gold catalysis which is accompanied by mechanistic insights at appropriate places.

Alumina-Mediated π-Activation of Alkynes

The ability to induce powerful atom-economic transformation of alkynes is the key feature of carbophilic π-Lewis acids such as gold- and platinum-based catalysts. The unique catalytic activity of these compounds in electrophilic activations of alkynes is explained through relativistic effects, enabling efficient orbital overlapping with π-systems. For this reason, it is believed that noble metals are indispensable components in the catalysis of such reactions. In this study, we report that thermally activated γ-Al₂O₃ activates enynes, diynes, and arene-ynes in a manner enabling reactions that were typically assigned to the softest π-Lewis acids, while some were known to be triggered exclusively by gold catalysts. We demonstrate the scope of these transformations and suggest a qualitative explanation of this phenomenon based on the Dewar-Chatt-Duncanson model confirmed by density functional theory calculations.

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.

Selective Synthesis of Phenanthrenes and Dihydrophenanthrenes via Gold-Catalyzed Cycloisomerization of Biphenyl Embedded Trienynes

Readily available o'-alkenyl-o-alkynylbiaryls, a particular type of 1,7-enynes, undergo a selective cycloisomerization reaction in the presence of a gold(I) catalyst to give interesting phenanthrene and dihydrophenanthrene derivatives in high yields. The solvent used provokes a switch in the evolution of the gold intermediate and plays a key role in the reaction outcome.

Gold-Catalyzed Synthesis of Small Rings

Three- and four-membered rings, widespread motifs in nature and medicinal chemistry, have fascinated chemists ever since their discovery. However, due to energetic considerations, small rings are often difficult to assemble. In this regard, homogeneous gold catalysis has emerged as a powerful tool to construct these highly strained carbocycles. This review aims to provide a comprehensive summary of all the major advances and discoveries made in the gold-catalyzed synthesis of cyclopropanes, cyclopropenes, cyclobutanes, cyclobutenes, and their corresponding heterocyclic or heterosubstituted analogs.

1,2,3-Triazolium-Derived Mesoionic Carbene Ligands Bearing Chiral Sulfur-Based Moieties: Synthesis, Catalytic Properties, and Their Role in Chirality Transfer

1,2,3-Triazole-derived mesoionic carbenes (MICs) having a chiral sulfur functional group at the C5 position are easily available through a CuAAC between chiral alkynyl sulfoxides and different azides. The MICs form complexes with several metals (Au, Ag, Ir, Rh, and Ru) that are enantiomerically pure. Moreover, enantiomerically pure MIC sulfinilimines are obtained from the corresponding sulfoxide retaining the chirality. Through this article, the participation of sulfoxide moieties in different catalytic and chirality transfer processes, as well as in discovering mechanistically new processes will be shown. The role of the sulfur chiral moiety in catalytic cycloisomerization and cycloisomerization-dimerization processes using Au-MIC catalysts is dual. The sulfur functional group either stabilizes intermediates in the catalytic cycle, allowing for the reaction to occur or significantly increases the selectivity of the cyclization processes. 1,2,3-Triazole MICs having chiral sulfoxides at C5 are extremely efficient in preparing chiral at the metal complexes by C-H insertion processes. The chiral at the metal half-sandwich complexes, having the enantiopure sulfur chiral group unaltered, experiences different reactions with complete retention of the configuration. Finally, mechanistically new processes, like the desulfinilation of 1,2,3-triazolium salts in Ag-MIC complexes have been uncovered. These still-nascent classes of compounds will offer opportunities for the discovery of novel catalytic applications and to study new mechanistically sound processes.

From Palladium to Gold Catalysis for the Synthesis of Crushed Fullerenes and Acenes

The quest for organic materials with improved optoelectronic properties has stimulated the development of new strategies for the preparation of polycyclic aromatic hydrocarbons. Within this context, transition metal catalysis offers unparalleled opportunities for the assembly of complex molecular architectures. The palladium-catalyzed direct C-H arylation provides straight access to biaryls without the need of prefunctionalization at the nucleophilic site, which is attractive from the perspective of the synthesis of polyarenes. Mechanistically, this reaction was found to be different from an electrophilic aromatic substitution, involving the abstraction of a proton by an external base in the key metalation step. Using readily available C27 truxene as the starting material, a concise synthetic route consisting of a threefold benzylation and subsequent palladium-catalyzed arylation led to C60 polyarenes, also referred to as "crushed fullerenes", which could be converted into C₆₀ fullerene by laser-induced cyclodehydrogenation in the gas phase or by thermal cyclodehydrogenation on a platinum surface. A conceptually related strategy based on the use of a highly electrophilic gold(I) complex as the catalyst for the threefold intramolecular hydroarylation of truxene derivatives was applied for the synthesis of decacyclenes. Using gold(I) catalysis, we have also developed a variety of synthetically useful protocols for the cycloisomerization of readily available 1,n-enynes as well as for the addition for nucleophiles to these unsaturated substrates. In one of these transformations, 1,7-enynes bearing aryl-substituted alkynes undergo formal [4 + 2] cycloaddition reactions via gold(I)-catalyzed 6-exo-dig cyclization and intramolecular Friedel-Crafts-type reaction to form tricyclic compounds bearing a dihydronaphthalene core. A related transformation led to a general synthesis of hydroacenes, which are known to be stabilized precursors of the corresponding conjugated acenes with enhanced solubility. A wide variety of dihydrotetracenes featuring electron-donating and electron-withdrawing groups, as well as dihydropentacene and dihydrohexacene could be easily obtained. A simple variation of our synthetic route led to tetrahydro-derivatives of higher acenes with up to 11 linearly fused six-membered rings. The dehydrogenation of tetrahydroacenes on a metallic substrate using the tip of a scanning tunneling microscopy instrument or by thermal annealing enabled the preparation of the whole series of higher acenes from heptacene up to previously unknown undecacene, whose structure was confirmed by noncontact atomic force microscopy. This work provided a unique opportunity for the analysis of the evolution of the transport gap in the acene series on Au(111). Furthermore, heptacene was also generated by on-surface dehydrogenation on Ag(001) from tetrahydroheptacene and a dibrominated derivative.

A rhodium catalyzed cycloisomerization and tandem Diels-Alder reaction for facile access to diverse bicyclic and tricyclic heterocycles

A rhodium catalyzed cycloisomerization of 1,6-allenenes successfully prepared six-membered ring exocyclic 1,3-dienes. Further tandem Diels–Alder reaction with various dienophiles provided diverse bicyclic and tricyclic heterocycles. This methodology is characterised by complete atom and step economy, low catalyst loading, and excellent chemo-, regio-, and diastereoselectivity.

A regioselective distal cycloisomerization of 1,6-allenenes was successfully developed to afford six-membered ring exocyclic 1,3-dienes employing a rhodium/diphosphine catalyst system. Deuterium labelling experiments and DFT calculations were performed to provide insights into the reaction mechanism of this unprecedented transformation. In addition, one-pot tandem Diels–Alder reactions with various dienophiles could readily construct diverse bicyclic and tricyclic nitrogen heterocycles, which are ubiquitous core scaffolds for a variety of natural products and bioactives. High efficiency and exclusive chemo and regioselectivities for a broad substrate scope were achieved under mild conditions using a low catalyst loading of 0.5 mol%.

Oxidative tandem annulation of 1-(2-ethynylaryl)prop-2-en-1-ones catalyzed by cooperative iodine and TBHP

Metal-free I₂-catalyzed tandem annulation of 1,6-enynes to access 1H-cyclopropa-[b]naphthalene-2,7-diones using TBHP is presented.

Gold-catalyzed heterocyclic syntheses through α-imino gold carbene complexes as intermediates

This review reports a comprehensive compilation of gold-catalyzed heterocyclic synthesis by using α-imino gold carbene complexes as the proposed intermediates.

Gold(i) catalysed regio- and stereoselective intermolecular hydroamination of internal alkynes: towards functionalised azoles

Vinyl- and saturated azoles were synthesised through gold(i) catalysed regio- and stereoselective intermolecular hydroamination of internal alkynes and subsequent hydrogenation.

Iridium-Catalyzed Asymmetric Allylic Substitution Reactions

In this review, we summarize the origin and advancements of iridium-catalyzed asymmetric allylic substitution reactions during the past two decades. Since the first report in 1997, Ir-catalyzed asymmetric allylic substitution reactions have attracted intense attention due to their exceptionally high regio- and enantioselectivities. Ir-catalyzed asymmetric allylic substitution reactions have been significantly developed in recent years in many respects, including ligand development, mechanistic understanding, substrate scope, and application in the synthesis of complex functional molecules. In this review, an explicit outline of ligands, mechanism, scope of nucleophiles, and applications is presented.

Diastereospecific Gold(I)-Catalyzed Cyclization Cascade for the Controlled Preparation of N- and N,O-Heterocycles

The reaction of oxime-tethered 1,6-enynes with a cationic gold(I) catalyst demonstrates a great potential for the synthesis of a range of heterocycles in a diastereospecific fashion. The control of the configuration of the oxime and the alkene of the enyne moiety is the key to selectively obtain dihydro-1,2-oxazines, isoxazolines or dihydropyrrole-N-oxides as single diastereoisomers. As supported by DFT calculations, these cascade reactions proceed stepwise, by the intramolecular addition of the O or N atom of the oxime onto cyclopropyl gold(I) carbene intermediates. In this study, a rare [3,3]-sigmatropic rearrangement of nitrones is also observed.

Dearomatization of Indole via Intramolecular [3 + 2] Cycloaddition: Access to the Pentacyclic Skeleton of Strychons Alkaloids

An efficient method to build various multisubstituted polycyclic indoline-annulated normal to medium-size rings through dearomatization of indole via a tandem 1,2-acyloxy migration/intramolecular [3 + 2] cycloaddition process is described. The pentacyclic skeleton of strychnine could be synthesized via this tandem cycloaddition and a further Mannich reaction. This approach would provide a novel strategy to the synthesis of strychons alkaloids.

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.

Revealing the Activity of π-Acid Catalysts using a 7-Alkynyl Cycloheptatriene

A compound that isomerizes into distinct products depending on the particular Lewis acid or Brønsted acid catalyst used is disclosed. One product can only be obtained with the softest π-acids, such as Au, Pt, Ga, or In complexes. Another is formed only with harder π-acids incorporating Ag or Cu salts. The formation of the third category requires even harder π-acids or protons. This simple benchmark reaction allows for prediction of the category of transformations that can be catalyzed by a new complex. It also informs whether protons have been unintentionally generated in the reaction mixture.