Gold-catalyzed cycloisomerizations of enynes: a mechanistic perspective

Deciphering substitution effects on reductive hydroalkoxylation of alkynyl aminols for stereoselective synthesis of morpholines and 1,4-oxazepanes: total synthesis of tridemorph and fenpropimorph

Acid catalysed reductive etherification of N-propargyl amino alcohols for the stereoselective synthesis of cis-2,5/2,6-disubstituted morpholines and cis-2,6/2,7-disubstituted oxazepanes has been developed. Mechanistic studies revealed that terminal alkynols gave morpholines via a 6-exo-dig hydroalkoxylation-isomerization-reduction cascade. Interestingly, an alkyne hydration-cyclization-reduction sequence is found to be involved in the formation of oxazepanes from alkyl substituted internal alkynols. The strategy was used as a key step in the total synthesis of fungicides tridemorph and fenpropimorph.

Synthesis and reactivity of N-heterocyclic carbene (NHC) gold-fluoroalkoxide complexes

We disclose a novel series of N-heterocyclic carbene (NHC) gold complexes with varied steric and electronic properties, bearing fluorinated alkoxide anions. Early reactivity studies involving these synthons, lead to the synthesis of various complexes of relevance to gold chemistry and catalysis.

Carbene-Assisted Arene Ring-Opening

Despite the significant achievements in dearomatization and C-H functionalization of arenes, the arene ring-opening remains a largely unmet challenge and is underdeveloped due to the high bond dissociation energy and strong resonance stabilization energy inherent in aromatic compounds. Herein, we demonstrate a novel carbene assisted strategy for arene ring-opening. The understanding of the mechanism by our DFT calculations will stimulate wide application of bulk arene chemicals for the synthesis of value-added polyconjugated chain molecules. Various aryl azide derivatives now can be directly converted into valuable polyconjugated enynes, avoiding traditional synthesis including multistep unsaturated precursors, poor selectivity control, and subsequent transition-metal catalyzed cross-coupling reactions. The simple conditions required were demonstrated in the late-stage modification of complex molecules and fused ring compounds. This chemistry expands the horizons of carbene chemistry and provides a novel pathway for arene ring-opening.

Copper-Catalyzed Directed Hydroindolation/Annulation Sequence of Alkynes with Indoles via Copper Carbenes

Described herein is an efficient copper-catalyzed tandem alkyne indolylcupration-initiated 1,2-indole migration/6π-electrocyclic reaction of allene-ynamides with indoles by the in situ-generated metal carbenes. This method allows the efficient synthesis of valuable indole-fused spirobenzo[f]indole-cyclohexanes with high regio- and stereoselectivity. In addition, this reaction affords rapid access to the functionalized spirobenzo[f]indole-cyclohexanes in the absence of indoles by a presumable 5-exo-dig cyclization/Friedel-Crafts alkylation via copper-containing all-carbon 1,4-dipoles.

1,2-Silyl Rearrangement in Gold Carbene Chemistry: Synthesis of Furyl-Decorated Tetrasubstituted Silylallene Derivatives

The gold-catalyzed reaction of 2-en-4-ynones with alkynylsilanes provides fully substituted allene derivatives bearing furyl and silyl groups. This transformation would involve generation of a gold furyl carbene intermediate, which regioselectively undergoes a nucleophilic attack by the alkynylsilane at the electrophilic carbene carbon atom with the formation of a β-gold vinyl cation species. The subsequent release of the gold catalyst, accompanied by a 1,2-silyl shift, leads to the formation of tetrasubstituted allene products.

NHC-Au-Catalyzed Isomerization of Propargylic B(MIDA)s to Allenes and Double Isomerization of Alkynes to 1,3-Dienes

The synthesis of allenyl boronates is an important yet challenging topic in organic synthesis. Reported herein is an NHC-gold-catalyzed 1,3-H shift toward allenyl boronates synthesis from simple propargylic B(MIDA)s. Mechanistic studies suggest dual roles of the boryl moiety in the reaction: to activate the substrate for isomerization and at the same time, to prevent the allene product from further isomerization. These effects should be a result of α-anion stabilization and α-cation destabilization conferred by the B(MIDA) moiety, respectively. The NHC-Au catalyst, which is commercially available, is also found to be reactive in alkyne-to-1,3-diene isomerization reactions in an atom-economic and base-free manner.

Cyclization Reactions of 1,5-Diynes: Mechanisms and the Role of the Central Linker

This study employed computational methods to elucidate the influence of structural features on the cyclization pathways of 1,5-diynes through the 5-endo-dig and 6-endo-dig mechanisms. The results revealed that the nature of the central linker played a significant role in dictating the preferred cyclization pathway. Notably, the capacity of this linker to extend delocalization appears to be the key factor governing the reaction pathway preference.

Copper-Catalyzed [2 + 2] Cyclization/Ring Expansion of Ene-Ynamides: Construction of Medium- and Large-Sized Rings

Catalytic cyclization of enynes is an efficient approach for the preparation of cyclic compounds, and a large variety of four- to six-membered rings could be synthesized using this method. However, it has been rarely employed for the construction of medium- and large-sized rings. Herein, we describe a copper-catalyzed cycloisomerization of ene-ynamides through a [2 + 2] cyclization/electrocyclic ring opening cascade, leading to the atom-economical assembly of indole-fused medium- and large-sized rings in moderate to excellent yields under mild reaction conditions. Importantly, the synthetic utility of this reaction was demonstrated by the convenient synthesis of iprindole.

Gold-Catalyzed Double Spirocyclization of 3-Ene-1,7-diyne Esters to Dispiroheterocycles

A synthetic method to prepare dispiroheterocycles containing two all-carbon quaternary centers efficiently that relies on the gold(I)-catalyzed double spirocyclization of 3-ene-1,7-diyne esters is described. The suggested mechanism delineates a rare example of a dispirocyclization featuring two 1,n-acyloxy shifts comprising a 1,3-acyloxy migration and an interrupted 1,5-acyl migration that was achieved with the assistance of residual water in the reaction media.

Palladium-catalyzed alkynylation of allylic gem-difluorides

Herein, a palladium-catalyzed regioselective alkynylation, esterification, and amination of allylic gem-difluorides via C-F bond activation/transmetallation/β-C elimination or nucleophilic attack has been achieved. This innovative protocol showcases an extensive substrate range and operates efficiently under mild reaction conditions, resulting in high product yields and Z-selectivity. Particularly noteworthy is its exceptional tolerance towards a wide array of functional groups. This developed methodology provides effective and convenient routes to access a diverse array of essential fluorinated enynes, esters and amines.

Gold-catalyzed endo-selective Ring-opening of Epoxides and its Application in Construction of Poly-ethers

Tetrahydropyran and tetrahydropyran-fused poly-ethers scaffolds are found in many classes of natural products and medicinally relevant small molecules. Here we describe a catalytic system for 6-endo selective ring-opening of epoxides by Au(I) or Au(III) catalyst that provides rapid access to various tetrahydropyran-derived motifs. It also could efficiently construct the subunits of marine ladder-like poly-ethers through emulating the Nakanishi's hypothesis on the biosynthesis of these toxins. The synthetic utility of this method is also demonstrated in the preparation of the tricyclic core of tetrahydropyran-containing macrolide natural products lituarines A-C.

Gold-Catalyzed Cascade Cycloisomerization of 3-Allyloxy-1,6-diynes to Cyclopropyl- and Cyclobutyl-Fused Benzofurans and Chromen-3a(1H)-ols

A synthetic method for the efficient preparation of partially hydrogenated benzo[f]cyclobuta[cd]cyclopenta[h]benzofurans and cyclopropa[c]chromen-3a(1H)-ols that relies on the gold(I)-catalyzed cascade cycloisomerization of 3-allyloxy-1,6-diynes is described.

Nickel-catalyzed switchable arylative/endo-cyclization of 1,6-enynes

Carbo- and heterocycles are frequently used as crucial scaffolds in natural products, fine chemicals, and biologically and pharmaceutically active compounds. Transition-metal-catalyzed cyclization of 1,6-enynes has emerged as a powerful strategy for constructing functionalized carbo- and heterocycles. Despite significant progress, the regioselectivity of alkyne functionalization is entirely substrate-dependent. And only exo-cyclization/cross-coupling products can be obtained, while endo-selective cyclization/cross-coupling remains elusive and still poses a formidable challenge. In this study, we disclose a nickel-catalyzed switchable arylation/cyclization of 1,6-enynes in which the nature of the ligand dictates the regioselectivity of alkyne arylation, while the electrophilic trapping reagents determine the selectivity of the cyclization mode. Specifically, using a commercially available 1,10-phenanthroline as a ligand facilitates trans-arylation/cyclization to obtain seven-membered ring products, while a 2-naphthyl-substituted bisbox ligand promotes cis-arylation/cyclization to access six-membered ring products. Diastereoselective cyclizations have also been developed for the synthesis of enantioenriched piperidines and azepanes, which are core structural elements of pharmaceuticals and natural products possessing important biological activities. Furthermore, experimental and density functional theory studies reveal that the regioselectivity of the alkyne arylation process is entirely controlled by the steric hindrance of the ligand; the reaction mechanism involves exo-cyclization followed by Dowd-Beckwith-type ring expansion to form endo-cyclization products.

Metal-free radical bicyclization/chloroalkylarylation of 1,6-enynes with chloroalkanes

Free radical initiated bicyclization of 1,6-enynes with chloralkanes, is achieved via selective activation of the C(sp3)-H bond of the chloralkane, resulting in diverse polychlorinated/chlorinated polyheterocycles. Two kinds of transformations and a scaled-up experiment were performed to test the synthetic importance of the organic chlorides. Finally, a range of radical inhibition operations and radical clock tests were explored to support the reaction process.

Divergent Geminal Alkynylation-Allylation and Acylation-Allylation of Carbenes: Evolution and Roles of Two Transition-Metal Catalysts

Cooperative bimetallic catalysis to access novel reactivities is a powerful strategy for reaction development in transition-metal-catalyzed chemistry. Particularly, elucidation of the evolution of two transition-metal catalysts and understanding their roles in dual catalysis are among the most fundamental goals for bimetallic catalysis. Herein, a novel three-component reaction of a terminal alkyne, a diazo ester, and an allylic carbonate was successfully developed via cooperative Cu/Rh catalysis with Xantphos as the ligand, providing a highly efficient strategy to access 1,5-enynes with an all-carbon quaternary center that can be used as immediate synthetic precursors for complex cyclic molecules. Notably, a Meyer-Schuster rearrangement was involved in the reactions using propargylic alcohols, resulting in an unprecedented acylation-allylation of carbenes. Mechanistic studies suggested that in the course of the reaction Cu(I) species might aggregate to some types of Cu clusters and nanoparticles (NPs), while the Rh(II)2 precursor can dissociate to mono-Rh species, wherein Cu NPs are proposed to be responsible for the alkynylation of carbenes and work in cooperation with Xantphos-coordinated dirhodium(II) or Rh(I)-catalyzed allylic alkylation.

Bismuth in Radical Chemistry and Catalysis

Whereas indications of radical reactivity in bismuth compounds can be traced back to the 19th century, the preparation and characterization of both transient and persistent bismuth-radical species has only been established in recent decades. These advancements led to the emergence of the field of bismuth radical chemistry, mirroring the progress seen for other main-group elements. The seminal and fundamental studies in this area have ultimately paved the way for the development of catalytic methodologies involving bismuth-radical intermediates, a promising approach that remains largely untapped in the broad landscape of synthetic organic chemistry. In this review, we delve into the milestones that eventually led to the present state-of-the-art in the field of radical bismuth chemistry. Our focus aims at outlining the intrinsic discoveries in fundamental inorganic/organometallic chemistry and contextualizing their practical applications in organic synthesis and catalysis.

Propargylic Dialkyl Effect for Cyclobutene Formation through Ir(III)-Catalyzed Cycloisomerization of 1,6-Enynes

The propargylic dialkyl effect (PDAE) has a significant impact on the cyclization reaction of enynes, partly reflected in changing the types of products. Herein, we described the influence of the propargylic dialkyl effect on the Ir(III)-catalyzed cycloisomerization of 1,6-enynes to provide strained cyclobutenes. A series of substituted 1,6-enynes were proved to be excellent substrate candidates in the presence of [Cp*IrCl2]2 in toluene. Mechanistic investigation, based on deuterium labeling experiments and control experiments, indicated that the propargylic dialkyl effect might boost C(sp)-H activation by preventing the coordination of active iridium species to the C(sp)≡C(sp) bond of enynes. This finding contributes to the fundamental understanding of enyne cyclization reactions and offers valuable insight into the propargylic dialkyl effect.

Sulfonium Cation in the Service of π-Acid Catalysis

While still rare, cationic ligands offer much promise as tunable electron-withdrawing ligands for π-acid catalysis. Recently, we introduced pincer-type sulfonium cations into the list of available strongly π-acidic ancillary ligands. However, the M-S bond in sulfonium complexes of these ligands was found highly labile, precluding their catalytic applications. Herein we demonstrate that this obstacle can be overcome by increasing the rigidity of the sulfonium pincer scaffold. X-ray analyses confirm that despite bearing a formal positive charge, the sulfur atom of this newly designed sulfonium ligand maintains its coordination to the Pt(II)-center, while DFT calculations indicate that by doing so it strongly enhances the electrophilic character of the metal. Kinetic studies carried out on three model cycloisomerization reactions prove that such a tris-cationic sulfonium-Pt(II) complex is highly reactive, compared to its thioether-based analogue. This proof-of-concept study presents the first example of employing sulfonium-based ligands in homogeneous catalysis.

Ag(I)-Mediated Annulation of 2-(2-Enynyl)pyridines and Propargyl Amines to Access 1-(2H-Pyrrol-3-yl)indolizines

An effective Ag(I)-mediated annulation of 2-(2-enynyl)pyridines and propargyl amines was developed, unexpectedly affording a broad range of functionalized 1-(2H-pyrrol-3-yl)indolizines in moderate to excellent yields. The developed method is characterized by operational simplicity, ready availability of starting materials, high regioselectivity, and broad substrate scope under mild reaction conditions. The Ag(I)-promoted cyclization of 2-(2-enynyl)pyridines and propargyl amines possibly results in the formation of the spiroindolizine, the ring-opening rearrangement of which may give the 1-(2H-pyrrol-3-yl)indolizine. Furthermore, a gram-scale reaction and synthetic transformations are also studied.

Cascade hydroarylation/Diels-Alder cycloaddition of alkynylindoles with electron-deficient alkynes and alkenes

Herein, a novel cascade gold(I)-catalyzed hydroarylation of alkynylindoles and subsequent Diels-Alder cycloaddition with electron-deficient alkynes and alkenes is described. A variety of azepino-fused hydrocarbazoles and carbazoles were obtained in moderate to excellent yields. Key features of this methodology are low catalyst loadings, high regioselectivity, broad functional group tolerances, access to important heterocycles, and 100% atom economy.

Pnictogen bonding at the service of gold catalysis: the case of a phosphinostiborane gold complex

The search for alternative gold catalyst activators has led us to consider the design of platforms in which a phosphine gold chloride moiety could be activated via formation of a pnictogen bond with a neighboring antimony unit. Here, we describe that such a system can be accessed from 4-(diphenylphosphino)-5-(diphenylstibino)-2,7-di-tert-butyl-9,9-dimethylxanthene, by oxidation of the stibine with 3,5-di-tert-butyl-o-benzoquinone and by coordination of an AuCl unit to the phosphine. This strategy affords a complex in which a Lewis acidic or pnictogen-bond donor catecholatostiborane unit flanks the adjacent gold chloride moiety. This design impacts the catalytic reactivity of the gold center, as reflected by the ability of this complex to catalyze propargyl amide cyclization reactions. Comparisons with a phosphinostiborane ferrocene analog and computations point to the formation of an intramolecular Au-Cl → Sb(V) interaction as responsible for the observed catalytic activity.

Gold(I)-Catalyzed Ring-Closing Alkyne-Carbonyl Metathesis for the Synthesis of Butenolides

Alkyne-carbonyl metathesis is a type of carbon-carbon forming reaction involving the construction a carbon-carbon double bond and a carbonyl group in one transformation. Herein, a Au(I)-catalyzed ring-closing alkyne-carbonyl metathesis protocol has been developed to make densely substituted γ-butenolides from propargyl α-ketoesters. It features 100 % atom economy, excellent substrate flexibility and benign functional group tolerance. Mechanistic studies demonstrate that the coordinative interaction between the gold catalyst and the alkyne might initiate the transfer of an oxygen atom and the formation of the carbon-carbon double bond. By using this gold-catalyzed ring-closing alkyne-carbonyl metathesis as a key step reaction, four naturally occurring butenolide-type compounds including decumbic acid (45 % yield for 3 steps), deoxyisosporothric acid (32 % yield for 5 steps), lichesterinic acid (34 % yield for 5 steps) and isomuronic acid (6 % yield for 8 steps) have been synthesized starting from commercially available starting materials.

Interrupted Furan-Yne Cyclization: Access to Unsaturated Dicarbonyl Compounds and Their Subsequent Transformation into Functionalized Pyridazines

The key carbenoid intermediate of transition-metal-catalyzed furan-yne cyclization in Hashmi phenol synthesis could be efficiently intercepted with water under the developed reaction conditions in order to provide access to functionalized unsaturated dicarbonyl compounds that might serve as convenient precursors for the straightforward synthesis of annulated pyridazines.

Gold(I)-Mediated Isomerization of Spring-loaded 1,7-Enynes seen through the Lens of Density Functional Theory

We propose a mechanism for the previously reported formation of benzobicyclo[3.2.0]heptane derivatives from 1,7-enyne derivatives bearing a terminal cyclopropane. -> A mechanism for the previously reported formation of benzobicyclo[3.2.0]heptane derivatives from 1,7-enyne derivatives bearing a terminal cyclopropane is proposed.

Deciphering the Primary Role of Au⋅⋅⋅H-X Hydrogen Bonding in Gold Catalysis

Au⋅⋅⋅H-X (X=N or C) hydrogen bonding is gaining increasing interest, both in the study of its intrinsic nature and in their operability in different fields. While the role of these interactions has been studied in the stabilization of gold(I) complexes, their role during the minimum free energy reaction pathway of a given catalytic process remains unexplored. We report herein that complex [Au(C≡CPh)(pip)] (pip=piperidine) catalyses the A3 -coupling reaction for the synthesis of propargylamines, thanks to the ability of Au(I) to promote weak hydrogen bonding interactions with the reactants along the free energy profile. Density Functional Theory (DFT) calculations show that these Au⋅⋅⋅H-X interactions play a directing role in the catalysed A3 -coupling. Topological non-covalent interactions (NCI), interaction region indicator (IRI) and quantum theory of atoms in molecules (QTAIM) analysis in real space of the electron density provide a description of these interactions accurately.

Reactivity of Cyclic (Alkyl)(amino)germylene towards Copper(I) and Gold(I) Complexes

The reactions of cyclic (alkyl)(amino)germylenes (CAAGe) with copper(I) and gold(I) complexes were investigated. CAAGe (1) reacts with CuBr(SMe2 ) leading to a tetrameric germylene complex [CAAGeCuBr]4 (2), whereas CAAGe (3) undergoes Au-Cl bond insertion with LAuCl (L=phosphine or N-heterocyclic carbene) to afford germanium gold(I) complexes (5 and 6). Chlorine abstraction of 6 gives the cationic germylene gold(I) complex 7.

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.

Regioselective Radical Cascade Cyclizations of Alkyne-Tethered Cyclohexadienones with Chalcogenides under Visible-Light Catalysis

Herein, we demonstrated a silver/K2S2O8-mediated highly regio- and diastereoselective 6/5-exo trig radical cascade cyclization of alkyne-tethered cyclohexadienones with sulfonyl hydrazides or sodium sulfinates and subsequent selenation to access 6,6-dihydrochromenone and 6,5-fused tetrahydro benzofuranone derivatives. This reaction protocol features high functional group compatibility and has a wide substrate scope providing a variety of dihydrochromenones and tetrahydro benzofuranone derivatives in good to excellent yields. The reaction proceeds via the attack of a sulfonyl radical to alkyne over the activated Michael acceptor. The TEMPO quenching experiment implies the presence of a radical intermediate. Further synthetic versatility of 6,6- and 5,6-fused derivatives is also showcased.

Mechanistic Insights into Oxazolone Synthesis by Bimetallic Au-Pd-Catalyzed Catalysis and Catalyst Design: DFT Investigations

Bimetallic synergistic catalysis is one of the most effective and powerful strategies for the synthesis of oxazolones, an important species in organic synthesis. In this work, the mechanism of AuCl(PMe₃)/AgOTf-Pd(0) ([Au-Pd]) bimetallic catalyst-catalyzed oxazolone synthesis using N-alkynyl carbamates as precursors was studied in detail by DFT calculations and the catalytic performances of a series of bimetallic catalysts were evaluated. The results show that the reaction begins from the [Au]-catalyzed cycloisomerization of N-alkynyl carbamates. After the five-membered intermediate is formed, the [Pd(0)]-catalyzed cycle starts, which contains three steps: oxidation addition, transmetalation, and reductive elimination. The whole reaction belongs to a catalyzed catalysis, and the reductive elimination is the rate-determining step. In the transmetalation process, both the [Pd(0)] catalyst and the ionic bridge are necessary. For the [Au-Pd]-catalyzed process, it is Cl^- as the bridge, not OTf^-. The cheaper metal compound, AgCl(PMe₃), can serve as the alternative of AuCl(PMe₃) to co-catalyze with the [Pd(0)] catalyst for the title reaction.

Enantioselective Catalysis with Pyrrolidinyl Gold(I) Complexes: DFT and NEST Analysis of the Chiral Binding Pocket

A new generation of chiral gold(I) catalysts based on variations of complexes with JohnPhos-type ligands with a remote C₂-symmetric 2,5-diarylpyrrolidine have been synthesized with different substitutions at the top and bottom aryl rings: from replacing the phosphine by a N-heterocyclic carbene (NHC) to increasing the steric hindrance with bis- or tris-biphenylphosphine scaffolds, or by directly attaching the C₂-chiral pyrrolidine in the ortho-position of the dialkylphenyl phosphine. The new chiral gold(I) catalysts have been tested in the intramolecular [4+2] cycloaddition of arylalkynes with alkenes and in the atroposelective synthesis of 2-arylindoles. Interestingly, simpler catalysts with the C₂-chiral pyrrolidine in the ortho-position of the dialkylphenyl phosphine led to the formation of opposite enantiomers. The chiral binding pockets of the new catalysts have been analyzed by DFT calculations. As revealed by non-covalent interaction plots, attractive non-covalent interactions between substrates and catalysts direct specific enantioselective folding. Furthermore, we have introduced the open-source tool NEST, specifically designed to account for steric effects in cylindrical-shaped complexes, which allows predicting experimental enantioselectivities in our systems.

Computational insights into the mechanisms and origins of switchable selectivity in gold(i)-catalyzed annulation of ynamides with isoxazoles via 6π-electrocyclizations of azaheptatrienyl cations

Electrocyclizations of acyclic conjugated π-motifs have emerged as a versatile and effective strategy for accessing various ring systems with excellent functional group tolerability and controllable selectivity. Typically, the realization of 6π-electrocyclization of heptatrienyl cations to afford seven-membered motif has proven difficult due to the high-energy state of the cyclizing seven-membered intermediate. Instead, it undergoes the Nazarov cyclization, affording a five-membered pyrrole product. However, the incorporation of a Au(i)-catalyst, a nitrogen atom and tosylamide group in the heptatrienyl cations unexpectedly circumvented the aforementioned high energy state to afford a seven-membered azepine product via 6π-electrocyclization in the annulation of 3-en-1-ynamides with isoxazoles. Therefore, extensive computational studies were carried out to investigate the mechanism of Au(i)-catalyzed [4+3] annulation of 3-en-1-ynamides with dimethylisoxazoles to produce a seven-membered 4H-azepine via the 6π-electrocyclization of azaheptatrienyl cations. Computational results showed that after the formation of the key α-imino gold carbene intermediate, the annulation of 3-en-1-ynamides with dimethylisoxazole occurs via the unusual 6π-electrocyclization to afford a seven-membered 4H-azepine exclusively. However, the annulation of 3-cyclohexen-1-ynamides with dimethylisoxazole occurs via the commonly proposed aza-Nazarov cyclization pathway to majorly generate five-membered pyrrole derivatives. The results from the DFT predictive analysis revealed that the key factors responsible for the different chemo-, and regio-selectivities observed are the cooperating effect of the tosylamide group on C¹, the uninterrupted π-conjugation pattern of the α-imino gold(i) carbene and the substitution pattern at the cyclization termini. The Au(i)-catalyst is believed to assist in the stabilization of the azaheptatrienyl cation.

Gold-based enantioselective bimetallic catalysis

Multimetallic catalysis is a powerful strategy to access complex molecular scaffolds efficiently from easily available starting materials. Numerous reports in the literature have demonstrated the effectiveness of this approach, particularly for capitalizing on enantioselective transformations. Interestingly, gold joined the race of transition metals very late making its use in multimetallic catalysis unthinkable. Recent literature revealed that there is an urgent need to develop gold-based multicatalytic systems based on the combination of gold with other metals for enabling enantioselective transformations that are not possible to capitalize with the use of a single catalyst alone. This review article highlights the progress made in the field of enantioselective gold-based bimetallic catalysis highlighting the power of multicatalysis for accessing new reactivities and selectivities which are beyond the reach of individual catalysts.

Metalla-Claisen Rearrangement in Gold-Catalyzed [4+2] Reaction: A New Elementary Reaction Suggested for Future Reaction Design

We report here computational evidence for a metalla-Claisen rearrangement (MCR) in the case of gold-catalyzed [4+2] cycloaddition reaction of yne-dienes. The [4+2] reaction starts from exo cyclopropanation, followed by MCR and reductive elimination. The cyclopropane moiety formed in the first step is crucial for a low barrier of the MCR step. In addition, the importance of an appropriate combination of the tether group and the terminal substituent on alkyne in the yne-diene substrates was studied. The mechanism of rhodium-catalyzed [4+2] reaction of yne-dienes was also investigated to see whether an MCR mechanism is involved or not. The findings and new understanding hereby reported represent an important advance in the catalysis field.

Gold-Catalyzed Addition of β-Oxo Enols at Tethered Alkynes via a Non-Conia-ene Pathway: Observation of a Formal 1,3-Hydroxymethylidene Migration

With the relay process of Ag(I)/Au(I) catalysts, a one-pot synthesis of skeletally rearranged (1-hydroxymethylidene)indene derivatives from 2-alkynylbenzaldehydes and α-diazo esters is described. This cascade sequence involves Au(I)-catalyzed 5-endo-dig attack of highly enolizable aldehydes at the tethered alkynes, leading to carbocyclizations with a formal 1,3-hydroxymethylidene transfer. On the basis of density functional theory calculations, the mechanism likely involves formation of cyclopropylgold carbenes, followed by an appealing 1,2-cyclopropane migration.

Cobalt-catalyzed enantioselective intramolecular reductive cyclization via electrochemistry

Transition-metal catalyzed asymmetric cyclization of 1,6-enynes has emerged as a powerful method for the construction of carbocycles and heterocycles. However, very rare examples worked under electrochemical conditions. We report herein a Co-catalyzed enantioselective intramolecular reductive coupling of enynes via electrochemistry using H₂O as hydride source. The products were obtained in good yields with high regio- and enantioselectivities. It represents the rare progress on the cobalt-catalyzed enantioselective transformation via electrochemistry with a general substrate scope. DFT studies explored the possible reaction pathways and revealed that the oxidative cyclization of enynes by LCo(I) is more favorable than oxidative addition of H₂O or other pathways.

Pd(0)-Catalyzed Asymmetric 7-Endo Hydroacyloxylative Cyclization of 1,6-Enyne Enabled by an Anion Ligand-Directed Strategy

Despite diversity in reaction mechanisms, the palladium-catalyzed cyclization of 1,6-enyne generally proceeds in a 5-exo manner. Herein, we report the development of a Pd(0)-catalyzed hydroacyloxylative cyclization of 1,6-enyne in either 7-endo-trig or 6-exo-trig fashion when paired with an appropriate dihaloacetic acid reactant, such as F₂HCCO₂H and Cl₂HCCO₂H. Using the combination of Pd₂(dba)₃ and a chiral phosphine ligand, the hydroacyloxylative cyclization of 1,6-enyne bearing a 1,1-disubstituted alkene moiety readily gives highly enantiopure seven-membered heterocycles while the reaction of those having a 1,2-disubstituted alkene affords six-membered rings with moderate enantioselectivity. Preliminary experimental studies suggest a reaction mechanism featuring an unusual E-to-Z vinyl-Pd(II) isomerization and alkene trans-oxypalladation, which is proven to be governed by the rationally selected carboxylate.

Gold-Catalyzed Alkyne Multifunctionalization through an Oxidation-Oxyalkylation-Aryloxylation Sequence

A gold-catalyzed oxidative three-component reaction of terminal alkynes with alcohols and quinone monoimines has been disclosed, affording α-ketoacetals in good to excellent yields. By using quinone monoimines as electrophiles for the interception of the in situ generated gold enolate intermediate, this one-pot process provides an unprecedented method for the polyfunctionalization of terminal alkynes through an oxidation-oxyalkylation-aryloxylation sequence, installing three oxygen atoms on the C-C triple bond.

Chiral oxalamide phosphine (COAP)-Pd-catalyzed enantioselective cascade formal [4 + 1] annulation for enantioenriched 2,3-disubstituted indolines and further DFT study on regio- and stereocontrol

COAP-Pd-catalyzed asymmetric cascade formal [4 + 1] annulation was developed between racemic vinyl benzoxazinones and N-tosylhydrazone sodium salts, affording trans-2,3-disubstituted indolines in good yields with high stereoselectivity.

Gold-catalyzed multicomponent reactions

Multicomponent reactions (MCRs) have emerged as an important branch in organic synthesis for the creation of complex molecular structures. This review is focused on gold-catalyzed MCRs with a special emphasis on the recent developments.

Relay Zn(ii)- and Au(i)-catalyzed aziridination/cyclization/ring expansion sequence to form 3-benzazepine derivatives

The synthesis of 3H-benzo[d]azepine-2-carboxylates from 2-alkynylphenyl aldimines and α-diazo esters using Zn(ii) and Au(i) catalysts is described.

Gold-Catalyzed Cyclization/Hydroboration of 1,6-Enynes: Synthesis of Bicyclo[3.1.0]hexane Boranes

The gold-catalyzed cyclization/hydroboration of 1,6-enynes offers facile, versatile, and atom-economical one-step access to bicyclo[3.1.0]hexane boranes. This new protocol proceeds in moderate to good yields under mild conditions. Different from bicyclo[3.1.0]hexane borates, these products are stable in air and during chromatography. Moreover, the borane moiety of the products can readily undergo a diverse array of transformations. The kinetic isotope effect experiment indicates that the hydrogen-transfer step is a fast process, which is not involved in the rate-limiting step.

A Density Functional Theory Study on the Cobalt-Mediated Intramolecular Pauson–Khand Reaction of Enynes Containing a Vinyl Fluoride Moiety

The Co2(CO)8-mediated intramolecular Pauson–Khand reaction (PKR) is an effective method for constructing polycyclic structures. Recently, our group reported a series of this type of reaction involving fluorinated enynes that proceed with reasonable reaction rates and yields. However, mechanistic studies involving these fluorinated derivatives in intramolecular PKR are scarce. In this study, density functional theory calculations are used to clarify the mechanism and reactivity of enynes containing a vinyl fluoride moiety for this reaction. In agreement with previous studies, alkene insertion is considered to be the rate-determining step for the overall Pauson–Khand reaction of enynes containing a vinyl fluoride moiety. The effect of the substituent on the Co2(CO)8-mediated intramolecular Pauson–Khand reaction has also been investigated. When introducing heteroatoms as tethering units, the fluorinated enynes exhibited lower reactivity than the malonate homologues, whereas the use of a sulfur-based tether was unsuccessful. This computational study provides detailed information about the PKR mechanism and transition-state structures, and the results are validated with previous experimental results.