Homogeneous and Heterogeneous Gold Catalysis for Materials Science

A [2.2]Isoindolinophanyl-Based Carbene (iPC) Ligand: Synthesis, Electronic and Photophysical Properties, and Application in Photocatalysis

Cyclic amino(alkyl) and cyclic amino(aryl) carbenes (cAACs/cAArCs) have been established as very useful ligands for catalytic and photonic applications of transition metal complexes. Herein, we describe the synthesis of a structurally related sterically demanding, electrophilic [2.2]isoindolinophanyl-based carbene (iPC) that bears a [2.2]paracyclophane moiety. The latter leads to more delocalized frontier orbitals and intense green fluorescence of (HiPC)OTf (2) from an intra-ligand charge transfer (1ILCT) state in the solid state. Base-promoted synthesis of the free carbene led to an unusual ring expansion and subsequent dimerization reaction, but the beneficial ligand properties can be exploited by trapping in situ at a metal center. The iPC ligand is a very potent π-chromophore, which participates in low energy metal-to-ligand (ML)CT transitions in [RhCl(CO)2(iPC)] (4) and IL-"through-space"-CT transitions in [Au(iPC)2]OTf (5). The steric demand of the iPC leads to high stability of 5 against air, moisture, or solvent attack, and ultralong-lived green phosphorescence with a lifetime of 185 μs is observed in solution. The beneficial photophysical and electronic properties of the iPC ligand, including a large accessible π surface area, were exploited by employing highly efficient energy transfer (EnT) photocatalysis in a [2+2] styrene cycloaddition reaction using 5, which outperformed other established photocatalysts in comparison.

Indium-Mediated Preparation of Bis(α-hydroxyallenes) or α,α'-Dihydroxyallenynes and Further Gold-Catalyzed Cyclizations

We present the regio- and diastereoselective Barbier-type allenylation reaction of glyoxals mediated by indium to furnish highly valuable syn-bis(α-hydroxyallenes) and syn-α,α'-dihydroxyallenynes. The gold-catalyzed controlled cyclization of these unsaturated diols enables the divergent preparation of three types of oxacycles.

Hydrophenoxylation of alkynes by gold catalysts: a mini review

CONTEXT

The field of chemistry has significantly evolved, with catalysis playing a crucial role in transforming chemical processes. From Valerius' use of sulfuric acid in the sixteenth century to modern advancements, catalysis has driven innovations across various industries. The introduction of gold as a catalyst marked a pivotal shift, expanding its applications beyond ornamentation to homogeneous catalysis. Gold's unique properties, such as its electrophilic nature and flexibility, have enabled its use in synthesizing complex molecules, including those in nanomedicine and sustainable chemical processes. The development of gold-based complexes, particularly in hydroalkoxylation and hydroamination reactions, showcases their efficiency in forming carbon-oxygen bonds under mild conditions. Recent studies on dual gold catalysis and heterobimetallic complexes further highlight gold's versatility in achieving high turnover rates and selectivity. This evolution underscores the potential of gold catalysis in advancing environmentally sustainable methodologies and enhancing the scope of modern synthetic chemistry. The debate about the nature of monogold and dual-gold catalysis is open.

METHODS

DFT calculations have played a key role in promoting the activation of alkynes, in particular the hydrophenoxylation of alkynes by metal-based catalysts. They not only help identify the most efficient and selective catalysts but also aid in screening for those capable of performing a dual metal catalytic mechanism. The most commonly used functionals are BP86 and B3LYP, with the SVP and 6-31G(d) basis sets employed for geometry optimizations, and M06 with TZVP or 6-311G(d,p) basis sets used for single-point energy calculations in a solvent. Grimme dispersion correction has been explicitly added either in the solvent single point energy calculations or in the gas phase geometry optimizations or in both. To point out that M06 implicitly includes part of this dispersion scheme.

Gold-Catalyzed Synthesis of 5H-Benzo[b]indeno[2,1-d]silines by Insertion of Vinyl Carbocations into the Si-H Bond

We have developed a gold-catalyzed cascade reaction of aryldiynes bearing a hydrosilyl group to afford a variety of unexplored 5H-benzo[b]indeno[2,1-d]silines. The reaction system is applicable to the synthesis of bidirectionally π-extended silacycles from tetra(alkynyl)aryl compounds. Computational studies suggest that 5H-benzo[b]indeno[2,1-d]silines are formed via the insertion of a vinyl carbocation intermediate into the Si-H bond.

Catalytic (4+2) Annulation via Regio- and Enantioselective Interception of in-situ Generated Alkylgold Intermediate

A regio- and stereoselective stepwise (4+2) annulation of N-propargylamides and α,β-unsaturated imines/ketones has been accomplished with synergetic catalysis by a combination of a gold-complex and a chiral quinine-derived squaramide (QN-SQA), leading to highly functionalized chiral tetrahydropyridines/dihydropyrans in good to high yields with generally excellent enantioselectivity. Mechanistic studies and DFT calculations indicate that the in situ formed alkylgold species is the key intermediate in this transformation, and the amide group served as a traceless directing group in this highly selective transformation. This method complements the enantioselective (4+2) annulation of allene reagents, providing the formal internal C-C π-bond cycloaddition products, which is challenging and remains elusive.

A Gold(I)-Acetylene Complex Synthesised using Single-Crystal Reactivity

Using single-crystal to single-crystal solid/gas reactivity the gold(I) acetylene complex [Au(L1)(η2-HC≡CH)][BArF 4] is cleanly synthesized by addition of acetylene gas to single crystals of [Au(L1)(CO)][BArF 4] [L1=tris-2-(4,4'-di-tert-butylbiphenyl)phosphine, ArF=3,5-(CF3)2C6H3]. This simplest gold-alkyne complex has been characterized by single crystal X-ray diffraction, solution and solid-state NMR spectroscopy and periodic DFT. Bonding of HC≡CH with [Au(L1)]+ comprises both σ-donation and π-backdonation with additional dispersion interactions within the cavity-shaped phosphine.

AuNP/MIL-88B-NH2 Nanocomposite for the Valorization of Nitroarene by Green Catalytic Hydrogenation

The efficiency of a catalytic process is assessed based on conversion, yield, and time effectiveness. However, these parameters are insufficient for evaluating environmentally sustainable research. As the world is urged to shift towards green catalysis, additional factors such as reaction media, raw material availability, sustainability, waste minimization and catalyst biosafety, need to be considered to accurately determine the efficacy and sustainability of the process. By combining the high porosity and versatility of metal organic frameworks (MOFs) and the activity of gold nanoparticles (AuNPs), efficient, cyclable and biosafe composite catalysts can be achieved. Thus, a composite based on AuNPs and the nanometric flexible porous iron(III) aminoterephthalate MIL-88B-NH2 was successfully synthesized and fully characterized. This nanocomposite was tested as catalyst in the reduction of nitroarenes, which were identified as anthropogenic water pollutants, reaching cyclable high conversion rates at short times for different nitroarenes. Both synthesis and catalytic reactions were performed using green conditions, and even further tested in a time-optimizing one-pot synthesis and catalysis experiment. The sustainability and environmental impact of the catalytic conditions were assessed by green metrics. Thus, this study provides an easily implementable synthesis, and efficient catalysis, while minimizing the environmental and health impact of the process.

A novel magnetic FSM-16 supported ionic liquid/Pd complex as a high performance and recyclable catalyst for the synthesis of pyrano[3,2-c]chromenes

In this work, Fe3O4@FSM-16/IL-Pd was successfully designed and synthesized via a new procedure of palladium(ii) complex immobilization onto magnetic FSM-16 using an ionic liquid, as a novel heterogeneous nanocatalyst. Multiple techniques were employed to characterize this magnetic nanocatalyst such as Fourier transform infrared spectroscopy (FT-IR), Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), Field Emission Scanning Electron Microscopy (FE-SEM), thermogravimetric analysis (TGA), Transmission electron microscopy (TEM), and Vibrating Sample Magnetometry (VSM). After complete characterization of the catalyst, its catalytic activity was used for the synthesis of pyrano[3,2-c]chromene-3-carbonitriles via the reaction of 4-hydroxycoumarin, aldehyde, and malononitrile under solvent-free conditions. Also, it can be recovered and reused several times without a significant decrease in its catalytic activity or palladium leaching.

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.

DFT-Enabled Development of Hemilabile (P∧N) Ligands for Gold(I/III) RedOx Catalysis: Application to the Thiotosylation of Aryl Iodides

Ligand-enabled oxidative addition of Csp2-X bonds to Au(I) centers has recently appeared as a valuable strategy for the development of catalytic RedOx processes. Several cross-coupling reactions that were previously considered difficult to achieve were reported lately, thus expanding the synthetic potential of gold(I) complexes beyond the traditional nucleophilic functionalization of π-systems. MeDalPhos has played an important role in this development and, despite several studies on alternative structures, remains, so far, the only general ligand for such process. We report herein the discovery and DFT-enabled structural optimization of a new family of hemilabile (P∧N) ligands that can promote the oxidative addition of aryl iodides to gold(I). These flexible ligands, which possess a common 2-methylamino heteroaromatic N-donor motif, are structurally and electronically tunable, beyond being easily accessible and affordable. The corresponding Au(I) complexes were shown to outperform the reactivity of (MeDalPhos)Au(I) in a series of alkoxy- and amidoarylations of alkenes. Their synthetic potential and comparatively higher reactivity were further highlighted in the thiotosylation of aryl iodides, a challenging unreported C-S cross-coupling reaction that could not be achieved under classical Pd(0/II) catalysis and that allows for general and divergent access to aryl sulfur derivatives.

Insertion of Molecular Oxygen into a Gold(III)-Hydride Bond

The use of molecular oxygen as an oxidant in chemical synthesis has significant environmental and economic benefits, and it is widely used as such in large-scale industrial processes. However, its adoption in highly selective homogeneous catalytic transformations, particularly to produce oxygenated organics, has been hindered by our limited understanding of the mechanisms by which O2 reacts with transition metals. Of particular relevance are the mechanisms of the reactions of oxygen with late transition metal hydrides as these metal centers are better poised to release oxygenated products. Homogeneous catalysis with gold complexes has markedly increased, and herein we report the synthesis and full characterization of a rare AuIII-H, supported by a diphosphine pincer ligand (tBuPCP = 2,6-bis(di-tert-butylphosphinomethyl)benzene). [(tBuPCP)AuIII-H]+ was found to cleanly react with molecular oxygen to yield a stable AuIII-OOH complex that was also fully characterized. Extensive kinetic studies on the reaction via variable temperature NMR spectroscopy have been completed, and the results are consistent with an autoaccelerating radical chain mechanism. The observed kinetic behavior exhibits similarities to that of previously reported PdII-H and PtIV-H reactions with O2 but is not fully consistent with any known O2 insertion mechanism. As such, this study contributes to the nascent fundamental understanding of the mechanisms of aerobic oxidation of late metal hydrides.

Direct synthesis of phenanthrenyl triflates from 1-biphenylyl-2-diazo-2-aryl ketones and triflic anhydride

A strategy for direct synthesis of phenanthrenyl triflates from 1-biphenylyl-2-diazo-2-aryl ketones and triflic anhydride is described. The reaction of 1-biphenylyl-2-diazo-2-aryl ketones with triflic anhydride proceeded smoothly in the presence of 2,6-di-tert-butylpyridine under mild conditions to produce phenanthrenyl triflates in high to excellent yields. The phenanthrenyl triflate products were demonstrated to be utilized as coupling partners in various coupling reactions. The proposed mechanism involves an intramolecular Friedel-Crafts reaction of a vinyl cation intermediate formed in situ.

Peptide Stereochemistry Effects from pKa-Shift to Gold Nanoparticle Templating in a Supramolecular Hydrogel

The divergent supramolecular behavior of a series of tripeptide stereoisomers was elucidated through spectroscopic, microscopic, crystallographic, and computational techniques. Only two epimers were able to effectively self-organize into amphipathic structures, leading to supramolecular hydrogels or crystals, respectively. Despite the similarity between the two peptides' turn conformations, stereoconfiguration led to different abilities to engage in intramolecular hydrogen bonding. Self-assembly further shifted the pKa value of the C-terminal side chain. As a result, across the pH range 4-6, only one epimer predominated sufficiently as a zwitterion to reach the critical molar fraction, allowing gelation. By contrast, the differing pKa values and higher dipole moment of the other epimer favored crystallization. The four stereoisomers were further tested for gold nanoparticle (AuNP) formation, with the supramolecular hydrogel being the key to control and stabilize AuNPs, yielding a nanocomposite that catalyzed the photodegradation of a dye. Importantly, the AuNP formation occurred without the use of reductants other than the peptide, and the redox chemistry was investigated by LC-MS, NMR, and infrared scattering-type near field optical microscopy (IR s-SNOM). This study provides important insights for the rational design of simple peptides as minimalistic and green building blocks for functional nanocomposites.

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.

Gold-Catalyzed 1,2-Aryl Shift and Double Alkyne Benzannulation

The tandem intramolecular hydroarylation of alkynes accompanied by a 1,2-aryl shift is described. Harnessing the unique electron-rich character of 1,4-dihydropyrrolo[3,2-b]pyrrole scaffold, we demonstrate that the hydroarylation of alkynes proceeds at the already occupied positions 2 and 5 leading to a 1,2-aryl shift. Remarkably, the reaction proceeds only in the presence of cationic gold catalyst, and it leads to heretofore unknown π-expanded, centrosymmetric pyrrolo[3,2-b]pyrroles. The utility is verified in the preparation of 13 products that bear six conjugated rings. The observed compatibility with various functional groups allows for increased tunability with regard to the photophysical properties as well as providing sites for further functionalization. Computational studies of the reaction mechanism revealed that the formation of the six-membered rings accompanied with a 1,2-aryl shift is both kinetically and thermodynamically favourable over plausible formation of products containing 7-membered rings. Steady-state UV/Visible spectroscopy reveals that upon photoexcitation, the prepared S-shaped N-doped nanographenes undergo mostly radiative relaxation leading to large fluorescence quantum yields. Their optical properties are rationalized through time-dependent density functional theory calculations. We anticipate that this chemistry will empower the creation of new materials with various functionalities.

Gold-Manganese Bimetallic Redox Coupling with Light

The classical Au(I)/Au(III) redox couple chemistry has been limited to constructing C-C and C-X bonds, and thus, the exploration of the elementary reaction of gold redox coupling is very significant to enrich its organometallic features. Herein, we report the first visible-light-mediated, external oxidant-free Au(I)/Au(III) redox couple using commercially available Mn2(CO)10 to generate Mn-Au(III)-Mn intermediates for bimetallic redox coupling. A wide range of structurally diverse heterodinuclear and polynuclear L-Au(I)-Mn-L' complexes (19 examples, up to >99% yields) are readily constructed, providing a robust strategy for the concise construction of Au-Mn complexes under mild reaction conditions. The mechanistic studies together with DFT calculations support the radical oxidative addition of •Mn(CO)5 to gold and bimetallic reductive elimination mechanisms from highly active Mn-Au(III)-Mn species, representing an important step toward an elementary reaction in gold chemistry research. Furthermore, the resulting Au-Mn complexes exhibit unique catalytic activity, with which divergent reductive coupling of nitroarenes can readily afford azoxybenzenes, azobenzenes, and hydrazobenzenes in moderate to good yields.

Hyperconjugative Aromaticity-Based Circularly Polarized Luminescence Enhancement in Polyaurated Heterocycles

Hyperconjugative aromaticity (HA) frequently appears in metalla-aromatics, but its effect on photophysical properties remains unexplored to date. Herein, we reveal two different HA scenarios in nearly isostructural triaurated indolium and benzofuranylium compounds. The biased HAs show a discernible effect on the spatial arrangement of metal atoms and thus tailor metal parentage in frontier orbitals and the HOMO-LUMO energy gap. Theoretical calculations and structural analyses demonstrate that HA not only influences the degree of electron delocalization over the trimetalated aromatic rings but also affects π-coordination of Au(I) and intercluster aurophilic interaction. Consequently, the triaurated benzofuranylium complex shows better photoluminescence performance (quantum yield up to 49.7%) over the indolium analogue. Furthermore, four pairs of axially chiral bibenzofuran-centered trinuclear and hexanuclear gold clusters were purposefully synthesized to correlate their HA-involved structures with the chiroptical response. The triaurated benzofuranylium complexes exhibit strong circular dichroism (CD) response in solution but CPL silence even in solid film. In contrast, the hexa-aurated homologues display strong CD and intense CPL signals in both aggregated state and solid film (luminescence anisotropy factor glum up to 10-3). Their amplified chiroptical response is finally ascribed to the dominant intermolecular exciton couplings of large assemblies formed through the HA-tailored aggregation of hexanuclear compounds.

Dinuclear Gold-Catalyzed para-Selective C-H Arylation of Undirected Arenes by Noncovalent Interactions

The regioselectivity of C-H functionalization is commonly achieved by directing groups, electronic factors, or steric hindrance, which facilitate the identification of reaction sites. However, such strategies are less effective for reactants such as simple monofluoroarenes due to their relatively low reactivity and the modest steric demands of the fluorine atom. Herein, we present an undirected gold-catalyzed para-C-H arylation of a wide array of monofluoroarenes using air-stable aryl silanes and germanes at room temperature. A high para-regioselectivity (up to 98 : 2) can be realized by utilizing a dinuclear dppm(AuOTs)2 (dppm=bis(diphenylphosphino)methane) as the catalyst and hexafluorobenzene as the solvent. This provides a general and practical protocol for the concise construction of structurally diverse para-arylated monofluoroarenes through C-H activation manner. It features excellent functional group tolerance and a broad substrate scope (>80 examples). Besides, this strategy is also robust for other simple monosubstituted arenes and heteroarenes. Our mechanistic studies and theoretical calculations suggest that para-C-H selectivity arises from highly electrophilic and structurally flexible dinuclear Ar-Au(III)-Au(I) species, coupled with noncovalent interaction induced by hexafluorobenzene.

Cascade Reactions of Aryl-Substituted Terminal Alkynes Involving in Situ-Generated α-Imino Gold Carbenes

An efficient, highly selective and divergent synthetic method to construct 2-substituted indoles and aryl-annulated carbazoles via the intermolecular generation of α-imino gold carbenes from terminal alkynes or diynes in combination with sulfilimines is disclosed. Importantly, the tandem reaction is proposed to proceed through an intermolecular gold carbene generation/C-H annulation followed by the activation of a second alkyne leading to 6-endo-dig cyclization, which is significantly different from previous dual activation or 1,6-carbene shift approaches for diyne systems. In the case of ortho-alkynylaniline as starting material, an unexpected regioselective formation of the indole moiety via the intermolecular path, instead of intramolecular hydroamination was discovered. This reactivity paved the way for a one-pot synthesis of the 11H-indolo [3,2-c] quinoline scaffold by exploiting the formed amino indole for a subsequent Pictet-Spengler reaction with aldehydes. The photophysical properties of the carbazoles indicated good violet-blue emission with quantum yields up to 40 %.

Remote Steric and Electronic Effects of N-Heterocyclic Carbene Ligands on Alkene Reactivity and Regioselectivity toward Hydrocupration Reactions: The Role of Expanded-Ring N-Heterocyclic Carbenes

The remote groups in N-heterocyclic carbene (NHC) ligands have a significant influence on metal-catalyzed reactions. We examine how remote bulkiness, electronic groups, and expanded-ring NHCs (ER-NHCs) influence alkene reactivity and regioselectivity toward hydrocupration using density functional theory calculations. The impact of remote steric bulkiness on the Cu-H insertion rate is analyzed, revealing a strong correlation between the steric substituent constant and rate ratio, where a bulky group increases the rate due to reduced steric effects in the transition state (TS). The steric properties of the examined catalysts (with a remote group R2 = CPh3, CHPh2, CH2Ph, CH3, and H) and their corresponding TSs are found to be modulated greatly by the remote steric substitution group and the ring size of the NHC ligand. Enhanced bulkiness enhances the nucleophilic Cu-H moiety. The remote electronic groups have a smaller impact on insertion barrier compared to that of steric hindrance. Furthermore, ER-NHC exploration indicates that NHCs with over five-membered rings have a significantly negative influence on the reaction rate. Finally, with a highly bulky group (R2 = CPh3), anti-Markovnikov insertion preference is attributed to high interaction energy and improved steric properties. Overall, our findings here provide valuable insights for the development of a more effective catalyst in metal-catalyzed reactions.

Mechanochemical preparation of strongly emissive monosubstituted triarylphosphane gold(i) compounds activated by hydrogen bonding driven aggregations

Gold(i) triarylphosphane compounds are a well-known class of coordination compounds displaying from mild to strong emissive properties. Mechanochemical approaches to the preparation, spectroscopic characterization, X-ray diffraction structural determination, and photophysical studies of green emissive neutral linear monophosphane or neutral pseudo-T-shaped or cationic bis-phosphane gold(i) compounds, are herein discussed. The mechanochemical approach to the preparation of gold(i) derivatives was particularly successful for ligands bearing the carboxylic group, while the preparation with esterified ligands yields better results with solvent-mediated methods. The introduction of carboxyl or ester substituents in one aryl group favors the ligand-centered emissions. The analysis of the origin of the emissions was elucidated on the basis of DFT calculations, addressing the emissive behavior to ligand-centered excited states, strongly affected by supramolecular reversible hydrogen bonding aggregation. The study indicates that the ligand with the carboxylic group is particularly suitable for the mechanochemical preparation of emissive gold(i) complexes for material science applications.

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.

Regio- and Stereoselective Carbon-Boron Bond Formation via Heterogeneous Palladium-Catalyzed Hydroboration of Enallenes

A highly efficient regio- and stereoselective heterogeneous palladium-catalyzed hydroboration reaction of enallenes was developed. Nanopalladium immobilized on microcrystalline cellulose (MCC) was successfully employed as an efficient catalyst for the enallene hydroboration reaction. The nanopalladium particles were shown by HAADF-STEM to have an average size of 2.4 nm. The cellulose-supported palladium catalyst exhibits high stability and provides vinyl boron products in good to high isolated yields (up to 90 %). The nanopalladium catalyst can be efficiently recycled and it was demonstrated that the catalyst can be used in 7 runs with a maintained high yield (>80 %). The vinylboron compounds prepared from enallenes are important synthetic intermediates that can be used in various organic synthetic transformations.

Gold-catalyzed endo-selective cyclization of alkynylcyclobutanecarboxamides: synthesis of cyclobutane-fused dihydropyridones

Cyclobutane-fused dihydropyridones can be efficiently synthesized by a completely endo-selective gold-catalyzed cyclization of alkynylcyclobutanes bearing an appended amide, which proceeds under mild conditions. The observed selectivity, which is reversed from that previously observed for the cyclization of related alcohols and acids, is supported by DFT calculations.

L-Shaped Heterobidentate Imidazo[1,5-a]pyridin-3-ylidene (N,C)-Ligands for Oxidant-Free AuI /AuIII Catalysis

In the last decade, major advances have been made in homogeneous gold catalysis. However, AuI /AuIII catalytic cycle remains much less explored due to the reluctance of AuI to undergo oxidative addition and the stability of the AuIII intermediate. Herein, we report activation of aryl halides at gold(I) enabled by NHC (NHC=N-heterocyclic carbene) ligands through the development of a new class of L-shaped heterobidentate ImPy (ImPy=imidazo[1,5-a]pyridin-3-ylidene) N,C ligands that feature hemilabile character of the amino group in combination with strong σ-donation of the carbene center in a rigid conformation, imposed by the ligand architecture. Detailed characterization and control studies reveal key ligand features for AuI /AuIII redox cycle, wherein the hemilabile nitrogen is placed at the coordinating position of a rigid framework. Given the tremendous significance of homogeneous gold catalysis, we anticipate that this ligand platform will find widespread application.

Construction of the quinobenzoxazine core via gold-catalyzed dual annulation of azide-tethered alkynones with anthranils

A new catalytic method for the construction of the quinobenzoxazine core has been developed employing the gold-catalyzed cyclization of o-azidoacetylenic ketones in the presence of anthranils. The overall process comprises of a gold-catalyzed 6-endo-dig cyclisation of o-azidoacetylenic ketone leading to a α-imino gold carbene and subsequent carbene transfer to anthranil leading to the 3-aryl-imino-quinoline-4-one intermediate, which undergoes 6π-electrocyclization and aromatization to form the central quinobenzoxazine core. This transformation provides a new approach to a diverse array of quinobenzoxazine structures, in addition to being scalable and having mild reaction conditions.

Mechanisms for Catalytic CO Oxidation on SiAun (n = 1-5) Cluster

Significant progress has been made in understanding the reactivity and catalytic activity of gas-phase and loaded gold clusters for CO oxidation. However, little research has focused on mixed silicon/gold clusters (SiAun) for CO oxidation. In the present work, we performed density function theory (DFT) calculations for a SiAun (n = 1-5) cluster at the CAM-B3LYP/aug-cc-pVDZ-PP level and investigated the effects on the reactivity and catalytic activity of the SiAun cluster for CO oxidation. The calculated results show that the effect is very low for the activation barriers for the formation of OOCO intermediates on SiAu clusters, SiAu3 clusters, and SiAu5 clusters in the catalytic oxidation of CO and the activation energy barriers for the formation of OCO intermediates on OSiAu3, OSiAu4, and OSiAu5. Our calculations show that, compared with the conventional small Au cluster, the incorporation of Si enhances the catalytic performance towards CO oxidation.

Regio- and stereospecific cis-hydrophenoxylation of ynamides with acidic phenols

Herein we describe a self-acid-enabled chemo-, regio-, and stereospecific cis-hydrophenoxylation of ynamides under reagent-free conditions. The presence of a non-polar solvent such as toluene was found to be beneficial to facilitate the rate-limiting proton transfer between phenols and ynamides to form an intimate ion pair, which is followed by a swift nucleophilic attack of the phenolate oxygen on keteniminium, fulfilling the overall hydrofunctionalization event. This protocol is operationally simple and easily scalable, tolerates a wide variety of functional groups, and shows good compatibility with the requirements of modern green chemistry.

Gold-Catalyzed Cascade Reaction of 2-Alkynyl Aryl Azides with Enecarbamates for Direct Synthesis of α-(3-Indolyl)ketones

α-(3-Indolyl)ketones are essential building blocks for the generation of biologically active molecules. We described a new method for the direct assembly of α-(3-indolyl)ketones through the cascade reaction of 2-alkynyl aryl azides with enecarbamates, in which the in situ generated α-imino gold carbene intermediate was trapped by enecarbamate to achieve umpolung reactivity of indole at the 3-position.

Integration of Gold Nanoparticles into Crosslinker-Free Polymer Particles and Their Colloidal Catalytic Property

This work demonstrates the incorporation of gold nanoparticles (AuNPs) into crosslinker-free poly(N-isopropylacrylamide), PNIPAM, particles in situ and the examination of their structural and catalytic properties. The formation process of the AuNPs across the crosslinker-free PNIPAM particles are compared to that of crosslinked PNIPAM particles. Given the relatively larger free volume across the crosslinker-free polymer network, the AuNPs formed by the in situ reduction of gold ions are detectably larger and more polydisperse, but their overall integration efficiency is slightly inferior. The structural features and stability of these composite particles are also examined in basic and alcoholic solvent environments, where the crosslinker-free PNIPAM particles still offer comparable physicochemical properties to the crosslinked PNIPAM particles. Interestingly, the crosslinker-free composite particles as a colloidal catalyst display a higher reactivity toward the homocoupling of phenylboronic acid and reveal the importance of the polymer network density. As such, the capability to prepare composite particles in a controlled polymer network and reactive metal nanoparticles, as well as understanding the structure-dependent physicochemical properties, can allow for the development of highly practical catalytic systems.

Impact of counteranions on N-heterocyclic carbene gold(i)-catalyzed cyclization of propargylic amide

N-Heterocyclic carbene (NHC) Au(i)-catalyzed organic synthesis has recently been receiving increasing attention, especially with the activation of alkynes. In contrast, counteranions, being widely problematic in Au(i)-catalyzed transformations, are commonly considered as innocent partners and are not respectably included in a computational model. Herein, we report density functional theory (DFT) investigations of the Au(i)-catalyzed cyclization of propargylic amides to exploit the mechanistic effect of several counteranions to shed some light for further future developments. Among the counteranions used in this study, NTf₂ ^-, ClO₄ ^-, TsO^-, TFA^-, TfO^-, MsO^-, and SbF₆ ^-, both the cyclization and protodeauration step favor the 5-exo-dig product over the 6-endo-dig product when the alkyne moiety is terminated with hydrogen. These anions reveal a crucial influence on the energy profile through lowering the barriers of the reaction. Mechanistically, the results obtained from all counteranions show that the protodeauration is slower than the cyclization. By using an energetic span model, the results clearly indicate that the rate-determining state is the protodeauration step for all counteranions, and thus protodeauration is the turnover-limiting step. The turnover frequency (TOF) results for the formation of the 5-exo-dig product show cyclization reactivity in the order of MsO^- > TFA^- > ClO₄ ^- > NTf₂ ^- > TfO^- > TsO^- ≫ SbF₆ ^-, whereas an order of TFA^- > MsO^- > NTf₂ ^- > TfO^- ≈ ClO₄ ^- > SbF₆ ^- ⋙ TsO^- is calculated for the protodeauration, suggesting that SbF₆ ^- and TsO^- are disfavored due to their slow protodeauration. In this regard, and for the 6-endo-dig pathway, our conclusions demonstrate an order of TfO^- > TFA^- > MsO^- > NTf₂ ^- > ClO₄ ^- > TsO^- ⋙ SbF₆ ^- for the cyclization and TFA^- > TsO^- > MsO^- > TfO^- > NTf₂ ^- > ClO₄ ^- ⋙ SbF₆ ^- for the protodeauration, advocating that the anions SbF₆ ^-, NTf₂ ^- and ClO₄ ^- are unlikely partners for the 6-endo-dig pathway because of their slow protodeauration. Finally, the findings here advise that any engineering of the counteranion to increase the efficiency of catalytic system would be more effective on the protodeauration step rather than the cyclization step.

An MCM-41-immobilized dichloro(pyridine-2-carboxylato)gold(III) complex: an efficient and recyclable catalyst for the annulation of anthranils and ynamides

A new mesoporous MCM-41-immobilized dichloro(pyridine-2-carboxylato)gold(III) complex [MCM-41-PicAuCl₂] was synthesized via an addition reaction of a dichloro(3-hydroxypyridine-2-carboxylato)gold(III) complex to triethoxy(3-isocyanatopropyl)silane, followed by immobilization on MCM-41 and was characterized by different physico-chemical techniques. In the presence of 5 mol% of MCM-41-PicAuCl₂, the annulation reaction between anthranils and ynamides proceeded smoothly under mild conditions to afford diverse 6- or 5-formylindoles with high atom economy and good to excellent yields. This new heterogenized gold(III) complex can be easily recovered through a simple filtration process and recycled more than seven times without any apparent loss of its catalytic efficiency.

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.

Versatile access to nitrogen-rich π-extended indolocarbazoles via a Pictet–Spengler approach

A bidirectional Pictet-Spengler Reaction allows easy access to nitrogen-rich aromatics with seven fused rings. Photophysical measurements and computational methods show significant differences to parent N-heteropolycycles with fewer nitrogen atoms.

Stable Monoareno-pentalenes with Two Olefinic Protons

A novel class of stable monoareno-pentalenes is introduced that have an olefinic proton on each five-membered ring of the pentalene subunit. Their synthesis was accomplished via a regioselective carbopalladation cascade reaction between ortho-arylacetyleno gem-dibromoolefins and TIPS-acetylene. These molecules could be experimental probes of magnetic (anti)aromaticity effects.

Gold-Catalyzed Intermolecular Alkyne Insertion into the N-S Bond in Sulfenamides

Gold-catalyzed reactions between sulfenamides and terminal alkynes proceeded via cis-insertion of alkynes into the N-S bond in sulfenamides, affording the corresponding β-sulfenylenamines in yields up to 90%. Mechanistic studies revealed that the reactions proceeded via nucleophilic attack of the sulfenamide nitrogen atom on the π-activated alkyne, followed by tosylate-assisted intermolecular transfer of the sulfenyl group.

Substituted Cyclopentannulated Tetraazapentacenes

Brominated pentannulated dihydrotetraazapentacenes were prepared by gold- or palladium-catalyzed 5-endo-dig cyclization of TIPS-ethynylated dihydrotetraazaacenes (TIPS = triisopropylsilyl). Post-functionalization was demonstrated by Sonogashira alkynylation and Rosenmund-von Braun cyanation. Calculations predict these species to act as n-type semiconductors, which was verified for two derivates through characterization in organic field-effect transistors.

Entropy-Induced Selectivity Switch in Gold Catalysis: Fast Access to Indolo[1,2-a]quinolines

New N-heterocyclic compounds for organic functional materials and their efficient syntheses are highly demanded. A surprising entropy-induced selectivity switch in the gold-catalyzed intramolecular hydroarylation of 2-ethynyl N-aryl indoles was found and its exploitation led to straightforward syntheses of indolo[1,2-a]quinolines. Experimental and computational mechanistic investigations gave insight into this uncommon selectivity phenomenon and into the special reactivity of the indolo[1,2-a]quinolines. The high functional group tolerance of this methodology enabled access to a diverse scope with high yields. In addition, bidirectional approaches, post-functionalization reactions, and π-extension of the core structure were feasible. An in-depth study of the photophysical properties explored the structure-effect relationship for different derivatives and revealed a high potential of these compounds for future applications as functional materials.

Access to Azepino-Annulated Benzo[c]carbazoles Enabled by Gold-Catalyzed Hydroarylation of Alkynylindoles and Subsequent Oxidative Cyclization

Herein, we report a facile and efficient synthetic method to construct azepino[1,2-a]indoles through a novel gold(I)-catalyzed intramolecular hydroarylation of alkynylindoles. A wide range of functional groups can be well tolerated in this transformation, and the corresponding highly functionalized azepino[1,2-a]indole skeletons were obtained in moderate to excellent yields. Subsequent oxidation of the products gave the interesting and valuable polycyclic carbazoles, which were widely used as the key building blocks in materials science.

Formation and Intramolecular Capture of α-Imino Gold Carbenoids in the Au(I)-Catalyzed [3 + 2] Reaction of Anthranils, 1,2,4-Oxadiazoles, and 4,5-Dihydro-1,2,4-Oxadiazoles with Ynamides

α-Imino gold carbenoid species have been recognized as key intermediates in a plethora of processes involving gold-activated alkynes. Here, we explored the pathways of the Au(I)-catalyzed [3 + 2] reaction between the mild nucleophiles: anthranil, 1,2,4-oxadiazole, or 4,5-dihydro-1,2,4-oxadiazole, and an ynamide, PhC≡C-N(Ts)Me, proceeding via the formation of the aforementioned α-imino gold carbene intermediate which, after intramolecular capture, regioselectively produces 2-amino-3-phenyl-7-acyl indoles, N-acyl-5-aminoimidazoles, or N-alkyl-4-aminoimidazoles, respectively. In all cases, the regioselectivity of the substituents at 2, 3 in the 7-acyl-indole ring and 4, 5 in the substituted imidazole ring is decided at the first transition state, involving the attack of nitrogen on the C1 or C2 carbon of the activated ynamide. A subsequent and steep energy drop furnishes the key α-imino gold carbene. These features are more pronounced for anthranil and 4,5-dihydro-1,2,4-oxadiazole reactions. Strikingly, in the 4,5-dihydro-1,2,4-oxadiazole reaction the significant drop of energy is due to the formation of an unstable α-imino gold carbene, which after a spontaneous benzaldehyde elimination is converted to a stabilized one. Compared to anthranil, the reaction pathways for 1,2,4-oxadiazoles or 4,5-dihydro-1,2,4-oxadiazoles are found to be significantly more complex than anticipated in the original research. For instance, compared to the formation of a five-member ring from the α-imino gold carbene, one competitive route involves the formation of intermediates consisting of a four-member ring condensed with a three-member ring, which after a metathesis and ring expansion led to the imidazole ring.