Au-Catalyzed Intermolecular [2+2] Cycloadditions between Chloroalkynes and Unactivated Alkenes

Palladium-Catalyzed Site-Selective Regiodivergent Carbocyclization of Di- and Trienallenes: A Switch between Substituted Cyclohexene and Cyclobutene

Nature efficiently produces a myriad of structurally diverse carbon ring frameworks from common linear precursors via cyclization reactions at specific olefinic sites in dienes or polyenes. In contrast, achieving the site-selective functionalization of diene or polyene substrates remains a formidable challenge in chemical synthesis. Herein, we report a pair of highly site-selective, regiodivergent carbocyclization reactions of dienallenes and trienallenes, enabling the efficient synthesis of cis-1,4-disubstituted cyclohexenes and trans-1,2-disubstituted cyclobutenes from a common precursor with high diastereoselectivity. Remarkably, simple achiral organophosphoric acids and amines are identified as powerful ligands for controlling these palladium-catalyzed regiodivergent carbocyclizations. This approach represents the first example of site-selective regiodivergent carbocyclization, providing a practical method for the stereospecific synthesis of thermodynamically disfavored cis-1,4-disubstituted cyclohexenes and fully substituted trans-1,2-cyclobutenes. Additionally, the methodology developed offers general insights into the development of metal-catalyzed site-selective, regiodivergent carbocyclizations of diene and polyene precursors, mimicking natural carbocyclization processes.

Stereoselective Alkyne-Alkene [2 + 2] Cycloaddition Enabled by Anion Relay Chemistry for Fully C-Substituted Cyclobutene Synthesis

Cyclobutenes serve as valuable scaffolds in pharmaceutical development, but synthesizing structurally diverse fully C-substituted cyclobutenes remains challenging. Here we report a stereoselective [2 + 2] cycloaddition between alkynyl 1,3-dithianes and cinnamate esters using a catalytic amount of KOtBu. Our approach enables the construction of synthetically challenging tetrasubstituted cyclobutenes via anion relay chemistry (ARC), exhibiting excellent trans-diastereoselectivities (dr > 20:1).

Sc-Catalyzed Asymmetric [2 + 2] Annulation of 2-Alkynylnaphthols with Dienes to Access Cyclobutene Frameworks

Herein, we introduce a scandium-catalyzed synthetic strategy that provides access to a diverse and functionalized array of cyclobutene frameworks adorned with a quaternary carbon center. This approach broadens the synthetic repertoire of 2-alkynylnaphthols with alkenes, offering a versatile platform for the construction of complex molecular architectures. The asymmetric catalytic [2 + 2] cycloaddition reaction demonstrates a wide substrate scope and an impressive functional group tolerance, yielding products with high efficiency, up to 97% yield, and excellent enantiomeric excess of up to 97%. The simplicity of scaling up this process, coupled with the ease of converting these cyclobutene frameworks into a variety of substituted products, significantly enhances the synthetic utility of this method.

The influential IPr: 25 years after its discovery

N-Heterocyclic carbenes (NHCs) have emerged as a privileged ligand family in organometallic chemistry, widely recognized for their unique steric and electronic properties. Among them, the 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene (IPr) ligand has become a cornerstone of NHC chemistry for its remarkable versatility, stability, and broad use. Since its discovery by the Nolan group in 1999, IPr has played a pivotal role in advancing catalytic transformations and facilitating the utilization of NHC ligands in various domains. This article highlights major contributions where IPr has helped shape modern organometallic chemistry, with a focus on its influence in transition metal catalysis and ligand design. Twenty five years after its discovery, the IPr ligand continues to be a benchmark ligand, inspiring and driving innovation.

Regiodivergent Gold-Catalyzed Rearrangement-Addition Reactions of Sulfenylated Propargylic Carboxylates with Indoles

Sulfenylated propargylic carboxylates were introduced to investigate the influence of sulfur substitution in gold-catalyzed alkyne activation pathways. Regiodivergent gold-catalyzed rearrangement and indole capture reactions proceed under mild conditions to give functionalized indole products bearing sulfenylated (Z)-enol carboxylate motifs. Pathways involving both 1,2- and 1,3-carboxylate migrations are achieved selectively, with indole being added in a 1,4 relationship to the sulfenyl group in each case. High levels of selectivity are influenced by the catalyst system, counterion, and carboxylate group.

Regiodivergent Synthesis of 4- and 5-Sulfenyl Oxazoles from Alkynyl Thioethers

The regiodivergent synthesis of 4- and 5-sulfenyl oxazoles from 1,4,2-dioxazoles and alkynyl thioethers has been achieved. Gold-catalysed conditions are used to favour the formation of 5-sulfenyl oxazoles via β-selective attack of the nitrenoid relative to the sulfenyl group. In contrast, 4-sulfenyl oxazoles are formed by α-selective reaction under Brønsted acid conditions from the same substrates. The nature of stabilising gold-sulfur interactions have been investigated by natural bond orbital analysis, showing that the S→Au interactions are significantly stronger in the intermediate that favours the 5-sulfenyl oxazoles. A kinetic survey identifies catalyst inhibition processes. This study into the regiodivergent methods includes the development of telescoped annulation-oxidation protocols for regioselective access to oxazole sulfoxides and sulfones.

π-Bonding of Group 11 Metals to a Tantalum Alkylidyne Alkyl Complex Promotes Unusual Tautomerism to Bis-alkylidene and CO2 to Ketenyl Transformation

A salt metathesis synthetic strategy is used to access rare tantalum/coinage metal (Cu, Ag, Au) heterobimetallic complexes. Specifically, complex [Li(THF)2][Ta(CtBu)(CH2tBu)3], 1, reacts with (IPr)MCl (M = Cu, Ag, Au, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) to afford the alkylidyne-bridged species [Ta(CH2tBu)3(μ-CtBu)M(IPr)] 2-M. Interestingly, π-bonding of group 11 metals to the Ta─C moiety promotes a rare alkylidyne alkyl to bis-alkylidene tautomerism, in which compounds 2-M are in equilibrium with [Ta(CHtBu)(CH2tBu)2(μ-CHtBu)M(IPr)] 3-M. This equilibrium was studied in detail using NMR spectroscopy and computational studies. This reveals that the equilibrium position is strongly dependent on the nature of the coinage metal going down the group 11 triad, thus offering a new valuable avenue for controlling this phenomenon. Furthermore, we show that these uncommon bimetallic couples could open attractive opportunities for synergistic reactivity. We notably report an uncommon deoxygenative carbyne transfer to CO2 resulting in rare examples of coinage metal ketenyl species, (tBuCCO)M(IPr), 4-M (M = Cu, Ag, Au). In the case of the Ta/Li analogue 1, the bis(alkylidene) tautomer is not detected, and the reaction with CO2 does not cleanly yield ketenyl species, which highlights the pivotal role played by the coinage metal partner in controlling these unconventional reactions.

Sulfur in Waste-Free Sustainable Synthesis: Advancing Carbon-Carbon Coupling Techniques

This review explores the pivotal role of sulfur in advancing sustainable carbon-carbon (C-C) coupling reactions. The unique electronic properties of sulfur, as a soft Lewis base with significant mesomeric effect make it an excellent candidate for initiating radical transformations, directing C-H-activation, and facilitating cycloaddition and C-S bond dissociation reactions. These attributes are crucial for developing waste-free methodologies in green chemistry. Our mini-review is focused on existing sulfur-directed C-C coupling techniques, emphasizing their sustainability and comparing state-of-the-art methods with traditional approaches. The review highlights the importance of this research in addressing current challenges in organic synthesis and catalysis. The innovative use of sulfur in photocatalytic, electrochemical and metal-catalyzed processes not only exemplifies significant advancements in the field but also opens new avenues for environmentally friendly chemical processes. By focusing on atom economy and waste minimization, the analysis provides broad appeal and potential for future developments in sustainable organic chemistry.

Palladium-Catalyzed Tandem Cyclization of Bromoalkynes, Anilines and CO: Access to 1,3-Substituted Maleimides

A novel palladium-catalyzed three-component carbonylation reaction for the assembly of various 1,3-substituted maleimide derivatives from haloalkynes and simple anilines. The nucleophilic addition reaction of haloalkynes, anilines and CO, and insertion of carbonyl have been achieved sequentially in this reaction. The high chemo- and regioselectivities, as well as no need of expensive ligands or additives further illustrate the synthetic value of this approach.

Copper-Catalyzed Regio- and Stereoselective Three-Component Coupling of Allenyl Ethers with gem-Dichlorocyclobutenones and B2pin2

The three-component coupling method for regio- and stereoselective difunctionalization of allenes with allenyl ethers, bis(pinacolato)diboron, and gem-dichlorocyclobutenones as electrophiles was reported, yielding a variety of highly functionalized cyclobutenone products tethering with an alkenylborate fragment. The polysubstituted cyclobutenone products also underwent diverse transformations.

Copper-Catalyzed Borylation and Silylation of Dichlorocyclobutenones

We report copper-catalyzed borylation and silylation of dichlorocyclobutenones, which furnish the boron-substituted and silicon-substituted polyfunctionalized cyclobutenones in high yields. The reactions proceed under mild reaction conditions, show broad substrate scope, and display high chemoselectivity. In addition, a series of transformations of the corresponding products has been realized.

Combination of gold and redox enzyme catalysis to access valuable enantioenriched aliphatic β-chlorohydrins

The synthesis of enantioenriched β-chlorohydrins is highly appealing due to their relevance as building-blocks in organic synthesis. However, the approximation to aliphatic derivatives is particularly challenging due to the difficulties to get access to the α-chloroketone precursors. Herein, we propose a straightforward and scalable approach combining in a concurrent manner gold(I) and redox enzyme catalysis through a hydration-bioreduction cascade. A total of nine aliphatic β-chlorohydrins bearing different functional groups were obtained with very high yields (63-88%) and stereoselectivities (>99% ee).

Palladium-catalyzed oxidative C-H activation/annulation of N-alkylanilines with bromoalkynes: access to functionalized 3-bromoindoles

A straightforward approach to the synthesis of 3-bromoindoles via palladium-catalyzed oxidative C-H activation/annulation of N-alkylanilines with bromoalkynes has been described. This protocol features high atom economy, excellent chemo- and regioselectivities, and good functional group tolerance. Moreover, the resultant 3-bromoindoles can be transformed to various functionalized indole derivatives, which demonstrates the practicability of this method in organic synthesis.

Acid-promoted intra- and intermolecular [2+2] cycloaddition of indoles to aryl alkynes to access cyclobutene-fused indolines

Herein, we have reported the first example of both intra- and intermolecular [2+2] cycloaddition of the electron-rich indoles and unactivated aryl alkynes promoted by the combination of Fe(NO₃)₃ and HNO₃, which highlights efficient and selective access to several different types of functionalized cyclobutene-fused indolines from readily available starting materials with cheap catalysts and simple operations.

Copper-catalyzed radical cascade reaction of simple cyclobutanes: synthesis of highly functionalized cyclobutene derivatives

Cyclobutenes as versatile and highly valuable synthons have been widely applied in synthesis. Although various methods for their synthesis have been well established, new strategies for the construction of the cyclobutene skeleton from simple substrates are still highly desirable. Starting from simple cyclobutanes, the construction of the cyclobutene skeleton especially introducing multiple functional groups simultaneously had never been achieved. Here, we developed a novel radical cascade strategy for the synthesis of highly functionalized cyclobutenes directly from cyclobutanes involving rare cleavage of four or five C–H bonds and formation of two C–N/C–S or three C–Br bonds. With copper as catalyst and N-fluorobenzenesulfonimide (NFSI) as oxidant, a wide range of diaminated, disulfonylated and tribrominated cyclobutene derivatives were efficiently synthesized.

A novel radical cascade strategy for the synthesis of highly functionalized cyclobutenes directly from cyclobutanes involving rare four or five C–H bonds cleavage and two C–N/C–S or three C–Br bonds formation has been successfully developed.

Steric- and Electronic-Controlled Intramolecular [2 + 2]-Cycloaddition of Cyclohexadienone-Containing 1,7-Enynes

Herein we have developed the silver-catalyzed electronic- and steric-controlled intramolecular formal [2 + 2]-cycloaddition of alkyne-tethered cyclohexadienones. Substrates with electron-rich alkynes and a less hindered quaternary carbon center afford tricyclic fused cyclobutenes through 1,7-enyne cyclization. In contrast, the formation of dihydrofurans was observed from electron-deficient alkynes via proton abstraction/C-O bond cleavage. The synthetic potential of this method was also broadened with a gram-scale reaction and various transformations on cyclobutene.

Synthesis of Cyclobutanones by Gold(I)-Catalyzed [2 + 2] Cycloaddition of Ynol Ethers with Alkenes

A broad scope synthesis of cyclobutanones by gold(I)-catalyzed [2 + 2] cycloaddition of ynol ethers with alkenes has been developed. We also found that internal aryl ynol ethers can undergo (4 + 2) cycloaddition reaction with alkenes leading to the corresponding chromanes.

Gold-Catalyzed Annulations with Nucleophilic Nitrenoids Enabled by Heteroatom-Substituted Alkynes

The combination of a nucleophilic nitrene equivalent, a triple bond and a π-acid catalyst has underpinned numerous efficient transformations for the preparation of azacycles. This personal account details our efforts in developing an annulation strategy. Adding a nucleophilic nitrenoid to an activated alkyne can generate carbenoid character that is then quenched by a cyclisation onto the nitrenoid substituent. The use and development of N-acyl and N-heterocyclic pyridinium-N-aminides as 1,3-N,O and 1,3-N,N-dipole equivalents is discussed in the context of oxazole and heterocycle-fused imidazole formation, respectively. The resulting processes are highly efficient, practically straightforward, and tolerate considerable structural and functional group variation. Our use of heteroatom-substituted alkynes as enabling tools for reaction discovery is discussed. The reactivity accessed from the strong donor-like properties of ynamides is complemented by that obtained from alkynyl thioethers, which are emerging as interesting substrates for π-acid catalysis.

Gold(I)-Catalyzed Intermolecular Formal [4+2] Cycloaddition of O-Aryl Ynol Ethers and Enol Ethers: Synthesis of Chromene Derivatives

Gold(I)-catalyzed formal [4+2] cycloaddition of O-aryl ynol ethers 1 and enol ethers 2 is described. This intermolecular reaction between two electron-rich unsaturated systems takes place, under mild conditions, in the presence of 5 mol% [IPrAu(CH3 CN)]SbF6 as catalyst giving chromene derivatives with good yields. The cycloaddition is completely regio- and stereoselective, as well as versatile for both reactives. Silyl enol ethers can also react in the same way and under the same reaction conditions with quantitative yields. A plausible mechanism through a selective addition of the enol ether to the alkyne gold activated complex followed by an intramolecular aromatic electrophilic substitution is proposed. Several experimental results support the presence of a cationic oxonium intermediate prior to the aromatic substitution. The reaction represents a new entry to the chromene core.

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.

Challenges in the Highly Selective [3 + 1]-Cycloaddition of an Enoldiazoacetamide to Form a Donor-Acceptor Cis-Cyclobutenecarboxamide

A substituted donor-acceptor cyclobutenecarboxamide is synthesized with modest enantiocontrol through a chiral copper(I) complex catalyzed [3 + 1]-cycloaddition reaction of α-acyl diphenylsulfur ylides with 3-siloxy-2-diazo-3-butenamides. With a methyl substituent on the 4-position of the 3-butenamide, the cis-vicinal-3,4-disubstituted cyclobutenecarboxamide is formed with >20:1 diastereocontrol. Donor-acceptor 3-methyl-2-siloxycyclopropenecarboxamide is rapidly formed from the reactant enoldiazoamide and undergoes catalytic ring opening to give only the Z-γ-substituted metallo-enolcarbene. Elimination from 3-siloxy-2-diazo-3-pentenamide to form the conjugated 3-siloxy-2,4-pentadienamide is competitive but minimized at low temperature.

Gold(I)-Catalyzed Synthesis of 3-Sulfenyl Pyrroles and Indoles by a Regioselective Annulation of Alkynyl Thioethers

The combination of nucleophilic nitrenoids and π-acid catalysis has emerged as a powerful tool in heterocycle synthesis. Accessing more varied heterocycle-substitution patterns by maintaining the same reaction pathways across different alkynes remains a challenge. Here we show that Au(I) catalysis of isoxazole-based nitrenoids with alkynyl thioethers provides controlled access to (3 + 2) annulation by a regioselective addition β to the sulfenyl group. The reaction with isoxazole-containing nitrenoids delivers sulfenylated pyrroles and indoles as single regioisomers bearing useful functional groups and structural variety.

Cycloisomerization of π-Coupled Heteroatom Nucleophiles by Gold Catalysis: En Route to Regiochemically Defined Heterocycles

Since the dawn of millennium, catalytic gold chemistry is at the forefront to set off diverse organic reactions via unique activation of π-bonded molecules. Within this purview, cycloisomerization of heteroatom nucleophiles linked to a π-system is one of the well recognized chemistry for the construction of numerous heterocyclic cores. Though the rudimentary aspects of this transformation are reviewed by several groups in different timeline, a holistic view on regiochemistry of such reactions went largely overlooked. Hence, this account emphasizes the gold catalyzed regioselective cycloisomerization of structurally distinctive π-connected hetero-nucleophiles leading to different heterocycles documented in the last two decades. From an application perspective, this account also highlights those methodologies which find a role in the total synthesis of natural products. Wherever appropriate, mechanistic details and contributing factors for selectivity are also discussed.

Gold Catalysis of Non-Conjugated Haloacetylenes

Gold-catalyzed reactions of conjugated haloacetylenes are well known and usually result in the formation of addition or dimerization products. Herein, we report a gold-catalyzed reaction of non-conjugated­ haloacetylenes, which leads exclusively to the halogenated cyclization products. Remarkable is the gold-catalyzed reaction of tritylhaloacetylenes to haloindene derivatives, as mechanistic studies reveal that an 1,2-aryl shift occurs in the initially formed gold complex. The potential functionalization at the halogen atom and the wide scope of these cyclization reactions make them an attractive method for the construction of cyclic systems.

Uncommon Four-Membered Building Blocks - Cyclobutenes, Azetines and Thietes

Strained ring systems have gained considerable importance over the last few years for their implication in natural product syntheses or in drug discovery programs. We present herein a recollection of our work on the construction and functionalization of unsaturated four-membered carbo- and heterocycles in the context of the literature, as well as their applications in further reactions.

Redirecting a Diels-Alder Reaction toward (2 + 2)-Cycloaddition

Recently, reactions of allylidenhydrazones with tetracyanoethylene were found to lead to cyclobutanes-products of usually unfavorable (2 + 2) cycloaddition. Herein we computationally demonstrate that the (4 + 2) product of this reaction is severely destabilized by incomplete C-N bond formation, arising from a complex interplay of substituent electronic effects. We show how destabilization of a single bond in the front-runner product averts its formation and redirects chemical reaction toward an uncharacteristic pathway.

Reactivity of ynamides in catalytic intermolecular annulations

Ynamides are unique alkynes with a carbon-carbon triple bond directly attached to the nitrogen atom bearing an electron-withdrawing group. The alkyne is strongly polarized by the electron-donating nitrogen atom, but its high reactivity can be finely tempered by the electron-withdrawing group. Accordingly, ynamides are endowed with both nucleophilic and electrophilic properties and their chemistry has been an active research field. The catalytic intermolecular annulations of ynamides, featuring divergent assembly of structurally important amino-heterocycles in a regioselective manner, have gained much attention over the past decade. This review aims to provide a comprehensive summary of the advances achieved in this area involving transition metal and acid catalysis. Moreover, the intermolecular annulations of ynamide analogs including ynol ethers and thioalkynes are also discussed, which can provide insights into the reactivity difference caused by the heteroatoms.

Catalytic Enantioselective [2+2] Cycloaddition of α-Halo Acroleins: Construction of Cyclobutanes Containing Two Tetrasubstituted Stereocenters

A catalytic enantioselective formal [2+2] cycloaddition between α-halo acroleins and electronically diverse arylalkenes is described. In the presence of (S)-oxazaborolidinium cation as the catalyst, densely functionalized cyclobutanes containing two vicinal tetrasubstituted stereocenters were produced in high yields and high diastereoselectivities with excellent enantioselectivities. Mechanistic studies revealed that the cis isomer could be transformed into the trans isomer via an enantiocontrolled process. A gram-scale reaction of this catalytic method was used to demonstrate its synthetic potential.

Au-Catalyzed intermolecular (3 + 2 + 1) and (5 + 2) cycloaddition for the synthesis of 1,4-dioxenes and 4,7-dihydrooxepines

1,4-Dioxenes and 4,7-dihydrooxepines present interesting potential as motifs for the incorporation of biologically relevant molecules, agrochemicals and materials. In this study, two efficient intermolecular (3 + 2 + 1) and (5 + 2) cycloadditions for the synthesis of 1,4-dioxenes and 4,7-dihydrooxepines are achieved with gold catalysis.

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.

Access to substituted cyclobutenes by tandem [3,3]-sigmatropic rearrangement/[2 + 2] cycloaddition of dipropargylphosphonates under Ag/Co relay catalysis

We present herein an unconventional tandem [3,3]-sigmatropic rearrangement/[2 + 2] cycloaddition of simple dipropargylphosphonates to deliver a range of bicyclic polysubstituted cyclobutenes and cyclobutanes under Ag/Co relay catalysis. An interesting switch from allene-allene to allene-alkyne cycloaddition was observed based on the substitution of the substrates, which further diversified the range of compounds accessible from this practical method. Significantly, preliminary biological screening of these new compounds identified promising candidates as suppressors of cellular proliferation.

Gold-Catalyzed Synthetic Strategies towards Four-Carbon Ring Systems

Four carbon ring systems are frequently present in natural products with remarkable biological activities such as terpenoids, alkaloids, and steroids. The development of new strategies for the assembly of these structures in a rapid and efficient manner has attracted the interest of synthetic chemists for a long time. The current research is focused mainly on the development of synthetic methods that can be performed under mild reaction conditions with a high tolerance to functional groups. In recent years, gold complexes have turned into excellent candidates for this aim, owing to their high reactivity, and are thus capable of promoting a wide range of transformations under mild conditions. Their remarkable efficiency has been thoroughly demonstrated in the synthesis of complex organic molecules from simple starting materials. This review summarizes the main synthetic strategies described for gold-catalyzed four-carbon ring formation, as well as their application in the synthesis of natural products.

Cyclopropenylmethyl Cation: A Concealed Intermediate in Gold(I)-Catalyzed Reactions

The last years have witnessed many gold-catalyzed reactions of alkynes. One of the most prominent species in the reaction of two alkyne units is the vinyl-substituted gold vinylidene intermediate. Here, we were able to show that the reaction of a haloacetylene and an alkyne proceeds via a hitherto overlooked intermediate, namely the cyclopropenylmethyl cation. The existence and relative stability of this concealed intermediate is verified by quantum chemical calculations and 13 C-labeling experiments. A comparison between the cyclopropenylmethyl cation and the well-known vinylidene intermediate reveals that the latter is more stable only for smaller cycles. However, this stability reverses in larger cycles. In the case of the smallest representative of both species, the vinylidene cation is the transition state en route to the cyclopropenylmethyl cation. The discovery of this intermediate should help to get a deeper understanding for gold-catalyzed carbon-carbon bond-forming reactions of alkynes. Furthermore, since enynes can be formed from the cyclopropenylmethyl cation, the inclusion of this intermediate should enable the development of new synthetic methods for the construction of larger cyclic halogenated and non-halogenated conjugated enyne systems.

Gold(I)-Catalyzed Haloalkynylation of Aryl Alkynes: Two Pathways, One Goal

Haloalkynylation reactions provide an efficient method for the simultaneous introduction of a halogen atom and an acetylenic unit. For the first time, we report a gold(I)‐catalyzed haloalkynylation of aryl alkynes that delivers exclusively the cis addition product. This method enables the simple synthesis of conjugated and halogenated enynes in yields of up to 90 %. Notably, quantum chemical calculations reveal an exceptional interplay between the place of the attack at the chloroacetylene: No matter which C−C bond is formed, the same enyne product is always formed. This is only possible through rearrangement of the corresponding skeleton. Hereby, one reaction pathway proceeds via a chloronium ion with a subsequent aryl shift; in the second case the corresponding vinyl cation is stabilized by a 1,3‐chlorine shift. ¹³C‐labeling experiments confirmed that the reaction proceeds through both reaction pathways.

Alkyne-Alkene [2 + 2] cycloaddition based on visible light photocatalysis

UV-activated alkyne-alkene [2 + 2] cycloaddition has served as an important tool to access cyclobutenes. Although broadly adopted, the limitations with UV light as an energy source prompted us to explore an alternative method. Here we report alkyne-alkene [2 + 2] cycloaddition based on visible light photocatalysis allowing the synthesis of diverse cyclobutenes and 1,3-dienes via inter- and intramolecular reactions. Extensive mechanistic studies suggest that the localized spin densities at sp2 carbons of alkenes account for the productive sensitization of alkenes despite their similar triplet levels of alkenes and alkynes. Moreover, the efficient formation of 1,3-dienes via tandem triplet activation of the resulting cyclobutenes is observed when intramolecular enyne cycloaddition is performed, which may serve as a complementary means to the Ru(II)-catalyzed enyne metathesis. In addition, the utility of the [2 + 2] cycloaddition has been demonstrated by several synthetic transformations including synthesis of various extended π-systems.

Recent Progress in N-Heterocyclic Carbene Gold-Catalyzed Reactions of Alkynes Involving Oxidation/Amination/Cycloaddition

Recent rapid development in homogeneous gold catalysis affords an alternative and particularly thriving strategy for the generation of gold carbenes through gold-catalyzed oxidation/amination/cycloaddition of alkynes, while it avoids the employment of hazardous and potentially explosive diazo compounds as starting materials for carbene generation. In addition to facile and secure operation, gold carbenes generated in this strategy display good chemoselectivity distinct from other metal carbenes produced from the related diazo approach. N-heterocyclic carbene (NHC) gold is a special metal complex that can be used as ancillary ligands, which provides enhanced stability and can also act as an efficient chiral directing group. In this review, we will present an overview of these recent advances in alkyne oxidation/amination/cycloaddition by highlighting their specificity and applicability, aiming to facilitate progress in this very exciting area of research.

Synthesis of 2H-Chromenes via Unexpected [4 + 2] Annulation of Alkynyl Thioethers with o-Hydroxybenzyl Alcohols

A novel Brønsted acid-catalyzed reaction of alkynyl thioethers with o-hydroxybenzyl alcohols via an unexpected formal [4 + 2] annulation has been developed. This metal-free protocol leads to the facile and practical synthesis of valuable polysubstituted 2H-chromenes in mostly good to excellent yields under mild reaction conditions and features a wide substrate scope and excellent functional group tolerance.