The role of cyclobutenes in gold(I)-catalysed skeletal rearrangement of 1,6-enynes

Puzzling Structure of the Key Intermediates in Gold(I)-catalyzed Cyclization Reactions of Enynes and Allenenes

We identify the dominant structures of the intermediates of gold(I)-catalyzed cyclizations of 1,5-enynes and 1,5-allenenes through computational analysis as gold(I) cyclopropylcarbenes, endocyclic vinylgold complexes and previously unreported non-classical carbocationic minima. In contrast to 1,6-enynes, the exocyclic carbocations are found to be less stable. Cyclopropylcarbene structures are consistently favoured as the most stable intermediates for all studied substitution patterns. We validate the computational methods used by using DLPNO-CCSD(T) energies as a benchmark, indicating that the B3LYP-D3 and M06-D3 functionals are most accurate for energy determination, while NPA charges are mostly insensitive to functional. The evolution of a 1,6-enyne in a single-cleavage or double-cleavage rearrangement is attributed to the barrierless evolution of a common cyclopropyl-gold(I) carbocation non-stationary geometry. Our findings provide insights into reaction pathways and substrate dependence of the cycloisomerization processes.

Gold(I)-Catalyzed 1,6-Enyne Single-Cleavage Rearrangements: The Complete Picture

We identify the factors that rule the selectivity in single-cleavage skeletal rearrangements promoted by gold(I) catalysts. We find that stereoconvergence is enabled by a rotational equilibrium when electron-rich substituents are used. The anomalous Z-selective skeletal rearrangement is found to be due to electronic factors, whereas endo-selectivity depends on both steric and electronic factors.

Gold-Catalyzed Amine Cascade Addition to Diyne-Ene: Enantioselective Synthesis of 1,2-Dihydropyridines

With the well-documented chemical and biological applications, piperidine and pyridine are among the most important N-heterocycles, and a new synthetic strategy, especially one with an alternative bond-forming design, is of general interest. Using the gold-catalyzed intermolecular condensation of amine and diyne-ene, we report herein the first example of enantioselective 1,2-dihydropyridine synthesis through a formal [3+2+1] fashion (up to 95 % yield, up to 99 % e.e.).

Facile synthesis of diverse hetero polyaromatic hydrocarbons (PAHs) via the styryl Diels–Alder reaction of conjugated diynes

The intramolecular styryl Diels–Alder reaction with conjugated diynes under thermally stable triazole-gold (TA–Au) catalytic conditions and the sequential transformation through alkyne activation to access various PAHs with high efficiency was reported for the first time.

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.

Synthetic Strategy for Construction of Highly Congested Tetracyclic Core (6-5-7-4) of Harziane Diterpenoids

The structurally intriguing tetracyclic core of complex harziane diterpenoid was constructed in 14 steps from commercially available 3-ethoxycyclohex-2-en-1-one. The key steps were a Mn/Cu-mediated oxidative 1,3-dicarbonyl radical cascade cyclization reaction, which diastereoselectively formed the core of dimethylbicyclo[3.2.1]octane structure, and a Au-catalyzed diastereoselective formal [2 + 2] cycloaddition for construction of the harziane diterpenoid tetracyclic framework. The developed method paves the way for achieving total synthesis of this type of complex natural product.

Alkynophilicity of Group 13 MX3 Salts: A Theoretical Study

The concept of alkynophilicity is revisited with group 13 MX₃ metal salts (M = In, Ga, Al, B; X = Cl, OTf) using M06-2X/6-31+G(d,p) calculations. This study aims at answering why some of these salts show reactivity toward enynes that is similar to that observed with late-transition-metal complexes, notably Au(I) species, and why some of them are inactive. For this purpose, the mechanism of the skeletal reorganization of 1,6-enynes into 1-vinylcyclopentenes has been computed, including monomeric ("standard") and dimeric (superelectrophilic) activation. Those results are confronted with deactivation pathways based on the dissociation of the M-X bond. The role of the X ligand in the stabilization of the intermediate nonclassical carbocation is revealed, and the whole features required to make a good π-Lewis acid are discussed.

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.

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.

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

The [2+2] cycloaddition is a versatile strategy for the synthesis of strained cyclobutenes of high synthetic value. In this study, two efficient intermolecular [2+2] cycloadditions between two different types of chloroalkynes and unactivated alkene are realized with gold catalysis. Of significance is that the reaction works with challenging monosubstituted unactivated alkenes, which is unprecedented in gold catalysis and scarcely documented in other metal-catalyzed/promoted reactions; moreover, the reaction exhibits excellent regioselectivities, which are much better than those reported in literature. With 1,2-disubstituted unactivated alkenes, the reaction is largely stereospecific. The cyclobutene products can be prepared in nearly gram scale and readily undergo further reactions including various cross-coupling reactions using the C(sp²)-Cl and/or C(sp²)-SPh bond, which in turn substantially broaden the scope of accessible cyclobutenes and enhance the synthetic utility of this bimolecular reaction.

Role of σ,π-Digold(I) Alkyne Complexes in Reactions of Enynes

Gold(I) acetylide and σ,π-digold(I) alkyne complexes derived from one prototypical 1,6-enyne and from 7-ethynyl-1,3,5-cycloheptatriene have been prepared and structurally characterized. Their possible role in gold(I)-catalyzed cycloisomerizations has been studied by experiment and by DFT calculations. Gold(I) acetylides are totally unproductive complexes in the absence of Brønsted acids. Similarly, no cyclizations were observed by heating σ,π-digold(I) alkyne digold(I) at least up to 130 °C. Theoretical studies provide a rationale for the much lower reactivity of digold species in reactions of enynes.

Cyclobutene vs 1,3-Diene Formation in the Gold-Catalyzed Reaction of Alkynes with Alkenes: The Complete Mechanistic Picture

The intermolecular gold(I)-catalyzed reaction between arylalkynes and alkenes leads to cyclobutenes by a [2 + 2] cycloaddition, which takes place stepwise, first by formation of cyclopropyl gold(I) carbenes, followed by a ring expansion. However, 1,3-butadienes are also formed in the case of ortho-substituted arylalkynes by a metathesis-type process. The corresponding reaction of alkenes with aryl-1,3-butadiynes, ethynylogous to arylalkynes, leads exclusively to cyclobutenes. A comprehensive mechanism for the gold(I)-catalyzed reaction of alkynes with alkenes is proposed on the basis of density functional theory calculations, which shows that the two pathways leading to cyclobutenes or dienes are very close in energy. The key intermediates are cyclopropyl gold(I) carbenes, which have been independently generated by retro-Buchner reaction from stereodefined 1a,7b-dihydro-1H-cyclopropa[a]naphthalenes.

Metal-catalyzed enyne cycloisomerization in natural product total synthesis

Enyne cycloisomerization has become a powerful and attractive strategy for the construction of cyclic compounds, thus possessing great potential for applications in total synthesis of natural products and pharmaceuticals.

Divergent reaction pathways in gold-catalyzed cycloisomerization of 1,5-enynes containing a cyclopropane ring: dramatic ortho substituent and temperature effects

A gold(i)-catalyzed cycloisomerization of easily available 1,5-enynes containing a cyclopropane ring has been developed, efficiently providing cyclobutane-fused 1,4-cyclohexadiene, tricyclic cyclobutene, biscyclopropane and 1,3-cyclohexadiene derivatives in moderate to excellent yields. When the phenyl group was not ortho substituted, 1,4-cyclohexadienes could be produced. With an ortho substituent, three different products could be selectively synthesized by control of the temperature and the used gold(i) catalyst. The 1,5-enyne substrate first undergoes a classical enyne cycloisomerization to form a tricyclic cyclobutene key intermediate, which undergoes subsequent transformation to produce the desired products. A plausible reaction mechanism was proposed according to deuterium labeling experiments and intermediate trapping experiments, as well as DFT calculations. In our current reaction, the ortho substituent on the phenyl group controls the reaction outcome and the ortho substituent effect was found to originate from steric and electronic factors.

Gold carbenes, gold-stabilized carbocations, and cationic intermediates relevant to gold-catalysed enyne cycloaddition

Cationic gold complexes in which gold is bound to a formally divalent carbon atom, typically formulated as gold carbenes or α-metallocarbenium ions, have been widely invoked in a range of gold-catalyzed transformations, most notably in the gold-catalyzed cycloisomerization of 1,n-enynes. Although the existence of gold carbene complexes as intermediates in gold-catalyzed transformations is supported by a wealth of indirect experimental data and by computation, until recently no examples of cationic gold carbenes/α-metallocarbenium ions had been synthesized nor had any cationic intermediates generated via gold-catalyzed enyne cycloaddition been directly observed. Largely for this reason, there has been considerable debate regarding the electronic structure of these cationic complexes, in particular the relative contributions of the carbene (LAu(+)[double bond, length as m-dash]CR2) and α-metallocarbenium (LAu-CR2(+)) forms, which is intimately related to the extent of d → p backbonding from gold to the C1 carbon atom. However, over the past ∼ seven years, a number of cationic gold carbene complexes have been synthesized in solution and generated in the gas phase and cationic intermediates have been directly observed in the gold-catalyzed cycloaddition of enynes. Together, these advances provide insight into the nature and electronic structure of gold carbene/α-metallocarbenium complexes and the cationic intermediates generated via gold-catalyzed enyne cycloaddition. Herein we review recent advances in this area.

Metal-catalyzed cycloisomerization as a powerful tool in the synthesis of complex sesquiterpenoids

Metal-catalyzed cycloisomerization reactions can be regarded as “artificial sesquiterpenoid cyclase” tools in modern organic synthesis, and their progress is reviewed.

Divergent Gold(I)-Catalyzed Skeletal Rearrangements of 1,7-Enynes

The gold(I) complex catalyzed cycloisomerization and skeletal rearrangement of 1,n-enynes (n=5-7) is a powerful methodology for the efficient synthesis of complex molecular architectures. In contrast to 1,6-enynes, readily accessible homologous 1,7-enynes are largely unexplored in such transformations. Here, the divergent skeletal rearrangement of all-carbon 1,7-enynes by catalysis with a cationic gold(I) complex is reported. Depending on electronic and steric factors, differently substituted 1,7-enynes react via different carbocations formed from a common gold carbene intermediate to yield on the one hand novel exocyclic allenes and on the other hand tricyclic hexahydro-anthracenes through a novel dehydrogenative Diels-Alder reaction.

Gold-Catalyzed Reactions via Cyclopropyl Gold Carbene-like Intermediates

Cycloisomerizations of 1,n-enynes catalyzed by gold(I) proceed via electrophilic species with a highly distorted cyclopropyl gold(I) carbene-like structure, which can react with different nucleophiles to form a wide variety of products by attack at the cyclopropane or the carbene carbons. Particularly important are reactions in which the gold(I) carbene reacts with alkenes to form cyclopropanes either intra- or intermolecularly. In the absence of nucleophiles, 1,n-enynes lead to a variety of cycloisomerized products including those resulting from skeletal rearrangements. Reactions proceeding through cyclopropyl gold(I) carbene-like intermediates are ideally suited for the bioinspired synthesis of terpenoid natural products by the selective activation of the alkyne in highly functionalized enynes or polyenynes.

Anatomy of gold catalysts: facts and myths

This review article covers the main types of gold(i) complexes used as precatalysts under homogeneous conditions in organic synthesis and discusses the different ways of catalyst activation as well as ligand, silver, and anion effects.

Ligand- and Brønsted acid/base-switchable reaction pathways in gold(i)-catalyzed cycloisomerizations of allenoic acids

Competing gold-catalyzed cycloisomerizations of γ-allenoic acids are optimized through ligand and Brønsted acid/base effects, affording three distinct classes of lactones.

Meeting the challenge of intermolecular gold(I)-catalyzed cycloadditions of alkynes and allenes

The development of gold(I)-catalyzed intermolecular carbo- and hetero-cycloadditions of alkynes and allenes has been more challenging than their intramolecular counterparts. Here we review, with a mechanistic perspective, the most fundamental intermolecular cycloadditions of alkynes and allenes with alkenes.

Gold-catalyzed cycloisomerization of 1,6-diyne esters to 1H-cyclopenta[b]naphthalenes, cis-cyclopenten-2-yl δ-diketones, and bicyclo[3.2.0]hepta-1,5-dienes

A synthetic method to chemoselectively prepare 1H-cyclopenta[b]naphthalenes, cis-cyclopenten-2-yl δ-diketones, and bicyclo[3.2.0]hepta-1,5-dienes efficiently by gold-catalyzed cycloisomerization of 1,6-diyne esters is described. These three product classes were accessed divergently by taking advantage of the electronic and steric differences between a phosphine and NHC (NHC = N-heterocyclic carbene) ligand in the respective gold(I) complexes and that of gold(III) complex combined with substrate substitution patterns and optimized reaction conditions. In the presence of [PhCNAuIPr](+)SbF6(-) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidine) as the catalyst, substrates with a pendant aryl group at the acetate alkynyl position were found to undergo preferential 1,3-acyloxy migration/5-exo-dig cyclization/Friedel-Crafts reaction to give 1H-cyclopenta[b]naphthalenes. In contrast, the analogous reactions with PicAuCl2 (Pic =2-picolinate) were found to proceed by selective 1,3-acyloxy migration/5-exo-dig cyclization/1,5-acyl migration to afford cis-cyclopenten-2-yl δ-diketones. Changing the catalyst to [MeCNAu(JohnPhos)](+)SbF6(-) (JohnPhos = (1,1'-biphenyl-2-yl)-di-tert-butylphosphine) and the acetate alkynyl position from an aryl to vinyl substituent in the starting ester led to 1,3-acyloxy migration/5-exo-dig cyclization/Prins-type [2 + 2]-cycloaddition to provide bicyclo[3.2.0]hepta-1,5-dienes.

Intriguing mechanistic labyrinths in gold(I) catalysis

Gold(i) controls complex transformations proceeding through carbocationic species by stabilising the key reactive intermediates.

Many mechanistically intriguing reactions have been developed in the last decade using gold(i) as catalyst. Here we review the main mechanistic proposals in gold-catalysed activation of alkynes and allenes, in which this metal plays a central role by stabilising a variety of complex cationic intermediates.

Gold-catalyzed rearrangements and beyond

Cycloisomerizations of enynes are probably the most representative carbon-carbon bond forming reactions catalyzed by electrophilic metal complexes. These transformations are synthetically useful because chemists can use them to build complex architectures under mild conditions from readily assembled starting materials. However, these transformations can have complex mechanisms. In general, gold(I) activates alkynes in the presence of any other unsaturated functional group by forming an (η(2)-alkyne)-gold complex. This species reacts readily with nucleophiles, including electron-rich alkenes. In this case, the reaction forms cyclopropyl gold(I) carbene-like intermediates. These can come from different pathways depending on the substitution pattern of the alkyne and the alkene. In the absence of external nucleophiles, 1,n-enynes can form products of skeletal rearrangement in fully intramolecular reactions, which are mechanistically very different from metathesis reactions initiated by the [2 + 2] cycloaddition of a Grubbs-type carbene or other related metal carbenes. In this Account, we discuss how cycloisomerization and addition reactions of substituted enynes, as well as intermolecular reactions between alkynes and alkenes, are best interpreted as proceeding through discrete cationic intermediates in which gold(I) plays a significant role in the stabilization of the positive charge. The most important intermediates are highly delocalized cationic species that some chemists describe as cyclopropyl gold(I) carbenes or gold(I)-stabilized cyclopropylmethyl/cyclobutyl/homoallyl carbocations. However, we prefer the cyclopropyl gold(I) carbene formulation for its simplicity and mnemonic value, highlighting the tendency of these intermediates to undergo cyclopropanation reactions with alkenes. We can add a variety of hetero- and carbonucleophiles to the enynes in the presence of gold(I) in intra- or intermolecular reactions, leading to the corresponding adducts with high stereoselectivity through stereospecific anti-additions. We have also developed stereospecific syn-additions, which probably occur through similar intermediates. The attack of carbonyl groups at the cyclopropyl carbons of the intermediate cyclopropyl gold(I) carbenes initiates a particularly interesting group of reactions. These trigger a cascade transformation that can lead to the formation of two C-C and one C-O bonds. In the fully intramolecular process, this stereospecific transformation has been applied for the synthesis of natural sesquiterpenoids such as (+)-orientalol F and (-)-englerin A. Intra- and intermolecular trapping of cyclopropyl gold(I) carbenes with alkenes leads to the formation of cyclopropanes with significant increase in the molecular complexity, particularly in cases in which this process combines with the migration of propargylic alkoxy and related OR groups. We have recently shown this in the stereoselective total synthesis of the antiviral sesquiterpene (+)-schisanwilsonene by a cyclization/1,5-acetoxy migration/intermolecular cyclopropanation. In this synthesis, the cyclization/1,5-acetoxy migration is faster than the alternative 1,2-acyloxy migration that would result in racemization.

Dissecting Anion Effects in Gold(I)-Catalyzed Intermolecular Cycloadditions

From a series of gold complexes of the type [t-BuXPhosAu(MeCN)]X (X=anion), the best results in intermolecular gold(I)-catalyzed reactions are obtained with the complex with the bulky and soft anion BAr4F- [BAr4F-=3,5-bis(trifluoromethyl)phenylborate] improving the original protocols by 10-30% yield. A kinetic study on the [2+2] cycloaddition reaction of alkynes with alkenes is consistent with an scenario in which the rate-determining step is the ligand exchange to generate the (η2-phenylacetylene)gold(I) complex. We have studied in detail the subtle differences that can be attributed to the anion in this formation, which result in a substantial decrease in the formation of unproductive σ,π-(alkyne)digold(I) complexes by destabilizing the conjugated acid formed.

Aerobic oxygenative cleavage of electron deficient C–C triple bonds in the gold-catalyzed cyclization of 1,6-enynes

Gold-catalyzed aerobic oxygenative cleavage of triple bonds that occurs under the ambient pressure of air and at room temperature is reported; radical inhibition tests suggest that oxygenation occurs via a gold-bound metalloradical intermediate.

Intermolecular reactions of gold(i)-carbenes with furans by related mechanisms

The intermolecular gold(i)-catalyzed reactions of propargyl carboxylates, 1,6-enynes, or 7-substituted 1,3,5-cycloheptatrienes with furans afford cyclopentenones, polyenes or polycyclic compounds by related mechanisms initiated by the electrophilic addition of gold(i) carbenes to furans followed by ring-opening.

Noninnocent counterion effect on the rearrangements of cationic intermediates in a gold(I)-catalyzed alkenylsilylation reaction

A mechanistic DFT study of the gold(I)-catalyzed alkenylsilylation reaction of a silyl-tethered 1,6-enyne system is reported. A novel pathway involving bistriflimide counterion-assisted rearrangements of carbocation and silyl cation intermediates corroborates the experimental observations. The results suggest the important role of the counterion in modulating the reactivity of cationic intermediates in gold catalysis.