Gold‐Catalyzed Cycloisomerizations of 1, n ‐Diyne Carbonates and Esters

Increasing the Donor Strength of Alkenylphosphines by Twisting the C=C Double Bond

Electron-rich phosphines play a crucial role in transition metal-based homogeneous catalysis. While alkyl groups have traditionally been employed to increase the phosphine donor strength, recent studies have shown that zwitterionic functional groups such as phosphorus ylides can result in a further enhancement. Herein we report the concept of twisting a C=C double bond to introduce a zwitterionic substituent by the synthesis and application of N-heterocyclic olefin phosphines with a sulfonyl substituent (sNHOP). This sulfonyl group enables the twisting of the olefin moiety due to steric and electronic stabilization of the carbanionic center. The resulting zwitterionic structure leads to a significant increase of the donor strength of the sNHOP ligands compared to conventional NHOP systems with a planar N-heterocyclic olefin moiety. The potential of this new ligand platform for catalysis is demonstrated by its application in the gold-catalyzed hydroamination and cyclo-isomerization of alkynes. Here, the ligands outperform the original NHOP ligands suggesting favorable properties for future catalysis applications.

Consecutive chirality transfer: efficient synthesis of chiral C,O-chelated BINOL/gold(iii) complexes for asymmetric catalysis and chiral resolution of disubstituted BINOLs

A novel approach for efficient synthesis of chiral C,O-chelated BINOL/gold(iii) complexes by diastereomeric resolution using enantiopure BINOL as a chiral resolving agent was demonstrated. The BINOL/gold(iii) diastereomers with different solubility were separated by simple filtration, providing optically pure BINOL/gold(iii) complexes with up to >99 : 1 dr. By combining this with an efficient BINOL ligand dissociation process, a simple and column-free method for chiral resolution of racemic gold(iii) dichloride complexes on a gram scale was established, affording their enantiopure forms in good yields. Conversely, the resolved enantiopure gold(iii) dichloride complexes could serve as chiral resolving agents to resolve disubstituted BINOL derivatives, achieving both BINOLs and gold(iii) complexes in good to excellent yields (overall 77-96% and 76-95%, respectively) with a high optical purity of up to 99% ee. Through a consecutive chirality transfer process, the chiral information from an inexpensive chiral source was transferred to highly valuable gold(iii) complexes, followed by sterically bulky BINOL derivatives. This work would open a new synthetic strategy facilitating the development of structurally diverse chiral gold(iii) complexes and gold(iii)-mediated chiral resolution of BINOL derivatives. In addition, this new class of C,O-chelated BINOL/gold(iii) complexes achieved asymmetric carboalkoxylation of ortho-alkynylbenzaldehydes with an excellent enantioselectivity of up to 99% ee.

Catalytic (4+2) Annulation via Regio- and Enantioselective Interception of in-situ Generated Alkylgold Intermediate

A regio- and stereoselective stepwise (4+2) annulation of N-propargylamides and α,β-unsaturated imines/ketones has been accomplished with synergetic catalysis by a combination of a gold-complex and a chiral quinine-derived squaramide (QN-SQA), leading to highly functionalized chiral tetrahydropyridines/dihydropyrans in good to high yields with generally excellent enantioselectivity. Mechanistic studies and DFT calculations indicate that the in situ formed alkylgold species is the key intermediate in this transformation, and the amide group served as a traceless directing group in this highly selective transformation. This method complements the enantioselective (4+2) annulation of allene reagents, providing the formal internal C-C π-bond cycloaddition products, which is challenging and remains elusive.

Chiral Auxiliary Approach for Gold(I)-Catalyzed Cyclizations

Two different classes of stereoselective cyclizations have been developed using a chiral auxiliary approach with commercially available [JohnPhosAu(MeCN)SbF6 ] as catalyst. First, a stereoselective cascade cyclization of 1,5-enynes was achieved using the Oppolzer camphorsultam as chiral auxiliary. In this case, a one-pot cyclization-hydrolysis sequence was developed to directly afford enantioenriched spirocyclic ketones. Then, the stereoselective alkoxycyclization of 1,6-enynes was mediated by an Evans-type oxazolidinone. A reduction-hydrolysis sequence was selected to remove the auxiliary to give enantioenriched β-tetralones. DFT studies confirmed that the steric clash between the chiral auxiliary and alkene accounts for the experimentally observed diastereoselective cyclization through the Si face.

Catalytic 4-exo-dig carbocyclization for the construction of furan-fused cyclobutanones and synthetic applications

Cyclobutanone is a strained motif with broad applications, while direct assembly of the aromatic ring fused cyclobutanones beyond benzocyclobutenone (BCB) skeletons remains challenging. Herein, we report a Rh-catalyzed formal [3+2] annulation of diazo group tethered alkynes involving a 4-exo-dig carbocyclization process, providing a straightforward access to furan-fused cyclobutanones. DFT calculations disclose that, by comparison to the competitive 5-endo-dig process, 4-exo-dig carbocyclization is mainly due to lower angle strain of the key sp-hybridized vinyl cationic transition state in the cyclization step. Using less reactive catalysts Rh2(carboxylate)4 is critical for high selectivity, which is explained as catalyst-substrate hydrogen bonding interaction. This method is proved successful to direct access previously inaccessible and unknown furan-fused cyclobutanone scaffolds, which can participate in a variety of post-functionalization reactions as versatile synthetic blocks. In addition, preliminary antitumor activity study of these products indicates that some molecules exhibite significant anticancer potency against different human cancer cell lines.

Deciphering the Primary Role of Au⋅⋅⋅H-X Hydrogen Bonding in Gold Catalysis

Au⋅⋅⋅H-X (X=N or C) hydrogen bonding is gaining increasing interest, both in the study of its intrinsic nature and in their operability in different fields. While the role of these interactions has been studied in the stabilization of gold(I) complexes, their role during the minimum free energy reaction pathway of a given catalytic process remains unexplored. We report herein that complex [Au(C≡CPh)(pip)] (pip=piperidine) catalyses the A3 -coupling reaction for the synthesis of propargylamines, thanks to the ability of Au(I) to promote weak hydrogen bonding interactions with the reactants along the free energy profile. Density Functional Theory (DFT) calculations show that these Au⋅⋅⋅H-X interactions play a directing role in the catalysed A3 -coupling. Topological non-covalent interactions (NCI), interaction region indicator (IRI) and quantum theory of atoms in molecules (QTAIM) analysis in real space of the electron density provide a description of these interactions accurately.

Gold-catalyzed carbocyclization and imidization of alkyne-tethered diazo compounds with nitrosoarenes for the synthesis of nitrones and naphthalene derivatives

A gold-catalyzed carbocyclization/imidization cascade reaction has been developed,leading a facile access to the synthesis of functionalized nitrones in moderate to good yields under mild conditions. The reaction initiated by a catalytic 6-endo-dig diazo-yne carbocyclization to form the key endocyclic vinyl carbene from alkyne-tethered diazo compounds, followed by addition with nitrosoarenes that features an imidization process. Notably, these resulting nitrone products could be smoothly converted into different substituted naphthalenol analogues, such as 4-aminonaphthalen-1-ol, naphthalene-1,4-dione, and naphthalene-1,4-diol derivatives, in high yields. Moreover, the generated products exhibited potential tumor suppression activity in tested cancer celllines; compound 3c (HCT116 cells, IC50 = 7.41 μM; MCF-7 cells, IC50 = 14.28 μM) exhibits higher anticancer potency than other tested compounds.

A Simple and Practical Bis-N-Heterocyclic Carbene as an Efficient Ligand in Cu-Catalyzed Glaser Reaction

Conjugated diyne derivatives are important scaffolds in modern organic synthetic chemistry. Using the Glaser reaction involves the coupling of terminal alkynes which can efficiently produce conjugated diyne derivatives, while the use of a stoichiometric amount of copper salts, strong inorganic base, and excess oxidants is generally needed. Developing an environmentally friendly and effective method for the construction of symmetrical 1,3-diynes compounds by Glaser coupling is still highly desirable. In this study, we present an economical method for the production of symmetric diynes starting from various terminal acetylenes in a Glaser reaction. A simple and practical bis-N-heterocyclic carbene ligand has been introduced as efficient ligands for the Cu-catalyzed Glaser reaction. High product yields were obtained at 100 °C for a variety of substrates including aliphatic and aromatic terminal alkynes and differently substituted terminal alkynes including the highly sterically hindered substrate 2-methoxy ethynylbenzene or 2-trifluoromethyl ethynylbenzene and a series of functional groups, such as trifluoromethyl group, ester group, carboxyl group, and nitrile group. The established protocol is carried out in air under base-free condition and is operationally simple. These research work suggest that bis-N-heterocyclic carbene could also an appealing ligand for Glaser reaction and provide a reference for the preparation of symmetric 1,3-diynes in industrial filed.

Silver/chiral pyrrolidinopyridine relay catalytic cycloisomerization/(2 + 3) cycloadditions of enynamides to asymmetrically synthesize bispirocyclopentenes as PDE1B inhibitors

Significant progress has been made in asymmetric synthesis through the use of transition metal catalysts combined with Lewis bases. However, the use of a dual catalytic system involving 4-aminopyridine and transition metal has received little attention. Here we show a metal/Lewis base relay catalytic system featuring silver acetate and a modified chiral pyrrolidinopyridine (PPY). It was successfully applied in the cycloisomerization/(2 + 3) cycloaddition reaction of enynamides. Bispirocyclopentene pyrazolone products could be efficiently synthesized in a stereoselective and economical manner (up to >19:1 dr, 99.5:0.5 er). Transformations of the product could access stereodivergent diastereoisomers and densely functionalized polycyclic derivatives. Mechanistic studies illustrated the relay catalytic model and the origin of the uncommon chemoselectivity. In subsequent bioassays, the products containing a privileged drug-like scaffold exhibited isoform-selective phosphodiesterase 1 (PDE1) inhibitory activity in vitro. The optimal lead compound displayed a good therapeutic effect for ameliorating pulmonary fibrosis via inhibiting PDE1 in vivo.

Computational insight into gold(I)-catalyzed intramolecular regioselectivity of tryptamine-ynamide cycloisomerizations

The regioselectivity for gold(I)-catalyzed intramolecular cycloisomerizations of tryptamine-ynamides has long been elusive despite various synthetic examples of similar substrates being available. Computational studies were carried out to provide insight into the mechanisms and the origin of the substrate-dependent regioselectivity of these transformations. Based on the analyses of non-covalent interactions, distortion/interaction, and energy decomposition on the interactions between the terminal substituent of alkynes and the gold(I) catalytic ligand, the electrostatic effect was determined to be the key factor for α-position selectivity while the dispersion effect was determined to be the key factor for β-position selectivity. Our computational results were consistent with the experimental observations. This study provides useful guidance for understanding other similar gold(I)-catalyzed asymmetric alkyne cyclization reactions.

Construction of Spirolactams by Unexpected Oxidative Cyclization of Alkenyl Diynes via Copper Catalysis

A copper-catalyzed oxidative cyclization of alkenyl N-propargyl ynamides is described. This protocol enables the practical synthesis of diverse spirocyclic γ-lactams bearing an exocyclic double bond with generally high Z/E selectivity in moderate to good yields. Importantly, this copper-catalyzed oxidative cyclization demonstrates a distinctive selectivity in comparison with the related gold catalysis.

Gold-Catalyzed Addition of Propargyl Acetates to Olefins via O-Acyl Migration/Cyclopropanation Sequence: Insight into the Diastereoselective Formation of the Alkene

This article discloses a study on the well-known addition of propargyl acetates to olefins via an O-acyl migration/cyclopropanation sequence. Herein, we show that the stereochemical outcome of the olefin is strongly dependent on the gold catalyst and reaction parameters (concentration, temperature, and alkene partner equivalents); the E- and Z-isomers can be selectively formed by the judicious choice of reaction conditions.

Divergent dehydroxyfluorination and carbonation of alcohols with trifluoromethyl trifluoromethanesulfonate

Preparation of alkyl fluorides and carbonates via divergent dehydroxyfluorination and carbonation of alcohols with trifluoromethyl trifluoromethanesulfonate (CF₃SO₂OCF₃) is described. The reactions performed with BTMG in THF provided alkyl fluorides in good yields, whereas those of two different alcohols with Et₃N in DCM formed asymmetric carbonates in moderate to excellent yields. CF₃SO₂OCF₃ was demonstrated to be either a "F" or a "CO" reagent in the reactions by changing the base, allowing the selective construction of alkyl fluorides and carbonates from the corresponding alcohols with high efficiency. Notably, the fluorine-containing asymmetric carbonates that are difficult to synthesize by other methods were comprehensively prepared by this method, which would have great application potential in both academic and industrial fields.

Gold-Catalyzed Alkyne Multifunctionalization through an Oxidation-Oxyalkylation-Aryloxylation Sequence

A gold-catalyzed oxidative three-component reaction of terminal alkynes with alcohols and quinone monoimines has been disclosed, affording α-ketoacetals in good to excellent yields. By using quinone monoimines as electrophiles for the interception of the in situ generated gold enolate intermediate, this one-pot process provides an unprecedented method for the polyfunctionalization of terminal alkynes through an oxidation-oxyalkylation-aryloxylation sequence, installing three oxygen atoms on the C-C triple bond.

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.

Ylide-Substituted Phosphines: A Platform of Strong Donor Ligands for Gold Catalysis and Palladium-Catalyzed Coupling Reactions

The development of homogeneous catalysts is strongly connected to the design of new, sophisticated ligands, which resolve limitations of a given reaction protocol by manipulating the electronic properties of the metal and its spatial environment. Phosphines are a privileged class of ligands that find applications in many catalytic transformations, ranging from hydrogenation reactions to hydroformylation and coupling chemistry. For many years, chemists have been trying to improve the efficiency, selectivity, and application of coupling reactions. The use of highly electron-rich and bulky phosphines was often associated with increased selectivity and efficiency and led to the development of a vast variety of electron-rich alkyl-substituted phosphines. However, this concept of increasing the ligand donor strength reaches its limits with the use of trialkyl-substituted phosphines with tri-tert-butylphosphine thus being one of the most active ligands for many years. In the course of our research efforts to use the special donor strength of ylides to stabilize electron-deficient, low-valent main group compounds, we realized that ylide-substituted phosphine (YPhos) ligands possess remarkably strong donor abilities. Moreover, the YPhos ligands are highly tunable by changing the nature of the groups on the phosphonium, phosphine, or central ylidic carbon atom. We thus obtained a ligand platform with donor capabilities ranging from PCy₃ to even stronger donor abilities than N-heterocyclic carbenes, while being more sterically demanding than simple phosphines as well as many well-known biarylphosphine ligands.These properties led us to explore the applicability of the YPhos ligands in catalysis. In a series of recent reports, our group applied YPhos ligands in gold and palladium catalyzed reactions at catalytic loadings applicable for medium- to large-scale applications. The increased donor strength and unique architecture allowed for remarkable activities in a series of transformations at mild reactions conditions. For gold(I)-catalyzed reactions, we obtained turnover numbers (TONs) for the hydroamination of phenylacetylene with aniline of over 20 000. Also, more complex reactions were easily catalyzed with efficiencies greater than those of other known gold(I) catalysts. Similar efficacies were found in a series of palladium-catalyzed coupling reactions. In Buchwald-Hartwig aminations, unprecedented activities for the amination of aryl chlorides were reached at room temperature. The speed of formation of the catalytically active mono-YPhos palladium species allowed for some of the amination reactions to be completed in only a few minutes. Adjustment of the ligand design enabled the use of a large variety of different aryl and alkyl amines of different steric demands. Furthermore, the YPhos ligands in general showed high activities and selectivity in the coupling of a variety of carbon nucleophiles with aryl chlorides, bromides, and triflates. This enabled the development of efficient reaction protocols for the α-arylation of unhindered ketones and the coupling of Grignard and zinc reagents as well as the first efficient coupling of chloroarenes with alkyllithium compounds. This Account summarizes the recent development of YPhos ligands and their application in gold and palladium catalysis. We also hope to stimulate further use of this ligand platform in catalysis in the future.

Rhodium(i)-catalysed cycloisomerisation/6π electrocyclisation of 5-(ethynylamino)pent-2-yn-1-yl esters to 2,3-dihydrobenzo[f]indoles

A rare instance in rhodium catalysis of an in situ formed rhodacycle that undergoes a formal 1,8-acyloxy migration-initiated reductive elimination.

Stereoselective synthesis of functionalized azepines via gold and palladium relay catalysis

We developed a cycloisomerization/asymmetric [4 + 3] cycloaddition cascade reaction via gold/palladium relay catalysis, furnishing enantioenriched furan-fused azepines efficiently.

Copper-Catalyzed Si-H Bond Insertion Reaction of N-Propargyl Ynamides with Hydrosilanes

Transition-metal-catalyzed Si-H bond insertion reactions are generally limited to stabilized diazo compounds. An efficient copper-catalyzed Si-H bond insertion reaction of N-propargyl ynamides with hydrosilanes is described, allowing practical and atom-economic construction of valuable organosilanes in generally moderate to excellent yields under mild reaction conditions. Notably, this reaction constitutes a new method of Si-H bond insertion reaction involving vinyl cations as key intermediates.

Copper-Catalyzed Asymmetric Diyne Cyclization via [1,2]-Stevens-Type Rearrangement for the Synthesis of Chiral Chromeno[3,4-c]pyrroles

Here, we report a copper-catalyzed asymmetric cascade cyclization/[1,2]-Stevens-type rearrangement via a non-diazo approach, leading to the practical and atom-economic assembly of various valuable chiral chromeno[3,4-c]pyrroles bearing a quaternary carbon stereocenter in generally moderate to good yields with wide substrate scope and excellent enantioselectivities (up to 99 % ee). Importantly, this protocol not only represents the first example of catalytic asymmetric [1,2]-Stevens-type rearrangement based on alkynes but also constitutes the first asymmetric formal carbene insertion into the Si-O bond.

An asymmetric oxidative cyclization/Mannich-type addition cascade reaction for direct access to chiral pyrrolidin-3-ones

An efficient gold and chiral phosphoric acid cooperatively catalyzed enantioselective oxidative cyclization/Mannich-type addition reaction of homopropargyl amides with nitrones has been developed, which provides chiral pyrrolidin-3-ones in high yields with excellent enantioselectivities under mild conditions. This reaction employed stable and readily available alkynes as non-diazo carbene precursors, which provides a 100% atom economy method with high bond formation efficiency.

Gold-Catalyzed Cycloisomerization of Propargyl Pyruvates Enabling Unified Access to Tricladolides C and D, Chaetomellic Anhydride A, and Tyromycin A

Gold-catalyzed cycloisomerization of propargyl pyruvates has been developed as a key reaction to prepare maleic anhydride-type natural products. By combining with chemoselective epoxidation of the formed γ-alkylidenebutenolides and oxidative cleavage of epoxides, the first synthesis of tricladolide D and racemic tricladolide C has been achieved in 52 and 16% overall yields with five to seven steps starting from commercially available compounds. Further catalytic hydrogenation of alkenylated maleic anhydrides derived from γ-alkylidenebutenolides produced chaetomellic anhydride A (19% yield for six steps) and tyromycin A (15% yield for six steps), which provides flexible synthetic approaches to these naturally occurring dialkylated maleic anhydrides distinct from the documented ones.

Enantioselective Oxidative Multi-Functionalization of Terminal Alkynes with Nitrones and Alcohols for Expeditious Assembly of Chiral α-Alkoxy-β-amino-ketones

Catalytic oxidative functionalization of alkynes has emerged as an effective method in synthetic chemistry in recent decades. However, enantioselective transformations via metal carbene intermediates are quite rare due to the lack of robust chiral catalysts, especially in the intermolecular versions. Herein, we report the first asymmetric three-component reaction of commercially available alkynes with nitrones and alcohols, which affords α-alkoxy-β-amino-ketones in good yields with high to excellent enantioselectivity using combined catalysis by an achiral gold complex and a chiral spiro phosphoric acid (CPA). Mechanistically, this atom-economic reaction involves a catalytic alkyne oxidation/ylide formation/Mannich-type addition sequence that uses nitrone as the oxidant and the leaving fragment imine as the electrophile, providing a novel method for multi-functionalization of commercially available terminal alkynes.

Enantioselective Rh(II)-Catalyzed Desymmetric Cycloisomerization of Diynes: Constructing Furan-Fused Dihydropiperidines with an Alkyne-Substituted Aza-Quaternary Stereocenter

Described herein is an enantioselective dirhodium(II)-catalyzed cycloisomerization of diynes achieved by the strategy of desymmetrization, which not only represents a new cycloisomerization reaction of diynes but also constitutes the first Rh(II)-catalyzed asymmetric intramolecular cycloisomerization of 1,6-diynes. This protocol provides a range of valuable furan-fused dihydropiperidine derivatives with an enantiomerically enriched alkynyl-substituted aza-quaternary stereocenter in high efficiency, complete atom economy, and excellent enantioselectivity (up to 98% ee). Besides, the highly functionalized products could be easily transformed into various synthetically useful building blocks and conjugated with a series of pharmaceutical molecules. The mechanism involving a concerted [3+2] cycloaddition/[1,2]-H shift of the Rh(II) carbenoid intermediate was elucidated by DFT calculations and mechanistic studies. More importantly, the first single crystal of alkyne-dirhodium(II) was obtained to show that a η²-coordinating activation of alkynal by dirhodium(II) was involved. Weak hydrogen bondings between the carboxylate ligands and alkynal were found, which probably made the well-defined paddlewheel-like dirhodium(II) distinctive from other metal complexes in catalyzing this transformation. Furthermore, the origin of the enantioselectivity was elucidated by a Rh₂(R-PTAD)₄-alkyne complex and additional calculational studies.

Au⋅⋅⋅H-C Hydrogen Bonds as Design Principle in Gold(I) Catalysis

Secondary ligand-metal interactions are decisive in many catalytic transformations. While arene-gold interactions have repeatedly been reported as critical structural feature in many high-performance gold catalysts, we herein report that these interactions can also be replaced by Au⋅⋅⋅H-C hydrogen bonds without suffering any reduction in catalytic performance. Systematic experimental and computational studies on a series of ylide-substituted phosphines featuring either a PPh3 (Ph YPhos) or PCy3 (Cy YPhos) moiety showed that the arene-gold interaction in the aryl-substituted compounds is efficiently compensated by the formation of Au⋅⋅⋅H-C hydrogen bonds. The strongest interaction is found with the C-H moiety next to the onium center, which due to the polarization results in remarkably strong interactions with the shortest Au⋅⋅⋅H-C hydrogen bonds reported to date. Calorimetric studies on the formation of the gold complexes further confirmed that the Ph YPhos and Cy YPhos ligands form similarly stable complexes. Consequently, both ligands showed the same catalytic performance in the hydroamination, hydrophenoxylation and hydrocarboxylation of alkynes, thus demonstrating that Au⋅⋅⋅H-C hydrogen bonds are equally suited for the generation of highly effective gold catalysts than gold-arene interactions. The generality of this observation was confirmed by a comparative study between a biaryl phosphine ligand and its cyclohexyl-substituted derivative, which again showed identical catalytic performance. These observations clearly support Au⋅⋅⋅H-C hydrogen bonds as fundamental secondary interactions in gold catalysts, thus further increasing the number of design elements that can be used for future catalyst construction.

New-Generation Ligand Design for the Gold-Catalyzed Asymmetric Activation of Alkynes

Gold(I) catalysts are ideal for the activation of alkynes under very mild conditions. However, unlike allenes or alkenes, the triple bond of alkynes cannot be prochiral. In addition, the linear coordination displayed by gold(I) complexes places the chiral ligand far away from the substrate resulting in an inefficient transfer of chiral information. This poses a significant challenge for the achievement of high enantiocontrol in gold(I)-catalyzed reactions of alkynes. Although considerable progress on enantioselective gold(I)-catalyzed transformations has recently been achieved, the asymmetric activation of non-prochiral alkyne-containing small molecules still represents a great challenge. Herein we summarize recent advances in intra- and intermolecular enantioselective gold(I)-catalyzed reactions involving alkynes, discussing new chiral ligand designs that lie at the basis of these developments. We also focus on the mode of action of these catalysts, their possible limitations towards a next-generation of more efficient ligand designs. Finally, square planar chiral gold(III) complexes, which offer an alternative to chiral gold(I) complexes, are also discussed.

(Z)-γ-Alkylidenebutenolide Synthesis through Au(I)-Catalyzed 1,3-Acyloxy Migration and the Carbonyl-Ene Reaction

A bimetallic protocol has been developed to construct (Z)-γ-alkylidenebutenolide compounds from readily available propargyl α-ketoesters. It involves a gold-catalyzed 1,3-acyloxy migration of propargyl α-ketoesters and a carbonyl-ene cyclization of in situ generated allenyl esters. DFT calculations suggest that the copper salt might play dual roles as both chloride abstractor facilitating the generation of highly active gold catalyst and Lewis acid promoting the stepwise intramolecular carbonyl-ene reaction.

The interplay of carbophilic activation and Au(I)/Au(III) catalysis: an emerging technique for 1,2-difunctionalization of C-C multiple bonds

Gold complexes have emerged as the catalysts of choice for various functionalization reactions of C-C multiple bonds due to their inherent carbophilic nature. In a parallel space, efforts to realize less accessible cross-coupling reactivity have led to the development of various strategies that facilitate the arduous Au(i)/Au(iii) redox cycle. The interplay of the two important reactivity modes encountered in gold catalysis, namely carbophilic activation and Au(i)/Au(iii) catalysis, has allowed the development of a novel mechanistic paradigm that sponsors 1,2-difunctionalization reactions of various C-C multiple bonds. Interestingly, the reactivity as well as selectivity obtained through this interplay could be complementary to that obtained by the use of various other transition metals that mainly involved the classical oxidative addition/migratory insertion pathways. The present review shall comprehensively cover all the 1,2-difunctionalization reactions of C-C multiple bonds that have been realized by the interplay of the two important reactivity modes and categorized on the basis of the method that has been employed to foster the Au(i)/Au(iii) redox cycle.

Copper-catalyzed asymmetric cyclization of alkenyl diynes: method development and new mechanistic insights

Metal carbenes have proven to be one of the most important and useful intermediates in organic synthesis, but catalytic asymmetric reactions involving metal carbenes are still scarce and remain a challenge. Particularly, the mechanistic pathway and chiral induction model in these asymmetric transformations are far from clear. Described herein is a copper-catalyzed asymmetric cyclization of alkenyl diynes involving a vinylic C(sp²)–H functionalization, which constitutes the first asymmetric vinylic C(sp²)–H functionalization through cyclopentannulation. Significantly, based on extensive mechanistic studies including control experiments and theoretical calculations, a revised mechanism involving a novel type of endocyclic copper carbene via remote-stereocontrol is proposed, thus providing new mechanistic insight into the copper-catalyzed asymmetric diyne cyclization and representing a new chiral control pattern in asymmetric catalysis based on remote-stereocontrol and vinyl cations. This method enables the practical and atom-economical construction of an array of valuable chiral polycyclic-pyrroles in high yields and enantioselectivities.

A copper-catalyzed asymmetric cyclization of alkenyl diynes involving a vinylic C(sp²)–H functionalization is reported, enabling the construction of various valuable chiral polycyclic-pyrroles in high yields and enantioselectivities.

Divergent Gold Catalysis: Unlocking Molecular Diversity through Catalyst Control

The catalyst-directed divergent synthesis, commonly termed as "divergent catalysis", has emerged as a promising technique as it allows chartering of structurally distinct products from common substrates simply by modulating the catalyst system. In this regard, gold complexes emerged as powerful catalysts as they offer unique reactivity profiles as compared to various other transition metal catalysts, primarily due to their salient electronic and geometrical features. Owing to the tunable soft π-acidic nature, gold catalysts not only evolved as superior contenders for catalyzing the reactions of alkynes, alkenes, and allenes but also, more intriguingly, have been found to provide divergent reaction pathways over other π-acid catalysts such as Ag, Pt, Pd, Rh, Cu, In, Sc, Hg, Zn, etc. The recent past has witnessed a renaissance in such examples wherein, by choosing gold catalysts over other transition metal catalysts or by fine-tuning the ligands, counteranions or oxidation states of the gold catalyst itself, a complete reactivity switch was observed. However, reviews documenting such examples are sporadic; as a result, most of the reports of this kind remained scattered in the literature, thereby hampering further development of this burgeoning field. By conceptualizing the idea of "Divergent Gold Catalysis (DGC)", this review aims to consolidate all such reports and provide a unified approach necessary to pave the way for further advancement of this exciting area. Based on the factors governing the divergence in product formation, an explicit classification of DGC has been provided. To gain a fundamental understanding of the divergence in observed reactivities and selectivities, the review is accompanied by mechanistic insights at appropriate places.