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

Cationic Bis(Gold) Indenyl Complexes

Reaction of (P)AuOTf [P=P(t-Bu)2o-biphenyl] with indenyl- or 3-methylindenyl lithium led to isolation of gold η1-indenyl complexes (P)Au(η1-inden-1-yl) (1 a) and (P)Au(η1-3-methylinden-1-yl) (1 b), respectively, in >65 % yield. Whereas complex 1 b is static, complex 1 a undergoes facile, degenerate 1,3-migration of gold about the indenyl ligand (ΔG≠ 153K=9.1±1.1 kcal/mol). Treatment of complexes 1 a and 1 b with (P)AuNTf2 led to formation of the corresponding cationic bis(gold) indenyl complexes trans-[(P)Au]2(η1,η1-inden-1,3-yl) (2 a) and trans-[(P)Au]2(η1,η2-3-methylinden-1-yl) (2 b), respectively, which were characterized spectroscopically and modeled computationally. Despite the absence of aurophilic stabilization in complexes 2 a and 2 b, the binding affinity of mono(gold) complex 1 a toward exogenous (P)Au+ exceed that of free indene by ~350-fold and similarly the binding affinity of 1 b toward exogenous (P)Au+ exceed that of 3-methylindene by ~50-fold. The energy barrier for protodeauration of bis(gold) indenyl complex 2 a with HOAc was ≥8 kcal/mol higher than for protodeauration of mono(gold) complex 1 a.

From Stable PH-Ylides to α-Carbanionic Phosphines as Ligands for Zwitterionic Catalysts

Although ylides are commonly used reagents in organic synthesis, the parent methylphosphine MePH2 only exists in its phosphine form in the condensed phase. Its ylide tautomer H3 P+ -CH2 - is considerably higher in energy. Here, we report on the formation of bis(sulfonyl)methyl-substituted phosphines of the type (RO2 S)2 C(H)-PR2, which form stable PH ylides under ambient conditions, amongst the first examples of an acyclic phosphine which only exists in its PH ylide form. Depending on the exact substitution pattern the phosphines form an equilibrium between the PH ylide and the phosphine form or exist as one of both extremes. These phosphines were found to be ideal starting systems for the facile formation of α-carbanionic phosphines. The carbanion-functionalization leads to a switch from electron-poor to highly electron-rich phosphines with strong donor abilities and high basicities. Thus, the phosphines readily react with different electrophiles exclusively at the phosphorus atom and not at the carbanionic center. Furthermore, the anionic nature of the phosphines allows the formation of zwitterionic complexes as demonstrated by the isolation of a gold(I) complex with a cationic metal center. The cationic gold center allows for catalytic activity in the hydroamination of alkyne without requiring a further activation step.

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(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.

Small rings in the bigger picture: ring expansion of three- and four-membered rings to access larger all-carbon cyclic systems

The release of the inherent ring strain of cyclobutane and cyclopropane derivatives allows a rapid build-up of molecular complexity. This review highlights the state-of-the-art of the ring expansions of three- and four-membered cycles and is organised by types of reactions with emphasis on the reaction mechanisms. Selected examples are discussed to illustrate the synthetic potential of this elegant synthetic tool.

Ligand-enforced intimacy between a gold cation and a carbenium ion: impact on stability and reactivity

Controlling the reactivity of transition metal complexes by positioning non-innocent functionalities around the catalytic pocket is a concept that has led to significant advances in catalysis. Here we describe our efforts toward the synthesis of dicationic phosphine gold complexes of general formula [(o-Ph₂P(C₆H₄)Carb)Au(tht)]²⁺ decorated by a carbenium moiety (Carb) positioned in the immediate vicinity of the gold center. While the most acidic examples of such compounds have limited stability, the dicationic complexes with Carb⁺ = 9-N-methylacridinium and Carb⁺ = [C(Ar^N)₂]⁺ (Ar^N = p-(C₆H₄)NMe₂) are active as catalysts for the cycloisomerization of N-propargyl-4-fluorobenzamide, a substrate chosen to benchmark reactivity. The dicationic complex [(o-Ph₂P(C₆H₄)C(Ar^N)₂)Au(tht)]²⁺, which also promotes hydroarylation and enyne cyclization reactions, displays a higher catalytic activity than its acridinium analog, indicating that the electrophilic reactivity of these complexes scales with the Lewis acidity of the carbenium moiety. These results support the role of the carbenium unit as a non-innocent functionality which can readily enhance the activity of the adjacent metal center. Finally, we also describe our efforts toward the generation and isolation of free γ-cationic phosphines of general formula [(o-Ph₂P(C₆H₄)Carb)]⁺. While cyclization into phosphonium species is observed for Carb⁺ = [C(Ar^N)₂]⁺, [C(Ph)(Ar^N)]⁺, and 9-xanthylium, [(o-Ph₂P(C₆H₄)-9-N-methylacridinium)]⁺ can be isolated as an air stable, biphilic derivative with uncompromised Lewis acidic and basic properties.

This work describes the synthesis of carbenium-based, γ-cationic phosphines and their coordination to Au(i) cations , leading to carbophilic catalysts whose activity is enhanced by the ligand-enforced convergence of the positively charged moieties.

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.

Annulative π-extension of BODIPYs made easy via gold(i)-catalyzed cycloisomerization

Here we report gold(i)-catalyzed cycloisomerization as a new powerful synthetic tool for the preparation of π-extended BODIPY derivatives. The catalytic system PPhF 3AuCl/AgSbF6 enables the synthesis of [b]-[2,1]naphtho-fused-BODIPYs (2a-2c) under mild conditions, in excellent yields and short reaction times. The reaction is totally regioselective to the 6-endo-dig product and for the α-position of the BODIPY, which is both the kinetically and thermodynamically favored pathway, as supported by the free energy profile calculated by means of Density Functional Theory (DFT). Moreover, this methodology also allows the synthesis of two new families of [b]-aryl-fused-BODIPYs, namely, [3,4]phenanthro- (2e and 2f) and [1,2]naphtho-fused (2g) BODIPYs. Their molecular and electronic structures were established by NMR and UV-vis spectroscopies as well as single-crystal X-ray diffraction analysis. As can be noted from the X-ray structures, 2a, 2e and 2g present interesting structural differences at both the molecular and packing level. Interestingly, despite being isomers, the UV/vis spectra of 2a and 2g revealed significant differences in their electronic structures. The origin of this finding was studied by Time-Dependent DFT calculations. Calculated DFT Nuclear Independent Chemical Shift (NICS(0)) values also supported the different electronic structures of 2a and 2g.

Acetylene as a Dicarbene Equivalent for Gold(I) Catalysis: Total Synthesis of Waitziacuminone in One Step

The gold(I)-catalyzed reaction of acetylene gas with alkenes leads to (Z,Z)-1,4-disubstituted 1,3-butadienes and biscyclopropanes depending on the donor ligand on gold(I). Acetylene was generated in situ from calcium carbide and water in a user-friendly procedure. Reaction of acetylene with 1,5-dienes gives rise stereoselectively to tricyclo[5.1.0.02,4 ]octanes. This novel double cyclopropanation has been applied to the one step total synthesis of the natural product waitziacuminone from acetylene and geranyl acetone.

Synthesis of tricyclo[7.2.1.09,10]dodecan-11-one core ring systems of norditerpenoid alkaloids and racemulosine

A synthetic strategy involving [2 + 2] enyne cycloaddition and pinacol rearrangement to construct common BCD ring systems of norditerpenoid alkaloids and racemulosine was developed.

Dinuclear gold(i) complexes: from bonding to applications

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

Improving Homogeneous Cationic Gold Catalysis through a Mechanism-Based Approach

Homogeneous gold catalysis is regarded as a landmark addition to the field of organic synthesis. It is the most effective way to activate alkynes for the addition of a diverse host of nucleophiles. However, the literature reveals that a relatively high catalyst loading is needed in many gold-catalyzed applications (1-10 mol %), which is impractical in large-scale synthesis or multistep synthesis because of the high price and recyclization difficulty of the gold. A more thorough understanding of the factors that operate on homogeneous gold catalysis can provide better guidelines for the future design of more efficient gold-catalyzed reactions. In this Account, we will summarize our group's extensive investigation of factors impacting cationic gold catalysis, namely, the effects of ligands, counterions, additives, and catalyst decay and deactivation, using a mechanism-based approach with the aim of improving the efficiency of homogeneous gold catalysis. Through NMR-assisted kinetic studies, we investigated the above factors. Our systematic ligand effect investigation provided a clearer understanding of how ligands influence each of the three stages in the gold catalytic cycle. On the basis of this study, we synthesized a novel phosphine ligand and achieved parts per million-level gold catalysis by manipulating the electron density of the substituents and the steric strain around phosphorus. Our investigation of counterion effects led to the design of a gold affinity index and hydrogen-bonding basicity index for counterions, which can forecast the reactivity of counterions in cationic gold catalysis. We studied the adverse silver effects in cationic gold catalyst activation and proposed a more efficient practical guide. Our additive effect investigation revealed that additives that are good hydrogen-bond acceptors increase the efficiency of gold-catalyzed reactions in those occurrences where protodeauration is the rate-determining step. The first detailed experimental analysis of gold catalyst decay and the influence of each component in the reaction system (substrate, counterion, solvent) on the decay process was also conducted. We found that high-gold-affinity impurities (halides, bases) in solvents, starting materials, filtration, or drying agents decrease the reactivity of a gold catalyst but that a suitable acid activator can reactivate the gold catalyst and enable the reaction to proceed smoothly at competitively low gold catalyst loadings. The effects of acid additives were also systematically investigated using typical reactions. We are convinced that better mechanistic understandings will offer clearer guidelines for the search for more efficient gold-catalyzed reactions.

Gold-Catalyzed Homogeneous (Cyclo)Isomerization Reactions

Gold is currently one of the most used metals in organometallic catalysis. The ability of gold to activate unsaturated groups in different modes, together with its tolerance to a wide range of functional groups and reaction conditions, turns gold-based complexes into efficient and highly sought after catalysts. Natural products and relevant compounds with biological and pharmaceutical activity are often characterized by complex molecular structures. (Cyclo)isomerization reactions are often a useful strategy for the generation of this molecular complexity from synthetically accessible reactants. In this review, we collect the most recent contributions in which gold(I)- and/or gold(III)-catalysts mediate intramolecular (cyclo)isomerization transformations of unsaturated species, which commonly feature allene or alkyne motifs, and organize them depending on the substrate and the reaction type.

Imidazolyl-phenyl (IMP) anions: a modular structure for tuning solubility and coordinating ability

The effect of counteranion upon a cation's solution-phase reactivity depends on a subtle interplay of weak interactions.

Direct Evidence for the Origin of Bis-Gold Intermediates: Probing Gold Catalysis with Mass Spectrometry

Gold-catalyzed alkyne hydration was studied by using in situ reacting mass spectrometry (MS) technology. By monitoring the reaction process in solution under different conditions (regular and very diluted catalyst concentrations, different pH values) and examining the reaction occurrence in the early reaction stage (1-2 ms after mixing) with MS, we collected a series of experimental evidence to support that the bis-gold complex is a potential key reaction intermediate. Furthermore, both experimental and computational studies confirmed that the σ,π-bis-gold complexes are not active intermediates toward nucleophilic addition. Instead, formation of geminally diaurated complex C is crucial for this catalytic process.

Gold-alkynyls in catalysis: alkyne activation, gold cumulenes and nuclearity

The use of cationic gold(i) species in the activation of substrates containing C[triple bond, length as m-dash]C bonds has become a valuable tool for synthetic chemists. Despite the seemingly simple label of 'alkyne activation', numerous patterns of reactivity and product structure are observed in systems employing related substrates and catalysts. The complications of mechanistic determination are compounded as the number of implicated gold(i) centres involved in catalysis increases and debate about the bonding in proposed intermediates clouds the number and importance of potential reaction pathways. This perspective aims to illustrate some of the principles underpinning gold-alkynyl interactions whilst highlighting some of the contentious areas in the field and offering some insight into other, often ignored, mechanistic possibilities based on recent findings.

Gold(i)-catalyzed cycloisomerization of ortho-(alkynyl) styrenes: DFT analysis of the crucial role of SbF6− in the elimination of protons

The mechanism, regioselectivity and role of SbF₆⁻ in the gold(i)-catalyzed reaction of o-(alkynyl) styrene are clarified through our DFT calculations.

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.

Enantioselective Synthesis of Cyclobutenes by Intermolecular [2+2] Cycloaddition with Non-C2 Symmetric Digold Catalysts

The enantioselective intermolecular gold(I)-catalyzed [2+2] cycloaddition of terminal alkynes and alkenes has been achieved using non-C2-chiral Josiphos digold(I) complexes as catalysts, by the formation of the monocationic complex. This new approach has been applied to the enantioselective total synthesis of rumphellaone A.

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.

O, Bao X. Computational insights into the mechanisms of Au(i)-catalysed intramolecular addition of the hydroxylamine group onto alkynes

Computational studies were carried out to understand the reaction mechanisms and the origin of the substrate-dependent chemo- and regio-selectivities of the Au(i)-catalysed intramolecular addition of the hydroxylamine group onto alkynes.

Gold-Catalyzed Vinyl Ether Hydroalkynylation: An Alternative Pathway for the Gold-Catalyzed Intermolecular Reaction of Alkenes and Alkynes

In this report, the gold-catalyzed intermolecular reaction of vinyl ethers and terminal alkynes is investigated. Utilizing a triazole gold catalyst lessens gold decomposition in the presence of the vinyl ether and affords an alkynylation product instead of the [2 + 2] product. This protocol has been expanded to include glycal substrates, which undergo a one-pot alkynylation-Ferrier reaction to produce functionalized sugars in moderate to excellent yields with high diastereoselectivity.

Ready Access to the Echinopines Skeleton via Gold(I)-Catalyzed Alkoxycyclizations of Enynes

The [3,5,5,7] tetracyclic skeleton of echinopines has been stereoselectively accessed through a gold(I)-catalyzed alkoxycyclization of cyclopropyl-tethered 1,6-enynes. The key bicyclo[4.2.1]nonane core of the enyne precursors was readily assembled by means of a Co-catalyzed [6 + 2] cycloaddition. Furthermore, the attempted alkoxycyclization of 1,5-enyne substrates revealed an uncovered cyclopropyl rearrangement that gives rise to [3,6,5,7] tetracyclic structures.

Synthesis and catalytic applications of 1,2,3-triazolylidene gold(i) complexes in silver-free oxazoline syntheses and C-H bond activation

A series of novel 1,2,3-triazolylidene gold(i) chloride complexes have been synthesised and fully characterised. Silver-free methodologies for chloride ion abstraction of these complexes were evaluated for their potential as Au-based catalyst precursors. Using simple potassium salts or MeOTf as chloride scavengers produced metal complexes that catalyse both the regioselective synthesis of oxazolines and the C-H activation of benzene or styrene for carbene transfer from ethyl diazoacetate. These results indicate that Ag-free activation of 1,2,3-triazolylidene gold(i) chloride complexes is feasible for the generation of catalytically active Au triazolylidene species. However, silver-mediated activation imparts substantially higher catalytic activity in oxazoline synthesis.