Nitrene Transfer and Carbene Transfer in Gold Catalysis

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.

Rhodium(III)-Catalyzed Intramolecular Cyclization and Sequential Aromatization of Ynamides with Propargyl Esters: Access to 2,5-Dihydropyrroles and Pyrroles

Disclosed herein is a rhodium(III)-catalyzed intramolecular cyclization of ynamides with propargyl esters. A variety of highly functionalized 2,5-dihydropyrroles were obtained in moderate to good yields with high E/Z selectivities. Subsequent oxidation of the products gave valuable pyrrole derivatives. Additionally, scale-up reactions and late-stage derivatizations highlight the potential synthetic utility of this methodology.

Synthesis and reactivity of an N-heterocyclic carbene-stabilized diazoborane

Diazo compounds and organic azides are widely used as reagents for accessing valuable molecules in multiple areas of fundamental and applied chemistry. Their capacity to undergo versatile chemical transformations arises from the reactive nature of an incipient dinitrogen molecule at the terminal position. In this work, we report the synthesis and characterization of an N-heterocyclic carbene (NHC)-stabilized diazoborane-a boron-centered analog of organic azides and diazoalkanes. The diazoborane displays a strong tendency to release dinitrogen, thus serving as a borylene source, in analogy to organic azides and diazoalkanes serving as nitrene and carbene sources, respectively. Also reminiscent of diazoalkane and organic azide reactivity, the diazoborane serves as a 1,3-dipole that undergoes uncatalyzed [3+2] cycloaddition with an unactivated terminal alkyne, affording a five-membered heterocycle after a two-step rearrangement.

Gold-catalyzed cyclization and cycloaddition in natural product synthesis

Covering: 2016 to mid 2023Transition metal catalysis, known for its remarkable capacity to expedite the assembly of molecular complexity from readily available starting materials in a single operation, occupies a central position in contemporary chemical synthesis. Within this landscape, gold-catalyzed reactions present a novel and versatile paradigm, offering robust frameworks for accessing diverse structural motifs. In this review, we highlighted a curated selection of publications in the past 8 years, focusing on the deployment of homogeneous gold catalysis in the ring-forming step for the total synthesis of natural products. These investigations are categorized based on the specific ring formations they engender, accentuating the prevailing gold-catalyzed methodologies applied to surmount intricate challenges in natural products synthesis.

Regiodivergent Synthesis of 4- and 5-Sulfenyl Oxazoles from Alkynyl Thioethers

The regiodivergent synthesis of 4- and 5-sulfenyl oxazoles from 1,4,2-dioxazoles and alkynyl thioethers has been achieved. Gold-catalysed conditions are used to favour the formation of 5-sulfenyl oxazoles via β-selective attack of the nitrenoid relative to the sulfenyl group. In contrast, 4-sulfenyl oxazoles are formed by α-selective reaction under Brønsted acid conditions from the same substrates. The nature of stabilising gold-sulfur interactions have been investigated by natural bond orbital analysis, showing that the S→Au interactions are significantly stronger in the intermediate that favours the 5-sulfenyl oxazoles. A kinetic survey identifies catalyst inhibition processes. This study into the regiodivergent methods includes the development of telescoped annulation-oxidation protocols for regioselective access to oxazole sulfoxides and sulfones.

Gold-Catalyzed Regio- and Stereoselective Alkenylation of Quinoline N-Oxides with Allenamides

A gold-catalyzed cycloaddition/ring opening of allenamides with quinoline N-oxides has been developed, which provides C2-alkenylated quinolines with high E selectivity in moderate to high yields. It is noted that quinoline N-oxides with a C8 or C7 substituent are crucial for this catalytic reaction.

Gold catalysis in quinoline synthesis

Quinolines are biologically and pharmaceutically important N-heterocyclic aromatic compounds, which have broad applications in medicinal chemistry. Thus, their efficient synthesis has attracted extensive attention, and a broad range of synthetic strategies have been established. Of note, gold-catalyzed methodologies for the synthesis of quinolines have greatly advanced this field. Various gold-catalyzed intermolecular annulation reactions, such as annulations of aniline derivatives with carbonyl compounds or alkynes, annulations of anthranils with alkynes, and annulations based on A3-coupling reactions, as well as intramolecular cyclization reactions of azide-tethered alkynes, 1,2-diphenylethynes, and 2-ethynyl N-aryl indoles, have been developed. This review provides an overview of this exciting research area. Typical achievements in reaction methodologies and plausible reaction mechanisms are summarized.

Advances in gold catalyzed synthesis of quinoid heteroaryls

This review explores recent advancements in synthesizing quinoid heteroaryls, namely quinazoline and quinoline, vital in chemistry due to their prevalence in natural products and pharmaceuticals. It emphasizes the rapid, highly efficient, and economically viable synthesis achieved through gold-catalyzed cascade protocols. By investigating methodologies and reaction pathways, the review underscores exceptional yields attainable in the synthesis of quinoid heteroaryls. It offers valuable insights into accessing these complex structures through efficient synthetic routes. Various strategies, including cyclization, heteroarylation, cycloisomerization, cyclo-condensation, intermolecular and intramolecular cascade reactions, are covered, highlighting the versatility of gold-catalyzed approaches. The comprehensive compilation of different synthetic approaches and elucidation of reaction mechanisms contribute to a deeper understanding of the field. This review paves the way for future advancements in synthesizing quinoid heteroaryls and their applications in drug discovery and materials science.

Kinetic resolution of 1-(1-alkynyl)cyclopropyl ketones via gold-catalyzed divergent (4 + 4) cycloadditions: stereoselective access to furan fused eight-membered heterocycles

Chiral eight-membered heterocycles comprise a diverse array of natural products and bioactive compounds, yet accessing them poses significant challenges. Here we report a gold-catalyzed stereoselective (4 + 4) cycloaddition as a reliable and divergent strategy, enabling readily accessible precursors (anthranils and ortho-quinone methides) to be intercepted by in situ generated gold-furyl 1,4-dipoles, delivering previously inaccessible chiral furan/pyrrole-containing eight-membered heterocycles with good results (56 examples, all >20 : 1 dr, up to 99% ee). Moreover, we achieve a remarkably efficient kinetic resolution (KR) process (s factor up to 747). The scale-up synthesis and diversified transformations of cycloadducts highlight the synthetic potential of this protocol. Computational calculations provide an in-depth understanding of the stereoselective cycloaddition process.

Light Induced Cobalt(III) Carbene Radical Formation from Dimethyl Malonate As Carbene Precursor

Radical-type carbene transfer catalysis is an efficient method for the direct functionalization of C-H and C=C bonds. However, carbene radical complexes are currently formed via high-energy carbene precursors, such as diazo compounds or iodonium ylides. Many of these carbene precursors require additional synthetic steps, have an explosive nature, or generate halogenated waste. Consequently, the utilization of carbene radical catalysis is limited by specific carbene precursors that access the carbene radical intermediate. In this study, we generate a cobalt(III) carbene radical complex from dimethyl malonate, which is commercially available and bench-stable. EPR and NMR spectroscopy were used to identify the intermediates and showed that the cobalt(III) carbene radical complex is formed upon light irradiation. In the presence of styrene, carbene transfer occurred, forming cyclopropane as the product. With this photochemical method, we demonstrate that dimethyl malonate can be used as an alternative carbene precursor in the formation of a cobalt(III) carbene radical complex.

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.

Cooperative Gold(I)/DMAP Catalysis Enabled (2 + 3) Cycloadditions of Yne-Enones with Oxindole-Derived MBH Carbonates

A cooperative gold(I)/DMAP system catalyzes the (2 + 3) cycloadditions of yne-enones with oxindole-derived Morita-Baylis-Hillman (MBH) carbonates, yielding diverse bispiro-cyclopentene oxindole products. The mild, scalable protocol demonstrates broad substrate scope and excellent chemo- and diastereoselectivity. Mechanistic study reveals pivotal roles of both catalysts in the unique (2 + 3) cycloaddition. This strategy showcases superiority in achieving transformation with unique chemoselectivity and excellent diastereoselectivity, unattainable through traditional monocatalytic methodologies.

Gold-Catalyzed Oxidation Reactions of Thioalkynes with Quinoline N-Oxides: A DFT Study

Mechanistic investigation of the gold-catalyzed oxidative reactions of thioalkynes with quinoline N-oxides was performed using density functional theory (DFT) calculations. For the oxidative rearrangement of thioalkynes with quinoline N-oxide to yield the same products, the Cβ-oxidation of thioalkynes was predicted to be competitive with Cα-oxidation, with the Cβ-oxidative process slightly more favorable. However, for the oxidative alkenylation of propargyl aryl thioethers with quinoline N-oxides, the Cβ-oxidation of thioether by quinoline N-oxide generated the product 3-hydroxy-1-alkylidene phenylthiopropan-2-one. Moreover, the ring opening of the four-membered sulfonium intermediate was achieved by the nucleophilic attack of quinoline N-oxide rather than a water molecule.

Synthesis of two nitrogen-containing polyaromatic compounds through gold catalysis/DBU-promoted cyclizations

This work reports an efficient synthesis of novel benzo[7,8]indolizino[2,3,4,5-ija]quinazoline derivatives between 2-(2-ethynylaryl)acetonitriles 1 and anthranils 2. The synthetic approach involves the initial formation of 7-formylindole intermediates that can be implemented by DBU to activate a novel indole-nitrile-aldehyde cyclization.

Gold Catalysis, Asymmetric Friedel-Crafts Alkylation Cascade for One-Pot Synthesis of Chiral Dihydrocarbazoles and Dihydrodibenzofurans

A one-pot gold-catalyzed acyl migration followed by ytterbium-catalyzed asymmetric Friedel-Crafts alkylation is disclosed, leading to the rapid synthesis of chiral dihydrocarbazoles and dihydrodibenzofuran in generally moderate to good overall yields with good to excellent enantioselectivities. The gold-catalyzed acyl migration of propargyl acetates generates α-ylidene-β-diketones with high E/Z ratios, which are then subjected to the ytterbium-catalyzed asymmetric Friedel-Crafts alkylation without any purification. Importantly, this protocol provides a new type of substrate for asymmetric Friedel-Crafts alkylation.

Gold/Chiral Amine Relay Catalysis Enables Asymmetric Synthesis of C2-Quaternary Indolin-3-ones

A mild and effective strategy for the asymmetric synthesis of C2-quaternary indolin-3-ones from 2-alkynyl arylazides and ketones by gold/chiral amine relay catalysis is described. In this reaction, 2-alkynyl arylazides undergo gold-catalyzed cyclization, nucleophilic attack, and oxidation to form intermediate 2-phenyl-3H-indol-3-ones, followed by an l-proline-catalyzed asymmetric Mannich reaction with ketones, to afford corresponding products in satisfactory yields with excellent enantio- and diastereoselectivities.

Catalytic Intermolecular Deoxygenative Coupling of Carbonyl Compounds with Alkynes by a Cp*Mo(II)-Catalyst

Carbonyl is highly accessible and acts as an essential functional group in chemical synthesis. However, the direct catalytic deoxygenative functionalization of carbonyl compounds via a putative metal carbene intermediate is a formidable challenge due to the requirement of a high activation energy for the cleavage of strong C═O double bonds. Here, we report a class of bench stable and readily available Cp*Mo(II)-complexes as efficient deoxygenation catalysts that could catalyze the direct intermolecular deoxygenative coupling of carbonyl compounds with alkynes. Enabled by this powerful Cp*Mo(II)-catalyst, various valuable heteroarenes (10 different classes) were obtained in generally good yields and remarkable chemo- and regioselectivities. Mechanistic studies suggested that this reaction might proceed via a sequence of C═O double bonds cleavage, carbene-alkyne metathesis, cyclization, and aromatization processes. This strategy not only provided a general catalytic platform for the rapid preparation of heteroarenes but also opened a new window for the applications of Cp*Mo(II)-catalysts in organic synthesis.

Nucleophile-Controlled Trapping of Gold Carbene by Nitriles and Water: Synthesis of 5H-Pyrimido[5,4-b]indoles and 2-Benzylidene-3-indolinones

A gold-catalyzed, nucleophile-controlled cascade reaction of N-(2-azidophenyl-ynyl)methanesulfonamides with nitriles and water is described that provides structurally diverse 5H-pyrimido[5,4-b]indoles and 2-benzylidene-3-indolinones in good to excellent yields. Mechanistic studies indicate that the β-sulfonamido-α-imino gold carbene is the key intermediate which is generated through the gold-catalyzed cyclization of N-(2-azidophenyl-ynyl)methanesulfonamides and undergoes formal [4 + 2] cascade annulation with nitriles and intramolecular SN2' type reaction with water, respectively.

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.

f-Element heavy pnictogen chemistry

The coordination and organometallic chemistry of the f-elements, that is group 3, lanthanide, and actinide ions, supported by nitrogen ligands, e.g. amides, imides, and nitrides, has become well developed over many decades. In contrast, the corresponding f-element chemisty with the heavier pnictogen analogues phosphorus, arsenic, antimony, and bismuth has remained significantly underdeveloped, due largely to a lack of suitable synthetic methodologies and also the inherent hard(f-element)-soft(heavier pnictogen) acid-base mismatch, but has begun to flourish in recent years. Here, we review complexes containing chemical bonds between the f-elements and heavy pnictogens from phosphorus to bismuth that spans five decades of endeavour. We focus on complexes whose identity has been unambiguously established by structural authentication by single-crystal X-ray diffraction with respect to their synthesis, characterisation, bonding, and reactivity, in order to provide a representative overview of this burgeoning area. By highlighting that much has been achieved but that there is still much to do this review aims to inspire, focus and guide future efforts in this area.

Phosphinidenes: Fundamental Properties and Reactivity

Phosphinidenes are heavy congeners of nitrenes that have been broadly used as in situ reagents in synthetic phosphorus chemistry and also serve as versatile ligands in coordination with transition metals. However, the detection of free phosphinidenes is largely challenged by their high reactivity and also the lack of suitable synthetic methods, rendering the knowledge about the fundamental properties of this class of low-valent phosphorus compounds limited. Recently, an increasing number of free phosphinidenes bearing prototype structural and bonding properties have been prepared for the first time, thus enabling the exploration of their distinct reactivity from the nitrene analogues. This Concept article will discuss the experimental approaches for the generation of the highly unstable phosphinidenes and highlight their distinct reactivity from the nitrogen analogues so as to stimuate future studies about their potential applications in phosphorus chemistry.

Gold-Catalyzed Cyclization of Yndiamides with Isoxazoles via α-Imino Gold Fischer Carbenes

Gold catalysis is an important method for alkyne functionalization. Here we report the gold-catalyzed formal [3+2] aminative cyclization of yndiamides and isoxazoles in a direct synthesis of polysubstituted diaminopyrroles, which are important motifs in drug discovery. Key to this process is the formation, and subsequent cyclization, of an α-imino gold Fischer carbene, which represents a new type of gold carbene intermediate. The reaction proceeds rapidly under mild conditions, with high regioselectivity being achieved by introducing a subtle steric bias between the nitrogen substituents on the yndiamide. DFT calculations revealed that the key to this regioselectivity was the interconversion of isomeric gold keteniminiun ions via a low-barrier π-complex transition state, which establishes a Curtin-Hammett scenario for isoxazole addition. By using benzisoxazoles as substrates, the reaction outcome could be switched to a formal [5+2] cyclization, leading to 1,4-oxazepines.

Catalyst-Controlled Divergent Generations and Transformations of α-Carbonyl Cations from Alkynes

α-Carbonyl cations are the umpolung forms of the synthetically fundamental α-carbonyl carbanions. They are highly reactive yet rarely studied and utilized species and their precursors were rather limited. Herein, we report the catalyst-controlled divergent generations of α-carbonyl cations from single alkyne functionalities and the interception of them via Wagner-Meerwein rearrangement. Two chemodivergent catalytic systems have been established, leading to two different types of α-carbonyl cations and, eventually, two different types of products, i.e. the α,β- and β,γ-unsaturated carbonyl compounds. Broad spectrum of alkynes including aryl alkyne, ynamide, alkynyl ether, and alkynyl sulfide could be utilized and the migration priorities of different groups in the Wagner-Meerwein rearrangement step was elucidated. Density functional theory calculations further supported the intermediacy of α-carbonyl cations via the N-O bond cleavage in both the two catalytic systems. Another key feature of this methodology was the fragmentation of synthetically inert tert-butyl groups into readily transformable olefin functionalities. The synthetic potential was highlighted by the scale-up reactions and the downstream diversifications including the formal synthesis of nicotlactone B and galbacin.

Copper-catalyzed atroposelective formal [4+1] annulation of 1,2-diketones with vinyl cations

The enantioselective transformation of easily accessible 1,2-diketones represents a quick pathway towards enantioenriched molecules. Herein, we disclose a copper-catalyzed atroposelective formal [4+1] annulation of 1,2-diketones with vinyl cations, enabling the efficient and atom-economical construction of axially chiral arylpyrroles bearing 1,3-dioxole moieties with good to excellent enantioselectivities under mild reaction conditions. Importantly, this methodology constitutes the first enantioselective formal [4+1] annulation of 1,2-diketones.

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.

Gold-Catalyzed Oxidative Cyclization/C-C Bond Cleavage of Ynones with External Nucleophiles: Synthesis of Linear Functionalized N-Tosylamides

A gold-catalyzed oxidative cyclization/nucleophilic addition/C-C bond cleavage reaction of ynones with various nucleophiles has been developed. This methodology allows for the formation of highly functionalized linear N-Ts amides with broad substrate scope, high efficiency, and general tolerance of functional groups. A wide range of nucleophiles such as alcohols, water, and amines including aryl and alkyl amines are compatible with the current method. The C-C triple bond cleavage of the ynone substrate was observed during the process.

Construction of Cyclic Nitrones Enabled by Photodriven and Gold-Catalyzed 1,3-Azaprotio Transfer of Allenyloximes

A protocol was developed to construct five- to seven-membered cyclic nitrones through the gold-catalyzed 1,3-azaprotio transfer of allenyloximes under photoirradiation. The photoisomerization of oximes was suggested to convert the inert stereoisomer to a reactive one. This photodriven and gold-catalyzed ring formation could be further extended to the thermodynamically stable aryl ketoximes with an E-configuration, which previously displayed chemical inertness in the absence of light irradiation.

Catalyst- and Substrate-Controlled Regiodivergent Synthesis of Carbazoles through Gold-Catalyzed Cyclizations of Indole-Functionalized Alkynols

A wide variety of regioselectively substituted carbazole derivatives can be synthesized by the gold-catalyzed cyclization of alkynols bearing an indol-3-yl and an additional group at the homopropargylic positions. The regioselectivity of the process can be controlled by both the oxidation state of the gold catalyst and the electronic nature of the substituents of the alkynol moiety. The 1,2-alkyl migration in the spiroindoleninium intermediate, generated after indole attack to the activated alkyne, is favored with gold(I) complexes and for electron-rich aromatic substituents at the homopropargylic position, whereas the 1,2-alkenyl shift is preferred when using gold(III) salts and for alkyl or non-electron-rich aromatic groups.

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 %.

Twofold Heteroannulation Reactions Enabled by Gold(I)/Zinc(II) Catalysts: Synthesis of Amine-Substituted Diaryl[c,h][1,6]naphthyridines

A straightforward and atom-economical one-pot protocol catalyzed by gold(I) and zinc(II) for the synthesis of amine-substituted diaryl[c,h][1,6]naphthyridines from two different aromatic nitriles has been showcased. This dual-catalytic strategy is highly efficient, offering an array of tetracyclic heteroaromatic products in good to excellent yields. Furthermore, the base can efficiently catalyze the second annulation step, yielding structurally unique thiophene-fused [1,6]naphthyridines in good yields.

Catalytic 4-exo-dig carbocyclization for the construction of furan-fused cyclobutanones and synthetic applications

Cyclobutanone is a strained motif with broad applications, while direct assembly of the aromatic ring fused cyclobutanones beyond benzocyclobutenone (BCB) skeletons remains challenging. Herein, we report a Rh-catalyzed formal [3+2] annulation of diazo group tethered alkynes involving a 4-exo-dig carbocyclization process, providing a straightforward access to furan-fused cyclobutanones. DFT calculations disclose that, by comparison to the competitive 5-endo-dig process, 4-exo-dig carbocyclization is mainly due to lower angle strain of the key sp-hybridized vinyl cationic transition state in the cyclization step. Using less reactive catalysts Rh2(carboxylate)4 is critical for high selectivity, which is explained as catalyst-substrate hydrogen bonding interaction. This method is proved successful to direct access previously inaccessible and unknown furan-fused cyclobutanone scaffolds, which can participate in a variety of post-functionalization reactions as versatile synthetic blocks. In addition, preliminary antitumor activity study of these products indicates that some molecules exhibite significant anticancer potency against different human cancer cell lines.

Gold-catalyzed carbocyclization and imidization of alkyne-tethered diazo compounds with nitrosoarenes for the synthesis of nitrones and naphthalene derivatives

A gold-catalyzed carbocyclization/imidization cascade reaction has been developed,leading a facile access to the synthesis of functionalized nitrones in moderate to good yields under mild conditions. The reaction initiated by a catalytic 6-endo-dig diazo-yne carbocyclization to form the key endocyclic vinyl carbene from alkyne-tethered diazo compounds, followed by addition with nitrosoarenes that features an imidization process. Notably, these resulting nitrone products could be smoothly converted into different substituted naphthalenol analogues, such as 4-aminonaphthalen-1-ol, naphthalene-1,4-dione, and naphthalene-1,4-diol derivatives, in high yields. Moreover, the generated products exhibited potential tumor suppression activity in tested cancer celllines; compound 3c (HCT116 cells, IC50 = 7.41 μM; MCF-7 cells, IC50 = 14.28 μM) exhibits higher anticancer potency than other tested compounds.

BN Analogue of Butadiyne: A Platform for Dinitrogen Release and Reduction

Exploration of the metallomimetic chemistry of main group elements is of the utmost importance from the perspective of both fundamental research and potential applications. Here, we report the synthesis, bonding analysis, and reactivities of an isolable diiminoborane, Mes*B≡N─N≡BMes* (Mes* = 2,4,6-tri-tert-butylphenyl) (1), a BN analogue of butadiyne. This species is characterized by a conjugated B≡N─N≡B moiety, a structural feature that enables the controlled release of N2 when it is exposed to organic nitriles. Furthermore, the N2 unit in 1 could be reduced to an ammonium salt via cleavage of the BN triple bond. Our work shows a rare example of an unsaturated BN system, serving as a platform for both the release and reduction of N2. This discovery opens new pathways and holds substantial influence on the future design of functional main group N2 species.

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).

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.

Regioselective access to polycyclic N-heterocycles via homogeneous copper-catalyzed cascade cyclization of allenynes

Polycyclic N-heterocycles are important structural motifs commonly found in bioactive compounds, however, their selective construction via the cyclization of allenynes remains challenging yet highly desirable. Here we show a homogeneous copper-catalyzed hetero Diels-Alder (HDA) reaction of allenynes with cis-diazenes (PTAD, 4-phenyl-1,2,4-triazoline-3,5-dione), allowing the practical and efficient synthesis of a diverse array of valuable polycyclic N-heterocycles. A temperature-controlled and stereocontrolled chemoselectivity of the reaction was observed, leading to the chemodivergent synthesis of tetracyclic pyrrolidines, pentacyclic triazepanes and tricyclic pyrrolidines. Compared with related Au-catalyzed cyclization of allenynes, this copper catalysis achieves cyclization of allenynes terminating in C-N bond formation via the HDA reaction.

Combining ligand-enhanced backdonation and steric shielding to stabilize a mono-substituted Au(I) carbene

A mono-substituted Au(I) carbene was prepared by reacting HC(N₂)(Dmp) (Dmp = 2,6-dimesitylphenyl) with an (o-carboranyl)-diphosphine AuNTf₂ complex. It is stable up to -10 °C and was characterized by NMR spectrocopy. According to DFT calculations, the chelating P^P ligand enhances Au → C_carb backdonation, while the Dmp substituent provides kinetic stabilization but does not bias the electronic structure of the carbene complex.

Gold-Catalyzed Anti-Markovnikov Oxidation of Au-Allenylidene to Generate Alkylidene Ketene

It remains a long-standing challenge to directly convert alkynes to carboxylic derivatives. Herein, a unexpectedly anti-Markovnikov oxidation of a unique Au-allenylidene pathway instead of a traditional α-oxo gold carbene routine is disclosed for in situ formation and transformation of highly unsaturated alkylidene ketenes, which are subsequently trapped by broad nucleophiles such as alcohols, phenols, water, amines, and sulfoximines to easily access α,β-unsaturated drugs and natural product derivatives by a multicomponent reaction. Based on this scenario, polyacrylate and polyacrylamide are efficiently afforded by corresponding multicomponent polymerization.

Mn(I)-catalyzed sigmatropic rearrangement of β, γ-unsaturated alcohols

Sigmatropic rearrangement provides a versatile strategy to site-selectively reorganize carbon-skeleton with high atom- and step-economy. Herein, we disclose a Mn(I)-catalyzed sigmatropic rearrangement of β, γ-unsaturated alcohols via C-C σ bond activation. A variety of α-aryl-allylic alcohols and α-aryl-propargyl alcohols could undergo in-situ 1,2- or 1,3- sigmatropic rearrangements to allow for converting to complex structural arylethyl- and arylvinyl- carbonyl compounds under a simple catalytic system. More importantly, this catalysis model can be further applied to assemble macrocyclic ketones through bimolecular [2n + 4] coupling-cyclization and monomolecular [n + 1] ring-extension. The presented skeleton rearrangement would be a useful tool complementary to the traditional molecular rearrangement.

Copper-Catalyzed Tandem Cyclization/Direct C(sp2)-H Annulation of Azide-Ynamides via α-Imino Copper Carbenes: Access to Azepino[2,3-b:4,5-b']diindoles

A novel copper-catalyzed tandem cyclization/direct C(sp²)-H annulation of phenyl azide-ynamides via α-imino copper carbenes has been developed, which provides a concise and flexible approach for the construction of a range of valuable azepino[2,3-b:4,5-b']diindoles in mostly good to excellent yields with high chemoselectivities. This tandem reaction also exhibits a broad substrate scope, excellent functional group tolerance, simple operation, and mild reaction conditions.

Modular Access to meta-Substituted Benzenes via Mo-Catalyzed Intermolecular Deoxygenative Benzene Formation

The substituted benzene derivatives are essential to organic synthesis, medicinal chemistry, and material science. However, the 1,3-di- and 1,3,5-trisubstituted benzenes are far less prevalent in small-molecule drugs than other substitution patterns, likely due to the lack of robust, efficient, and convenient synthetic methods. Here, we report a Mo-catalyzed intermolecular deoxygenative benzene-forming reaction of readily available ynones and allylic amines. A wide range of unsymmetric and unfunctionalized 1,3-di- and 1,3,5-trisubstituted benzenes were obtained in up to 88% yield by using a commercially available molybdenum catalyst. The synthetic potential of the method was further illustrated by synthetic transformations, a scale-up synthesis, and derivatization of bioactive molecules. Preliminary mechanistic studies suggested that this benzene-forming process might proceed through a Mo-catalyzed aza-Michael addition/[1,5]-hydride shift/cyclization/aromatization cascade. This strategy not only provided a facile, robust, and modular approach to various meta-substituted benzene derivatives but also demonstrated the potential of molybdenum catalysis in the challenging intermolecular deoxygenative cross-coupling reactions.

Gold self-relay catalysis enabling annulative oxygenation of propargylic alcohols with O-nucleophiles

A new gold(I) self-relay catalysis reaction enabling the annulative oxygenation of propargylic alcohols with various O-nucleophiles, such as carboxylic acids, alcohols and TBHP, is reported, producing a series of functionalized benzofurans in moderate to good yields under mild conditions. This protocol benefits from the π- and σ-Lewis acid capability of gold complexes, demonstrating high molecular convergence, broad substrate flexibility, high functional group compatibility and mild conditions.

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, Au^I /Au^III catalytic cycle remains much less explored due to the reluctance of Au^I to undergo oxidative addition and the stability of the Au^III 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 Au^I /Au^III 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 Spirolactams by Unexpected Oxidative Cyclization of Alkenyl Diynes via Copper Catalysis

A copper-catalyzed oxidative cyclization of alkenyl N-propargyl ynamides is described. This protocol enables the practical synthesis of diverse spirocyclic γ-lactams bearing an exocyclic double bond with generally high Z/E selectivity in moderate to good yields. Importantly, this copper-catalyzed oxidative cyclization demonstrates a distinctive selectivity in comparison with the related gold catalysis.

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.

PPh3-Mediated Cascade Reaction of 2-Alkynylnitrobenzenes and Thioureas for the Construction of Imidazo[4,5-b]indole-2-thiones

A simple method for the preparation of imidazo[4,5-b]indole-2-thiones from 2-alkynylnitrobenzenes and thioureas is described. In the reaction, a Wittig-like process was triggered by PPh₃ and followed by a cyclization step. The products were afforded in yields of 70-98% under mild conditions. Additionally, the 2-alkynylnitrobenzenes were stable and could be prepared via a simple coupling step.

Copper-Catalyzed Enantioselective Doyle-Kirmse Reaction of Azide-Ynamides via α-Imino Copper Carbenes

[2,3]-Sigmatropic rearrangement reaction involving sulfonium ylide (Doyle-Kirmse reaction) generated from metal carbenes represents one of the powerful methods for the construction of C(sp³ )-S and C-C bonds. Although significant advances have been achieved, the asymmetric versions via the generation of sulfonium ylides from metal carbenes have been rarely reported to date, and they have so far been limited to diazo compounds as metal carbene precursors. Here, we describe a copper-catalyzed enantioselective Doyle-Kirmse reaction via azide-ynamide cyclization, leading to the practical and divergent assembly of an array of chiral [1,4]thiazino[3,2-b]indoles bearing a quaternary carbon stereocenter in generally moderate to excellent yields and excellent enantioselectivities. Importantly, this protocol represents a unique catalytic asymmetric Doyle-Kirmse reaction via a non-diazo approach and an unprecedented asymmetric [2,3]-sigmatropic rearrangement via α-imino metal carbenes.

Advancements in Gold-Catalyzed Cascade Reactions to Access Carbocycles and Heterocycles: An Overview

This review summarizes recent developments (from 2006 to 2022) in numerous important and efficient carbo- and heterocycle generations using gold-catalyzed cascade protocols. Herein, methodologies involve selectivity, cost-effectiveness, and ease of product formation being controlled by the ligand as well as the counter anion, catalyst, substrate, and reaction conditions. Gold-catalyzed cascade reactions covered different strategies through the compilation of various approaches such as cyclization, hydroarylation, intermolecular and intramolecular cascade reactions, etc. This entitled reaction is also useful for the synthesis of spiro, fused, bridged carbo- and heterocycles.