Alkyne Trifunctionalization via Divergent Gold Catalysis: Combining π-Acid Activation, Vinyl-Gold Addition, and Redox Catalysis

Gold-Catalyzed Alkyne Oxidative Cyclization/Mannich-Type Addition Cascade Reaction of Ynamides with 1,3,5-Triazinanes

Herein, a gold-catalyzed alkyne oxidative cyclization/Mannich-type addition cascade reaction of ynamides with 1,3,5-triazinanes in the presence of a Brønsted acid has been presented. A class of functionalized fluorenes bearing a quaternary carbon center was synthesized directly with moderate to excellent yields via in situ formed α-oxo carbenes using quinoline N-oxide as the oxidant under mild reaction conditions.

Straightforward Access to Isoindoles and 1,2-Dihydrophthalazines Enabled by a Gold-Catalyzed Three-Component Reaction

We describe herein a gold-catalyzed three-component reaction of o-alkynylbenzaldehydes, aryldiazonium salts, and trimethoxybenzene. This process enables the one-pot formation of valuable isoindoles and 1,2-dihydrophathalazines. The regioselectivity of the reaction is dictated by the nature of the aryldiazonium salt. Noticeably, the reaction is performed at room temperature under mild conditions and tolerates a variety of functional groups on both the o-alkynylbenzaldehyde and the aryldiazonium salt. Experimental mechanistic studies suggest that it is catalyzed by arylAu(III) species.

Integrating Au Catalysis and Engineered Amine Dehydrogenase for the Chemoenzymatic Synthesis of Chiral Aliphatic Amines

Direct synthesis of aliphatic amines from alkynes is highly desirable due to its atom economy and high stereoselectivity but still challenging, especially for the long-chain members. Here, a combination of Au-catalyzed alkyne hydration and amine dehydrogenase-catalyzed (AmDH) reductive amination was constructed, enabling sequential conversion of alkynes into chiral amines in aqueous solutions, particularly for the synthesis of long-chain aliphatic amines on a large scale. The production of chiral aliphatic amines with more than 6 carbons reached 36-60 g/L. A suitable biocatalyst [PtAmDH (A113G/T134G/V294A)], obtained by data mining and active site engineering, enabled the transformation of previously inactive long-chain ketones at high concentrations. Computational analysis revealed that the broader substrate scope and tolerance with the high substrate concentrations resulted from the additive effects of mutations introduced to the three gatekeeper residues 113, 134, and 294.

Azobenzene-Integrated NHC Ligands: A Versatile Platform for Visible-Light-Switchable Metal Catalysis

A new class of photoswitchable NHC ligands, named azImBA, has been developed by integrating azobenzene into a previously unreported imidazobenzoxazol-1-ylidene framework. These rigid photochromic carbenes enable precise control over confinement around a metal's coordination sphere. As a model system, gold(I) complexes of these NHCs exhibit efficient bidirectional E-Z isomerization under visible light, offering a versatile platform for reversibly photomodulating the reactivity of organogold species. Comprehensive kinetic studies of the protodeauration reaction reveal rate differences of up to 2 orders of magnitude between the E and Z isomers of the NHCs, resulting in a quasi-complete visible-light-gated ON/OFF switchable system. Such a high level of photomodulation efficiency is unprecedented for gold complexes, challenging the current state-of-the-art in photoswitchable organometallics. Thorough investigations into the ligand properties paired with structure-reactivity correlations underscored the unique ligand's steric features as a key factor for reactivity. This effective photocontrol strategy was further validated in gold(I) catalysis, enabling in situ photoswitching of catalytic activity in the intramolecular hydroalkoxylation and -amination of alkynes. Given the significance of these findings and its potential as a widely applicable, easily customizable photoswitchable ancillary ligand platform, azImBA is poised to stimulate the development of adaptive, multifunctional metal complexes.

Enantioselective Synthesis of C2-Quaternary Indolin-3-ones by Pt-Catalyzed Alkynylation of 2-Aryl-3H-indol-3-one with Alkynylsilanes

An efficient method for the synthesis of five-membered chiral propargylic amines from 2-aryl-3H-indol-3-one and alkynylsilanes has been developed. The reaction proceeded under the catalytic system of PtCl4, oxazoline-based ligand L11, Zn(CF3COO)2, and AcOH in DCE at 95 °C via in situ desilylation of TMS-alkynes. This methodology also highlights a new protocol for the in situ desilylation of alkynylsilanes. The reaction showed a broad substrate scope with good yields and enantioselectivity.

Advancing Gold Redox Catalysis: Mechanistic Insights, Nucleophilicity-Guided Transmetalation, and Predictive Frameworks for the Oxidation of Aryl Gold(I) Complexes

Gold redox catalysis, often facilitated by hypervalent iodine(III) reagents, offers unique reactivity but its progress is mainly hindered by an incomplete mechanistic understanding. In this study, we investigated the reaction between the gold(I) complexes [(aryl)Au(PR3 )] and the hypervalent iodine(III) reagent PhICl2 , both experimentally and computationally and provided an explanation for the formation of divergent products as the ligands bonded to the gold(I) center change. We tackled this essential question by uncovering an intriguing transmetalation mechanism that takes place between gold(I) and gold(III) complexes. We found that the ease of transmetalation is governed by the nucleophilicity of the gold(I) complex, [(aryl)Au(PR3 )], with greater nucleophilicity leading to a lower activation energy barrier. Remarkably, transmetalation is mainly controlled by a single orbital - the gold dx 2 -y 2 orbital. This orbital also has a profound influence on the reactivity of the oxidative addition step. In this way, the fundamental mechanistic basis of divergent outcomes in reactions of aryl gold(I) complexes with PhICl2 was established and these observations are reconciled from first principles. The theoretical model developed in this study provides a conceptual framework for anticipating the outcomes of reactions involving [(aryl)Au(PR3 )] with PhICl2 , thereby establishing a solid foundation for further advancements in this field.

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.

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.

Gold-Catalyzed 1,2-Dicarbofunctionalization of Alkynes with Organohalides

Herein, we report the first gold-catalyzed 1,2-dicarbofunctionalization of alkynes using organohalides as non-prefunctionalized coupling partners. The mechanism of the reaction involves an oxidative addition/π-activation mechanism in contrast to the migratory insertion/cis-trans isomerization pathway that is predominantly observed with other transition metals yielding products with anti-selectivity. Mechanistic insights include several control experiments, NMR studies, HR-MSMS analyses, and DFT calculations that strongly support the proposed mechanism.

Diastereo- and atroposelective synthesis of N-arylpyrroles enabled by light-induced phosphoric acid catalysis

The C-N axially chiral N-arylpyrrole motifs are privileged scaffolds in numerous biologically active molecules and natural products, as well as in chiral ligands/catalysts. Asymmetric synthesis of N-arylpyrroles, however, is still challenging, and the simultaneous creation of contiguous C-N axial and central chirality remains unknown. Herein, a diastereo- and atroposelective synthesis of N-arylpyrroles enabled by light-induced phosphoric acid catalysis has been developed. The key transformation is a one-pot, three-component oxo-diarylation reaction, which simultaneously creates a C-N axial chirality and a central quaternary stereogenic center. A broad range of unactivated alkynes were readily employed as a reaction partner in this transformation, and the N-arylpyrrole products are obtained in good yields, with excellent enantioselectivities and very good diastereoselectivities. Notably, the N-arylpyrrole skeletons represent interesting structural motifs that could be used as chiral ligands and catalysts in asymmetric catalysis.

Asymmetric Hydrative Aldol Reaction (HAR) via Vinyl-Gold Promoted Intermolecular Ynamide Addition to Aldehydes

Herein, we reported an intermolecular asymmetric hydrative aldol reaction through vinyl-gold intermediate under ambient conditions. This tandem alkyne hydration and sequential nucleophilic addition afforded a "base-free" approach to β-hydroxy amides with high efficiency (up to 95 % yields, >50 examples). Vinyl gold intermediate was applied as reactive nucleophile and Fe(acac)3 was used as the critical co-catalyst to prevent undesired protodeauration, allowing this transformation to proceed under mild conditions with good functional group tolerance and excellent stereoselectivity (>20 : 1 d.r. and up to 99 % ee).

Metal-free trifunctionalization of phenylacetylenes: an efficient one-pot two-step synthesis of gem-bis(dithiocarbamates)

The synthesis of phenacyl-bis(dithiocarbamates) has been reported by metal-free trifunctionalization of phenylacetylene systems by following a one-pot two-step strategy. Phenyl acetylene undergoes molecular bromine-mediated oxidative bromination followed by nucleophilic substitution with the freshly prepared dithiocarbamate salt which is prepared by the prompt reaction of amine and CS₂ in the presence of triethylamine base. A series of gem-bis(dithiocarbamates) are prepared using various secondary amines and phenylacetylene systems containing different substituents.

Site-Selective Arylation of Carboxamides from Unprotected Peptides

The amidated peptides are an important class of biologically active compounds due to their unique biological properties and wide applications as potential peptide drugs and biomarkers. Despite the abundance of free amide motifs (Asn, Gln, and C-terminal amide) in native peptides, late-stage modification of the amide unit in naturally occurring peptides remains very rare because of the intrinsically weak nucleophilicity of amides and the interference of multiple competing nucleophilic residues, which generally lead to undesired side reactions. Herein, chemoselective arylation of amides in unprotected polypeptides has been developed under an air atmosphere to afford the N-aryl amide peptides bearing various functional motifs. Its success relies on the combination of gold catalysis and silver salt to differentiate the relative inert amide among a collection of reactive nucleophilic amino acid residues (e.g., -NH₂, -OH, and -COOH), favoring the C-N bond coupling toward amides over other more nucleophilic groups. Experimental and DFT studies reveal a crucial role of the silver cation, which serves as a transient coordination mask of the more reactive reaction sites, overcoming the inherently low reactivity of amides. The excellent biocompatibility of this strategy has been applied to functionalize a wide range of peptide drugs and complex peptides. The application could be further extended to peptide labeling and peptide stapling.

Gold-catalyzed four-component multifunctionalization of alkynes

The alkyne unit is a versatile building block in organic synthesis and the development of selective multifunctionalization of alkynes is an important object of research in this field. Herein, we report an interesting gold-catalyzed, four-component reaction that achieves the oxo-arylfluorination or oxo-arylalkenylation of internal aromatic or aliphatic alkynes, efficiently breaking a carbon-carbon triple bond and forming four new chemical bonds. The reaction divergence can be controlled by site-directing functional groups in the alkynes; the presence of a phosphonate unit favors the oxo-arylfluorination, while the carboxylate motif benefits oxo-arylalkenylation. This reaction is enabled by an Au(I)/Au(III) redox coupling process using Selectfluor as both an oxidant and a fluorinating reagent. A wide range of structurally diverse α,α-disubstituted ketones, and tri- or tetra-substituted unsaturated ketones have been prepared in synthetically valuable yields and with excellent chemo-, regio- and stereoselectivity. The gram-scale preparation and late-stage application of complex alkynes have further enhanced their synthetic value.

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.

Copper-Catalyzed 6-endo-dig Cyclization-Coupling of 2-Bromoaryl Ketones and Terminal Alkynes toward Naphthyl Aryl Ethers in Water

The cyclization-coupling reaction of 2-bromoaryl ketones and terminal alkynes is first realized by copper catalysis, which produces polyfunctional naphthyl aryl ethers in moderate to excellent yields with broad substrate scope and good functional group tolerance. This reaction proceeds via 6-endo-dig cyclization and C(sp²)-O coupling using green H₂O as the unique solvent and 5-bromopyrimidin-2-amine as the critical additive. Mechanistically, a unique Cu(III)-acetylide probably is the key intermediate, which allows exclusive 6-endo-dig selectivity.

Chiral Hemilabile P,N-Ligand-Assisted Gold Redox Catalysis for Enantioselective Alkene Aminoarylation

Enantioselective, intermolecular alkene arylamination was achieved through gold redox catalysis. Screening of ligands revealed chiral P,N ligands as the optimal choice, giving alkene aminoarylation with good yields (up to 80 %) and excellent stereoselectivity (up to 99 : 1 er). As the first example of enantioselective gold redox catalysis, this work confirmed the feasibility of applying a chiral ligand at the gold(I) stage, with the stereodetermining step (SDS) at the gold(III) intermediate, thus opening up a new way to conduct gold redox catalysis with stereochemistry control.

One-Pot Difunctionalization of Aryldiazonium Salts for Synthesis of para-Azophenols

A novel difunctionalization of aryldiazonium salts was realized for the one-step generation of symmetric and asymmetric p-azophenols. This approach is proceeded by the sequentially regioselective aromatic C-O and C-N bond construction under mild reaction conditions, unlocking a new reaction strategy to facilitate the synthesis of p-azophenols.

Facile synthesis of diverse hetero polyaromatic hydrocarbons (PAHs) via the styryl Diels–Alder reaction of conjugated diynes

The intramolecular styryl Diels–Alder reaction with conjugated diynes under thermally stable triazole-gold (TA–Au) catalytic conditions and the sequential transformation through alkyne activation to access various PAHs with high efficiency was reported for the first time.

Switchable Divergent Synthesis in Gold-Catalyzed Difunctionalizations of o-Alkynylbenzenesulfonamides with Aryldiazonium Salts

Gold-catalyzed difunctionalizations of o-alkynylbenzenesulfonamides with aryldiazonium salts are reported herein. Upon irradiation with the blue LEDs, benzosultam products were formed via aminoarylation accompanied by the release of N₂. Without irradiation, aryldiazonium salts were engaged as efficient electrophiles, facilitating electrophilic deaurations of the vinyl-Au(I) intermediates, followed by tautomerization to give the N-aryl-substituted α-imino (E)-hydrazones. The regioselectivities of 6-endo-dig and 5-exo-dig cyclizations were excellent.

Gold-Catalyzed Transformation of Ynamides

Ynamide, a unique species with inherited polarization of nitrogen lone pair electron to triple bond, has been largely used for the developement of novel synthetic methods and the construction of unusual N-bearing heterocycles. The reaction versatility of ynamide on umpolung reactivity, radical reactions and asymmetric synthesis have been recently reviewed. This review provides an overall scenic view into the gold catalyzed transformation of ynamides. The ynamides reactivity towards nitrogen-transfer reagents, such as azides, nitrogen ylides, isoxazoles, and anthranils; oxygen atom-transfer reagents, like nitrones, sulfoxides, and pyridine N-oxides; and carbon nucleophiles under gold catalysis are herein uncovered. The scope as well the mechanistic insights of each reaction is also briefed.

The interplay of carbophilic activation and Au(I)/Au(III) catalysis: an emerging technique for 1,2-difunctionalization of C-C multiple bonds

Gold complexes have emerged as the catalysts of choice for various functionalization reactions of C-C multiple bonds due to their inherent carbophilic nature. In a parallel space, efforts to realize less accessible cross-coupling reactivity have led to the development of various strategies that facilitate the arduous Au(i)/Au(iii) redox cycle. The interplay of the two important reactivity modes encountered in gold catalysis, namely carbophilic activation and Au(i)/Au(iii) catalysis, has allowed the development of a novel mechanistic paradigm that sponsors 1,2-difunctionalization reactions of various C-C multiple bonds. Interestingly, the reactivity as well as selectivity obtained through this interplay could be complementary to that obtained by the use of various other transition metals that mainly involved the classical oxidative addition/migratory insertion pathways. The present review shall comprehensively cover all the 1,2-difunctionalization reactions of C-C multiple bonds that have been realized by the interplay of the two important reactivity modes and categorized on the basis of the method that has been employed to foster the Au(i)/Au(iii) redox cycle.