Ligand-Accelerated Gold-Catalyzed Addition of in Situ Generated Hydrazoic Acid to Alkynes under Neat Conditions

Bifunctional Ligand Enables Gold-Catalyzed Propargyl C-H Functionalization via Reactive Gold Allenylidene Intermediate

Gold allenylidene species have been seldom exploited as reactive intermediates in synthetically versatile catalytic reactions. By employing alkynylbenziodoxoles as the substrates and bifunctional WangPhos as the metal ligand, this work demonstrated ready catalytic access to these intermediates of general substitution patterns and their electrophilic reactivities at the γ-carbon center with a diverse range of nucleophiles. The reaction is driven by the reductive decomposition of the benziodoxole moiety and achieves the replacement of a propargylic proton with an N/O/C-based nucleophile, hence realizing reactivity umpolung. Corroborated by Density Functional Theory (DFT) calculations, the reaction mechanism involves a mild propargylic deprotonation. In contrast to prior works employing a tertiary amine functionality, a weakly BrØnsted-basic amide group in WangPhos is surprisingly effective in deprotonation at the propargylic position under a gold-ligand cooperation regime.

Gold-Catalyzed Homo/Heterodimerization of Terminal Alkynes Facilitated by Metal-Ligand Cooperation

Gold-catalyzed dimerization of terminal alkynes is achieved under mild reaction conditions and in excellent yields. In addition to homodimerizations, heterodimerizations between TBS acetylene and a range of terminal alkynes were realized using the syringe pump technique. The reaction tolerates various functional groups. The rate acceleration experienced in the reactions is enabled by metal-ligand cooperation. A binaphthyl-2-ylphosphine ligand featuring a 3'-diisopropylphosphoryl group plays a pivotal role in facilitating alkyne attack by accommodating and/or deprotonating its terminal proton.

Copper-Catalyzed Azide-Alkyne Cycloaddition of Hydrazoic Acid Formed In Situ from Sodium Azide Affords 4-Monosubstituted-1,2,3-Triazoles

We report a copper-catalyzed cycloaddition of hydrogen azide (hydrazoic acid, HN3) with terminal alkynes to form 4-substituted-1H-1,2,3-triazoles in a sustainable manner. Hydrazoic acid was formed in situ from sodium azide under acidic conditions to react with terminal alkynes in a copper-catalyzed reaction. Using polydentate N-donor chelating ligands and mild organic acids, the reactions were realized to proceed at room temperature under aerobic conditions in a methanol-water mixture and with 5 mol % catalyst loadings to afford 4-substituted-1,2,3-triazoles in high yields. This method is amenable on a wide range of alkyne substrates, including unprotected peptides, showing diverse functional group tolerance. It is applicable for late-stage functionalization synthetic strategies, as demonstrated in the synthesis of the triazole analogue of losartan. The preparation of orthogonally protected azahistidine from Fmoc-l-propargylglycine was realized on a gram scale. The hazardous nature of hydrazoic acid has been diminished as it forms in situ in <6% concentrations at which it is safe to handle. Reactions of distilled solutions of hydrazoic acid indicated its role as a reactive species in the copper-catalyzed reaction.

Three-component addition of terminal alkynes, carboxylic acids, and tert-butyl hypochlorite

This paper described the unprecedented three-component addition of terminal alkynes, carboxylic acids, and tert-butyl hypochlorite. This new type of addition proceeds smoothly to produce gem-dichloroalkane derivatives in satisfactory to excellent yields via successive two-time additions under mild conditions. The synthetically useful functional groups, such as Cl, Br, CN, and NO₂, remained intact in the final products.

Main Avenues in Gold Coordination Chemistry

In this contribution, we provide an overview of the main avenues that have emerged in gold coordination chemistry during the last years. The unique properties of gold have motivated research in gold chemistry, and especially regarding the properties and applications of gold compounds in catalysis, medicine, and materials chemistry. The advances in the synthesis and knowledge of gold coordination compounds have been possible with the design of novel ligands becoming relevant motifs that have allowed the preparation of elusive complexes in this area of research. Strong donor ligands with easily modulable electronic and steric properties, such as stable singlet carbenes or cyclometalated ligands, have been decisive in the stabilization of gold(0) species, gold fluoride complexes, gold hydrides, unprecedented π complexes, or cluster derivatives. These new ligands have been important not only from the fundamental structure and bonding studies but also for the synthesis of sophisticated catalysts to improve activity and selectivity of organic transformations. Moreover, they have enabled the facile oxidative addition from gold(I) to gold(III) and the design of a plethora of complexes with specific properties.

Optimizing Catalyst and Reaction Conditions in Gold(I) Catalysis-Ligand Development

This review considers phosphine and N-heterocyclic carbene complexes of gold(I) that are used as (pre)catalysts for a range of reactions in organic synthesis. These are divided according to the structure of the ligand, with the narrative focusing on studies that offer a quantitative comparison between the ligands and readily available or widely used existing systems.

Chiral Bifunctional Phosphine Ligand-Enabled Cooperative Cu Catalysis: Formation of Chiral α,β-Butenolides via Highly Enantioselective γ-Protonation

α,β-Butenolides with ≥96% enantiomeric excess are synthesized from β,γ-butenolides via a novel Cu(I)-ligand cooperative catalysis. The reaction is enabled by a chiral biphenyl-2-ylphosphine ligand featuring a remote tertiary amino group. Density functional theory studies support the cooperation between the metal center and the ligand basic amino group during the initial soft deprotonation and the key asymmetric γ-protonation. Remarkably, other coinage metals, that is, Ag and Au, can readily assume the same role as Cu in this asymmetric isomerization chemistry.

Recent Advances in the Development of Chiral Gold Complexes for Catalytic Asymmetric Catalysis

Asymmetric gold catalysis has been rapidly developed in the past ten years. Breakthroughs have been made by rational design and meticulous selection of chiral ligands. This review summarizes newly developed gold-catalyzed enantioselective organic transformations and recent progress in ligand design (since 2016), organized according to different types of chiral ligands, including bisphosphine ligands, monophosphine ligands, phosphite-derived ligands, and N-heterocyclic carbene ligands for asymmetric gold(I) catalysis as well as heterocyclic carbene ligands and oxazoline ligands for asymmetric gold(III) catalysis.

Towards the rational design of ylide-substituted phosphines for gold(i)-catalysis: from inactive to ppm-level catalysis

The implementation of gold catalysis into large-scale processes suffers from the fact that most reactions still require high catalyst loadings to achieve efficient catalysis thus making upscaling impractical. Here, we report systematic studies on the impact of the substituent in the backbone of ylide-substituted phosphines (YPhos) on the catalytic activity in the hydroamination of alkynes, which allowed us to increase the catalyst performance by orders of magnitude. While electronic changes of the ligand properties by introduction of aryl groups with electron-withdrawing or electron-donating groups had surprisingly little impact on the activity of the gold complexes, the use of bulky aryl groups with ortho-substituents led to a remarkable boost in the catalyst activity. However, this catalyst improvement is not a result of an increased steric demand of the ligand towards the metal center, but due to steric protection of the reactive ylidic carbon centre in the ligand backbone. The gold complex of the thus designed mesityl-substituted YPhos ligand Y_MesPCy₂, which is readily accessible in one step from a simple phosphonium salt, exhibited a high catalyst stability and allowed for turnover numbers up to 20 000 in the hydroamination of a series of different alkynes and amines. Furthermore, the catalyst was also active in more challenging reactions including enyne cyclisation and the formation of 1,2-dihydroquinolines.

Access to Cycloalkeno[c]-Fused Pyridines via Pd-Catalyzed C(sp2)-H Activation and Cyclization of N-Acetyl Hydrazones of Acylcycloalkenes with Vinyl Azides

A novel Pd(II)-catalyzed vinylic C-H activation and cyclization has been developed, reacting a series of small, medium, and large N-acetyl hydrazones of acylcycloalkenes with vinyl azides to access diverse cycloalkeno[c]-fused pyridine scaffolds. This protocol provides progress in C(sp²)-H bond activation of medium to large cycloalkenes, and the target products can be obtained in a specific regioselectivity with good functional group tolerance and a broad substrate scope.

Bifunctional phosphine ligand-enabled gold-catalyzed direct cycloisomerization of alkynyl ketones to 2,5-disubstituted furans

An efficient synthesis of 2,5-disubstituted furans directly from alkynyl ketones has been developed via tandem gold(i)-catalyzed isomerization of alkynyl ketones to allenyl ketones and cycloisomerization. The key to the success of this chemistry is the use of a biphenyl-2-ylphosphine ligand featuring a critical remote tertiary amino group.

Bifunctional Phosphine Ligand-Enabled Gold(I)-Catalyzed O-Nucleophilic Addition of N-Hydroxybenzo[1,2,3]-triazin-4(3H)-ones to Alkynes Followed by [3,3]-Rearrangement: Simultaneous Formation of C-O and C-N Bonds

We describe a gold (I)-catalyzed tandem O-nucleophilic addition/[3,3]-rearrangement reaction of N-hydroxybenzo[1,2,3]-triazin-4(3H)-ones with alkynes enabled by a biphenyl-2-yl phosphine ligand featuring a pendant amide moiety (L1). A variety of 1-(2-oxo-2-arylethyl)benzo [d][1,2,3]triazin-4(1H)-one derivatives were synthesized in good to excellent yields. The present protocol gives a rare example of simultaneous formation of C-O and C-N bonds in the gold(I)-catalyzed [3,3]-sigmatropic rearrangements.

Silver-Catalyzed Activation of Terminal Alkynes for Synthesizing Nitrogen-Containing Molecules

Alkynes are one of the most abundant chemicals in organic chemistry, and therefore the development of catalytic reactions to transform alkynes into other useful functionalities is of great value. In recent decades, extraordinary advances have been made in this area with transition-metal catalysis, and silver-based reagents are ideal for the activation of alkynes. This high reactivity is probably due to the superior π-Lewis acidic, carbophilic behavior of silver(I), allowing it to selectively activate carbon-carbon triple bonds (C≡C) through the formation of a silver-π complex. Within this field, we have been interested in the activation and subsequent reactions of readily accessible terminal alkynes for the synthesis of nitrogen-containing compounds, which has generally received less attention than methods involving internal alkynes. This is possibly due to the lack of suitable reactive reaction partners that are compatible under transition metals. Therefore, a thorough understanding of the factors that influence homogeneous silver catalysis and the identification of the appropriate reaction partners can provide a powerful platform for designing more efficient silver-catalyzed reactions of terminal alkynes. In this context, we envisioned that using readily available, environmentally benign, and inexpensive trimethylsilyl azide (TMSN₃) or an isocyanide as the nitrogen-donor would be the key to develop novel reactions of terminal alkynes.This Account describes our efforts since 2013 toward the development of novel silver-catalyzed tandem reactions of terminal alkynes with either TMSN₃ or isocyanides for the assembly of various nitrogen-containing compounds. The first section of this Account discusses the initial developments in the silver-catalyzed hydroazidation of terminal alkynes with TMSN₃ and the subsequent advances made in our laboratory. We first describe the discovery and experimental and computational mechanistic investigations of silver-catalyzed hydroazidation reactions, which is the most efficient strategy reported to date for accessing vinyl azides. Mechanistic study of this hydroazidation reaction provides an alternative activation mode for terminal alkyne conversion in transition metal catalysis. We then present the chemistry of in situ generated vinyl azides, including one-pot tandem radical addition/cyclization or migration reactions of terminal alkynes to access a variety of nitrogen-containing molecules. Finally, we discuss the one-pot, multistep tandem hydroazidation and 1,2-azide migratory gem-difluorination of terminal alkynes for the synthesis of β-difluorinated alkyl azides. The second section describes the silver-catalyzed coupling reactions between terminal alkynes and isocyanides, which offer a straightforward method for accessing synthetically useful building blocks, such as pyrroles, allenamides, benzofuran, vinyl sulfones, indazolines, propiolonitriles, and pyrazoles. The high efficiency, mild conditions, low cost, broad substrate scope, high chemo- and regioselectivity, step economy, and ecofriendliness of the developed approaches make them attractive and practical. The progress in this area provides guiding principles for designing new reactions of terminal alkynes that can be extended to various nitrogen-containing molecules of interest to medicinal and materials chemists.

Synthesis of Chiral Bifunctional NHC Ligands and Survey of Their Utilities in Asymmetric Gold Catalysis

The synthesis and characterization of the chiral bifunctional NHC ligands based on the imidazo[1,5-a]pyridine (ImPy) scaffold are described. These ligands possess a fluxional biaryl axis and a chiral center. The configurational stability of the biaryl axis in their gold(I) complexes is investigated. The application of these axially chiral ImPy-based AuCl complexes in a series of gold catalysis is explored, and varying degrees of asymmetric induction are observed. In most cases, the ligand (aS,R)-L8-H with its cyclohexyl group pointing to the reaction site and hence exerting asymmetric steric influence is more effective in asymmetric induction.

Switchable Reactivity between Vinyl Azides and Terminal Alkyne by Nano Copper Catalysis

A novel nano copper-catalyzed substrate-dependent chemodivergent transformation of vinyl azides with a terminal alkyne is disclosed. 2,5-Disubstituted pyrroles were selectively obtained in high yield with aryl alkynes and aliphatic alkynes, whereas 2,3,4-trisubstituted pyrroles were formed with silylated alkynes. This switchable method provides a controllable and facile access to both multisubstituted pyrrole scaffolds with high efficiency, excellent regioselectivity, and good functional group compatibility.

Iron-catalyzed carboazidation of alkenes and alkynes

Carboazidation of alkenes and alkynes holds the promise to construct valuable molecules directly from chemical feedstock therefore is significantly important. Although a few examples have been developed, there are still some unsolved problems and lack of universal methods for carboazidation of both alkenes and alkynes. Here we describe an iron-catalyzed rapid carboazidation of alkenes and alkynes, enabled by the oxidative radical relay precursor t-butyl perbenzoate. This strategy enjoys success with a broad scope of alkenes under mild conditions, and it can also work with aryl alkynes which are challenging substrates for carboazidation. A large number of diverse structures, including many kinds of amino acid precursors, fluoroalkylated vinyl azides, other specific organoazides, and 2H-azirines can be easily produced.

Carboazidation of alkenes and alkynes holds the promise to construct valuable, functionalized molecules. Here, the authors report a general and efficient iron-catalyzed carboazidation of both alkenes and alkynes enabled by t-butyl perbenzoate.

Copper-catalyzed regioselective cyclization of vinyl azides by gem-difluoromethylene for trisubstituted pyridines

A novel copper-catalyzed cyclization of readily available vinyl azides with gem-difluoromethylene is described.

Bifunctional Biphenyl-2-ylphosphine Ligand Enables Tandem Gold-Catalyzed Propargylation of Aldehyde and Unexpected Cycloisomerization

Despite extensive studies in gold catalysis, σ-allenylgold species have not been invoked as catalytic intermediates and their reactivities not studied. This work reports for the first time they are generated in situ and undergo nucleophilic addition to activated aldehydes in a bifunctional phosphine ligand-enabled gold catalysis. This development reveals a broad range of opportunities to achieve propargylic C-H functionalization for the first time under catalytic and mild conditions. The homopropargylic alcohols generated undergo ligand-enabled cycloisomerizations involving an unexpected silyl migration.

Designed Bifunctional Phosphine Ligand-Enabled Gold-Catalyzed Isomerizations of Ynamides and Allenamides: Stereoselective and Regioselective Formation of 1-Amido-1,3-dienes

By using designed biphenyl-2-ylphosphines functionalized with a remote basic groups as ligands, N-alkynyl-o-nosylamides are directly converted to (1E,3E)-1-amido-1,3-dienes with excellent diastereoselectivities under gold catalysis. With allenamides as substrates, the gold-catalyzed isomerizations are high yielding and applicable to a broad substrate scope including various nitrogen protecting groups and exhibit unprecedented (3E)-selectivities for the distal C-C double bond and good regioselectivities. Combining this gold catalysis with one-pot Diels-Alder reactions leads to rapid assembly of valuable bicyclic compounds.

Computational Exploration of Counterion Effects in Gold(I)-Catalyzed Cycloisomerization of ortho-(Alkynyl)styrenes

A detailed theoretical analysis of the mechanism and chemoselectivity for gold(I)-catalyzed reaction of o-(alkynyl)styrene containing an isopropyl and a methyl at the terminal position of the alkene has been reported in this work. Two different counterions (SbF₆ ^- and OTs^-) were studied as model catalysts. According to our calculation, for SbF₆ ^-, the reaction pathway is more prone to direct 1,2-H shifts (isopropyl H) than the elimination and ring expansion pathway. However, an elimination pathway affords the indenyl derivative by forming p-toluenesulfonic acid (HOTs), which may be the main pathway in the presence of OTs^-. The chemoselectivity for the title reaction is mainly determined by the electronic effect of the counterion and the substituent rather than the steric effect. In other words, less basic SbF₆ ^- mainly provides the charge separation effect rather than assisted proton elimination. However, the more basic OTs^- mainly assist proton elimination through the formation of HOTs. In addition to the good agreement with the experimental data, the density functional theory results also provide a significant contribution to the understanding of the reaction mechanism.

Bifunctional Ligand Enables Efficient Gold-Catalyzed Hydroalkenylation of Propargylic Alcohol

Using the previously designed biphenyl-2-ylphosphine ligand, featuring a remote tertiary amino group, the first gold-catalyzed intermolecular hydroalkenylation of alkynes has been developed. Synthetically valuable conjugated dienyl alcohols are formed in moderate to good yields. A range of alkenyltrifluoroborates are allowed as the alkenyl donor, and no erosion of alkene geometry and/or the propargylic configuration are detected. DFT calculations confirm the critical role of the remote basic group in the ligand as a general-base catalyst for promoting this novel gold catalysis with good efficiency.

Divergent access to N-hydroxypyrroles and isoxazoles via the gold(i)- or Brønsted acid-catalysed regioselective cyclization of N-(2-trifluoromethyl-3-alkynyl) oximes

The divergent regioselective cyclization of N-(2-trifluoromethyl-3-alkynyl) oximes by a suitable choice of a gold(i) or a Brønsted acid catalyst, leading to 4-trifluoromethyl-N-hydroxypyrroles or 4-trifluoromethyl-5-alkylisoxazoles was developed. In order to avoid the tedious separation of unstable N-(2-trifluoromethyl-3-alkynyl) oximes, an easy two-step, one-pot synthesis of 4-trifluoromethyl-5-alkylisoxazoles was realized via the sequential oxidation of the corresponding hydroxylamines and subsequent treatment with the reductant, sodium thiosulfate (Na₂S₂O₃)_. This two-step, one-pot procedure is a complementary method for the synthesis of 4-trifluoromethyl-5-alkylisoxazole from those unstable N-(2-trifluoromethyl-3-alkynyl) oximes.

α-Substituted vinyl azides: an emerging functionalized alkene

Vinyl azides are highly versatile synthons that provide access to numerous N-heterocycles and other functional groups.