Ag(I)-catalyzed diastereoselective oxidative cyclopropanation of prochiral alkyne-tethered 1,3-dicarbonitriles

Herein, we developed a highly diastereoselective silver-catalyzed intramolecular cyclopropanation of prochiral alkyne-tethered 1,3-dicarbonitriles using perchloric acid as an effective oxidizing agent. This method facilitates the construction of densely functionalized complex [6.6.5.3] frameworks having three all-carbon quaternary stereocenters in high yields. The significance of the reaction was demonstrated by a gram-scale reaction and post-synthetic modifications of the product.

Pd-Catalyzed Enantioselective Desymmetrizing 1,7-Enyne Cycloisomerization of Alkyne-Tethered Cyclopentenes

An enantioselective Pd-catalyzed intramolecular desymmetrizing cycloisomerization of N-(cyclopent-3-en-1-yl)propiolamides has been developed by employing a new chiral phosphoramidite ligand. A series of structurally unique bridged azabicycles are achieved in moderate to excellent yields with good E/Z selectivity and high enantioselectivity. Synthetic transformations are conducted to demonstrate the practical utility of this reaction.

Revised Mechanism of Gold-Catalyzed Thioallylation of Propiolates

Gold-catalyzed enantioselective thioallylation of propiolates proved effective in delivering highly enantio-enriched α-allyl-β-thioacrylates. In this work, we report a revised mechanism for this process based on the new mechanistic experiments and kinetic data in the presence of a competitive inhibitor. The employment of thioethers as nucleophiles inevitably involves their competitive binding to the only catalytic site of the Au(I) catalyst, which may inhibit the activity. We developed a modified Hammett plot in the presence of a dummy thioether inhibitor, which revealed a true kinetic profile, excluding the effect of inhibition. A revised mechanism suggested that the conjugate addition of thioethers to the Au(I)-activated alkynes is the turnover-limiting step, and the subsequent [3,3]-rearrangement occurs quickly, suggesting the efficacy of the sulfonium-based approach in accelerating Claisen rearrangement. In addition, the enantioselectivity was suggested to be determined during the sigmatropic rearrangement by discriminating the prochiral olefin faces of the allyl group in the σ-bound Au(I) complex.

The Analysis of Two Distinct Strategies toward the Enantioselective Formal Total Synthesis of (+)-Gelsenicine

A full account of a formal enantioselective total synthesis of (+)-gelsenicine is described. Separate strategies based on catalytic cycloisomerization as the central step are considered. One plan involves chirality transfer from enantioenriched substrates, while the other employs asymmetric catalysis. The chirality transfer strategy is less effective, while in the latter, phosphoramidite- and bisphosphine-gold complexes are tested and ultimately provide a key intermediate in high enantiopurity in our Gelsemium alkaloid syntheses.

Silver(I)-Catalyzed Oxidative Cyclopropanation of 1,6-Enynes: Synthesis of 3-Aza-bicyclo[3.1.0]hexane Derivatives

A simple Ag(I)-catalyzed oxidative cyclopropanation of heteroatom-tethered 1,6-enynes for the establishment of valuable functionalized 3-aza-bicyclo[3.1.0]hexane is presented, which allows the formation of multiple chemical bonds in one step under 20 mol % silver(I) catalysts and air conditions. This approach is highly atom economical, easy to perform, and free of external oxidants and features good to excellent yields and gram-scale synthesis. The preliminary study showed that an uncommon silver carbenoid intermediate might be involved in this process.

H-Bonded Counterion-Directed Catalysis: Enantioselective Gold(I)-Catalyzed Addition to 2-Alkynyl Enones as a Case Study

H-bonded counterion-directed catalysis (HCDC) is a strategy wherein a chiral anion that is hydrogen-bonded to the achiral ligand of a metal complex is responsible for enantioinduction. In this article we present the application of H-bonded counterion-directed catalysis to the Au(I)-catalyzed enantioselective tandem cycloisomerization-addition reaction of 2-alkynyl enones. Following the addition of C-, N- or O-centered nucleophiles, bicyclic furans were obtained in moderate to excellent yield and enantioselectivity (28 examples, 59-96 % yield, 62 : 38 to 95 : 5 er). The optimal catalytic system, comprising a phosphinosquaramide Au(I) chloride complex and a BINOL-derived phosphoramidate Ag(I) salt, was selected in a combinatorial fashion from a larger library with the help of high-throughput screening. An enantioselectivity switch of ca. 120 Δee% was observed upon addition of the achiral Au(I) component to the Ag(I) salt.

Chirality Transfer and Asymmetric Catalysis: Two Strategies toward the Enantioselective Formal Total Synthesis of (+)-Gelsenicine

Two strategies are described en route to an enantioselective total synthesis of gelsenicine. One approach centers on a chirality transfer cycloisomerization that ultimately fell short. Separately, an asymmetric catalysis route utilizing bisphosphine-gold-catalyzed cycloisomerization was pursued. A catalytic system was identified that provided a synthetic intermediate in our Gelsemium alkaloid syntheses in high enantiopurity and with absolute configuration determined by electronic circular dichroism, thus representing an enantioselective formal total synthesis of (+)-gelsenicine.

Helical Chiral N-Heterocyclic Carbene Ligands in Enantioselective Gold Catalysis

The first chiral helicene-NHC gold(I) complexes efficient in enantioselective catalysis were prepared. The L-shaped chiral ligand is composed of an imidazo[1,5-a]pyridin-3-ylidene (IPy) scaffold laterally substituted by a configurationally stable [5]-helicenoid unit. The chiral information was introduced in a key post-functionalization step of a NHC-gold(I) complex bearing a symmetrical anionic fluoreno[5]helicene substituent, leading to a racemic mixture of complexes featuring three correlated elements of chirality, namely central, axial and helical chirality. After HPLC enantiomeric resolution, X-ray crystallography and theoretical calculations enabled structural and stereochemical characterization of these configurationally stable NHC-gold(I) complexes. The high potential in asymmetric catalysis is demonstrated in the benchmark cycloisomerization of N-tethered 1,6-enynes with up to 95 : 5 er.

H-Bonded Counterion-Directed Enantioselective Au(I) Catalysis

A new strategy for enantioselective transition-metal catalysis is presented, wherein a H-bond donor placed on the ligand of a cationic complex allows precise positioning of the chiral counteranion responsible for asymmetric induction. The successful implementation of this paradigm is demonstrated in 5-exo-dig and 6-endo-dig cyclizations of 1,6-enynes, combining an achiral phosphinourea Au(I) chloride complex with a BINOL-derived phosphoramidate Ag(I) salt and thus allowing the first general use of chiral anions in Au(I)-catalyzed reactions of challenging alkyne substrates. Experiments with modified complexes and anions, ¹H NMR titrations, kinetic data, and studies of solvent and nonlinear effects substantiate the key H-bonding interaction at the heart of the catalytic system. This conceptually novel approach, which lies at the intersection of metal catalysis, H-bond organocatalysis, and asymmetric counterion-directed catalysis, provides a blueprint for the development of supramolecularly assembled chiral ligands for metal complexes.

Gold-Catalyzed Reactions Towards Diversity: From Simple Substrates to Functionalized Carbo- and Heterocycles

The field of gold catalysis has been in constant expansion during the last twenty years. Based on the precept of π-activation of unsaturated simple substrates, several new rearrangements have been discovered, implying aryl, alkyne, alkene or keto derivatives as key partners. In this personal account, the main contributions in the field of gold catalysis from our group will be highlighted, emphasizing the recent reports, starting from 1,6- and 1,5-enynes and then moving to keto-ynes derivatives. The gold-catalyzed reactions will be presented starting from classical skeletal rearrangements (cycloisomerization) and then domino processes. In each part, the presentation of asymmetric versions will be highlighted.

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.

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.

When Gold Meets Perfumes: Synthesis of Olfactive Compounds via Gold-Catalyzed Cycloisomerization Reactions

An efficient, and mild synthetic route for the preparation of functionalized volatile oxa-bicyclo[4.1.0]-hept-4-ene (29 compounds, 44-98% isolated yields) has been developed relying on the association of IPrAuCl with NaBArF. The remarkable selectivity was demonstrated on a 1 g and 25 g scale with low catalyst loadings. The synthetic utility of these low-molecular-weight enols was further demonstrated by the derivatization of some adducts and by the unprecedented olfactory evaluation of all bicyclic derivatives.

Triflic Acid-Catalyzed Cycloisomerization of 1,6-Enynes: Facile Access to Carbo- and Azaheterocycles

A new and efficient strategy for enynes cyclization catalyzed by triflic acid has been described. Various valuable carbocycle-fused and heterocycle-fused ketones were easily accessed by the formation of new C-C and C-O bond under benign reaction conditions. This protocol also provides another opportunity to construct polycyclic single-nitrogen ketones via a cation-induced cascade cyclization of polyenynes. Furthermore, antiviral bioassays revealed that a few compounds exhibited good antiviral activity against tobacco mosaic virus at a concentration of 200 μg mL^-1.

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.

Gold(I)/Xiang-Phos-Catalyzed Asymmetric Intramolecular Cyclopropanation of Indenes and Trisubstituted Alkenes

The first intramolecular enantioselective cyclopropanation of indenes and trisubstituted alkenes was accomplished by using new chiral phosphine X5 derived gold(I) complexes. This reaction is a straightforward, efficient method for constructing [5-3-6] fused-ring compounds with two vicinal all-carbon quaternary stereogenic centers, a core structure shared by numerous pharmacological products, and bioactive compounds. The salient features of this transformation include high enantioselectivity (up to >98% ee), excellent yield (>97%), and nice functional group tolerance.

An enyne cycloisomerization/[5+1] reaction sequence to synthesize tetrahydroisoquinolinones from enyne-enes and CO

An enyne cycloisomerization/[5+1] reaction sequence was developed to synthesize tetrahydroisoquinolinones from linear enyne-enes and CO. The first step is a gold(i)-catalyzed enyne cycloisomerization, generating six-membered-ring-fused vinylcyclopropanes. The second step is a rhodium(i)-catalyzed [5+1] reaction of vinylcyclopropanes with CO. This two-step reaction could also be carried out in one-pot without isolating the cycloisomerization product generated from the first step of this sequence.

Recent advances in enantioselective gold catalysis

Interest in homogeneous gold catalysis has undergone a marked increase. As strong yet air- and moisture-tolerant π-acids, cationic gold(i) complexes have been shown to catalyze diverse transformations of alkenes, alkynes and allenes, opening new opportunities for chemical synthesis. The development of efficient asymmetric variants is required in order to take full advantage of the preparative potential of these transformations. During the last few years, the chemical community has achieved tremendous success in the area. This review highlights the updated progress (2011-2015) in enantioselective gold catalysis. The discussion is classified according to the π-bonds activated by gold(i), in an order of alkynes, allenes and alkenes. Other gold activation modes, such as σ-Lewis acid catalyzed reactions and transformations of diazo compounds are also discussed.

Selectivity-switchable construction of benzo-fused polycyclic compounds through a gold-catalyzed reaction of enyne-lactone

A gold-catalyzed selectivity-switchable reaction of enyne-lactone is reported. The choice of the gold complex is the key for the chemoselectivity.

Well-Defined Chiral Gold(III) Complex Catalyzed Direct Enantioconvergent Kinetic Resolution of 1,5-Enynes

The development of a gold(III) catalyzed direct enantioconvergent 1,5-enyne cycloisomerization and kinetic resolution reaction is described. The transformation results in highly enantioenriched bicyclo[3.1.0]hexenes at all levels of conversion, with no racemization or symmetrization taking place during the course of the reaction, and simultaneously affords optically enriched 1,5-enynes. This report marks the first highly enantioselective transformation catalyzed by a well-defined cationic gold(III) catalyst and demonstrates the unique potential of gold(III) complexes in enantioselective catalysis.

C-C Bond Migration in the Cycloisomerization of 1,6-Enynes

A full account of our investigation of C-C bond migration in the cycloisomerization of oxygen-tethered 1,6-enynes is described. Under Pt(II) and/or Ir(I) catalysis, cyclic and acylic alkyl groups were found to undergo 1,2-shifts into metal carbenoid intermediates. Interestingly, this process does not appear to be driven by the release of ring strain, and thus provides access to large carbocyclic frameworks. The beneficial effect of CO on the Pt(II) and Ir(I) catalytic systems is also evaluated.

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.

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.

Atropisomeric [(diphosphine)Au2 Cl2 ] complexes and their catalytic activity towards asymmetric cycloisomerisation of 1,6-enynes

X-ray crystal structures of two [(diphosphine)Au2 Cl2 ] complexes (in which diphosphine=P-Phos and xylyl-P-Phos; P-Phos=[2,2',6,6'-Tetramethoxy-4,4'-bis(diphenylphosphino)-3,3'-bipyridine]) were determined and compared to the reported structures of similar atropisomeric gold complexes. Correlations between the Au⋅⋅⋅Au distances and torsional angles for the biaryl series of ligands (MeOBIPHEP, SEGPhos, and P-Phos; BIPHEP=2,2'-bis(diphenylphosphino)-1,1'-biphenyl, SEGPhos=[(4,4'-bi-1,3-benzodioxole)-5,5'-diyl]bis[diphenylphosphine]) can be made; these measurements appear to be very dependent upon the phosphorous substituent. Conversely, the same effect was not observed for ligands based on the binaphthyl (BINAP) series. The catalytic activity of these complexes was subsequently assessed in the enantioselective cycloisomerisation of 1,6-enynes and revealed an over-riding electronic effect: more-electron-rich phosphines promote greater enantioselectivity. The possibility of silver acting as a (co-)catalyst was ruled out in these reactions.

Gold(I)-catalyzed asymmetric desymmetrization of meso-alkynyl diols and kinetic resolution of the corresponding DL-diols: effects of celite filtration and silver salts

The asymmetric desymmetrization of meso-2-alkynylbenzenediols through the use of a combination of axially chiral diphosphine(AuCl)2 precatalysts and silver salt co-catalysts gave optically active isochromene compounds with high enantioselectivities in good yields. The corresponding DL-diol isomers underwent efficient kinetic resolution to give the cyclized isochromenes and recovered diols with high enantioselectivities under similar conditions. The high reactivity and selectivity in the desymmetrization of the meso-diols is independent of the combination of axially chiral diphosphine(AuCl)2 precatalyst and silver salt co-catalyst, whereas the corresponding tricarbonylchromium complexes of alkynylbenzenediols were affected by the combination of the diphosphine(AuCl)2 and silver salt. The reactivity was largely dependent on the nature of the gold(I) species.

Palladium-catalyzed oxidative 6-exo-trig cyclization of 1,6-enynes: facile synthesis of bicyclo[4.1.0]heptan-5-ones

We here describe a new palladium-catalyzed oxidative 6-exo-trig cyclization of 1,6-enynes at room temperature using tBuONO as an oxidant for the synthesis of 3-bicyclo[4.1.0]heptan-5-ones.

Gold-catalyzed cyclopropanation reactions using a carbenoid precursor toolbox

This review highlights recent advances in gold-catalyzed selective cyclopropanation divided by the type of carbenoid precursors.