α-Imino Gold Carbenes from 1,2,4-Oxadiazoles: Atom-Economical Access to Fully Substituted 4-Aminoimidazoles

Synthesis of 3-Aminoquinazolinones via a SnCl2-Mediated ANRORC-like Reductive Rearrangement of 1,3,4-Oxadiazoles

Herein, we developed a new SnCl2-mediated ANRORC (addition of nucleophile, ring-opening, and ring-closure)-like rearrangement for the synthesis of 3-amino-2-substituted-quinazolin-4(3H)-one from 2-(2-nitrophenyl)-5-substituted-1,3,4-oxadiazole. The new method is solvent-dependent and features the use of a green solvent system (i.e., ethanol/water), high yields, and simple workup. The reduced product could be exclusively synthesized by changing the solvent to acetonitrile.

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

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

Preparation of polysubstituted imidazoles using AC-SO3H/[Urea]7[ZnCl2]2 as an efficient catalyst system: a novel method, and α-glucosidase inhibitor activity

Deep eutectic solvents (DESs) act as both an organic solvent and a useful catalyst for organic synthesis reactions, especially the synthesis of heterocyclic compounds containing the element nitrogen. DESs exhibit many important properties namely large liquid fields, biodegradability, outstanding thermal stability, and moderate vapor pressure. Amorphous carbon-bearing sulfonic acid groups (AC-SO₃H) are one of the new-generation solid acids showing strong acid activity. Based on the simultaneous presence of acidic functional groups such as carboxylic acid, phenolic, and sulfonic acid groups, they exhibit many important activities namely strong Brønsted acid, high surface area, high stability, reusability, and recyclability. In this study, AC-SO₃H was made from rice husk via the carbonization and sulfonation processes, and the surface properties and structure were examined using contemporary methods such as FT-IR, P-XRD, TGA, SEM, and EDS. And, [Urea]₇[ZnCl₂]₂ was synthesized from urea and ZnCl₂ with a mole ratio of 7 : 2; the structure is defined using FT-IR and TGA. By combining AC-SO₃H and [Urea]₇[ZnCl₂]₂ we aim to form an effective catalyst/solvent system for the preparation of polysubstituted imidazole derivatives through the multi-component cyclization reaction from nitrobenzenes, benzil, aldehydes, and ammonium acetate. The major products are obtained with high isolation yields above 60%. To assess the catalyst system's activity, the recovery and reusability of the AC-SO₃H/[Urea]₇[ZnCl₂]₂ system were examined with hardly any performance modification. In an effort to create potential enzyme α-glucosidase inhibitors, several novel polysubstituted imidazoles were created. Five of these compounds showed good enzyme α-glucosidase inhibitor activity. The most effective substances were IMI-13, IMI-15, and IMI-20, with IC₅₀ values that were greater than the acarbose at 16.5, 15.8, and 11.6 μM, respectively - the acarbose (IC₅₀, 214.5 μM) as the positive control.

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.

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.

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.

DFT Rationalization of Gold(I)-Catalyzed Couplings between Alkynyl Thioether and Nitrenoid Derivatives: Mechanism, Selectivity Patterns, and Effects of Substituents

The present work focuses on a theoretical investigation of mechanistic features, chemoselectivity, regioselectivity, and effect of substituents in the gold-catalyzed reactions of alkynyl thioethers and isoxazoles. The DFT calculations reveal that the nucleophilic attack of isoxazole to a β-position of catalyst-bound ynamide forms a vinyl intermediate. This undergoes cleavage of the N-O isoxazole bond and isomerizes to form an α-imino α'-sulfenyl gold carbene complex with stabilization of the Au-S interaction. For 3,5-dimethylisoxazole, the reaction follows a formal [3 + 2] cycloaddition pathway and a 1,3-H migration to give the pyrrole products. Replacement of 3,5-dimethylisoxazole by 3,5-dimethyl-4-phenylisoxazole leads to the formation of deacylative annulation products and desulfenylated 3-acylated pyrroles. Reactions with 4-methyl-3,5-diphenylisoxazoles induce the formation of N-acylated pyrroles and desulfenylated 3-acylated pyrroles. For the minor pathway, the α-addition and 1,2-S migration result in sulfur-substituted β-keto enamide derivatives. In addition, the unique features of regio- and chemoselectivity were rationalized by the distortion and interaction analysis. Apart from fully rationalizing the experimental results, the theoretical DFT data give an important support for comprehending related types of reaction mechanisms.

Insights into gold-catalyzed formation of aza-heterocycles using benzofuroxans as nitrene transfer reagents: mechanism and origins of chemoselectivity

Density functional theory (DFT) calculations have been performed to reveal the mechanism of gold(i)-catalyzed annulation of N-allylynamides and benzofuroxans as nitrene transfer reagents to construct azaheterocyclic compounds. The calculated results revealed that the reaction mechanism mainly undergoes eight processes. Among the reaction steps, intramolecular nucleophilic attack of the imino N atom on the α-position of activated gold keteniminium is a rate-determining process, which is different from that proposed previously by experiment. The chemoselectivity of the products is controlled by competition between the cyclopropanation of α-imino gold carbenes and intramolecular nucleophilic attack of the phenyl ring on α-imino gold carbenes, and could be explained by NPA charge. The different yields of cyclopropanated product in different solvents are dictated by the relative polarity leading to the different energy barriers of the rate-determining steps. The present work expounds the experimental observation at the molecular level and is informative for exploring efficient syntheses of 3-azabicyclo[3.1.0]hexanes.

DFT calculations were employed to study the mechanism and origin of chemoselectivity of gold-catalyzed annulation of N-allylynamides and benzofuroxans as nitrene transfer reagents.

Gold-Catalyzed Annulations with Nucleophilic Nitrenoids Enabled by Heteroatom-Substituted Alkynes

The combination of a nucleophilic nitrene equivalent, a triple bond and a π-acid catalyst has underpinned numerous efficient transformations for the preparation of azacycles. This personal account details our efforts in developing an annulation strategy. Adding a nucleophilic nitrenoid to an activated alkyne can generate carbenoid character that is then quenched by a cyclisation onto the nitrenoid substituent. The use and development of N-acyl and N-heterocyclic pyridinium-N-aminides as 1,3-N,O and 1,3-N,N-dipole equivalents is discussed in the context of oxazole and heterocycle-fused imidazole formation, respectively. The resulting processes are highly efficient, practically straightforward, and tolerate considerable structural and functional group variation. Our use of heteroatom-substituted alkynes as enabling tools for reaction discovery is discussed. The reactivity accessed from the strong donor-like properties of ynamides is complemented by that obtained from alkynyl thioethers, which are emerging as interesting substrates for π-acid catalysis.

Acyclic Twisted Amides

In this contribution, we provide a comprehensive overview of acyclic twisted amides, covering the literature since 1993 (the year of the first recognized report on acyclic twisted amides) through June 2020. The review focuses on classes of acyclic twisted amides and their key structural properties, such as amide bond twist and nitrogen pyramidalization, which are primarily responsible for disrupting nN to π*C═O conjugation. Through discussing acyclic twisted amides in comparison with the classic bridged lactams and conformationally restricted cyclic fused amides, the reader is provided with an overview of amidic distortion that results in novel conformational features of acyclic amides that can be exploited in various fields of chemistry ranging from organic synthesis and polymers to biochemistry and structural chemistry and the current position of acyclic twisted amides in modern chemistry.

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.

Gold(I)-Catalyzed Synthesis of 3-Sulfenyl Pyrroles and Indoles by a Regioselective Annulation of Alkynyl Thioethers

The combination of nucleophilic nitrenoids and π-acid catalysis has emerged as a powerful tool in heterocycle synthesis. Accessing more varied heterocycle-substitution patterns by maintaining the same reaction pathways across different alkynes remains a challenge. Here we show that Au(I) catalysis of isoxazole-based nitrenoids with alkynyl thioethers provides controlled access to (3 + 2) annulation by a regioselective addition β to the sulfenyl group. The reaction with isoxazole-containing nitrenoids delivers sulfenylated pyrroles and indoles as single regioisomers bearing useful functional groups and structural variety.

Metal-Free [3+2] Annulation of Ynamides with Anthranils to Construct 2-Aminoindoles

A novel metal-free [3+2] annulation of ynamides with anthranils provides a facile, flexible, environmentally friendly, and atom-economical route to 2-aminoindoles. This synthetic process proceeds with efficiency, excellent regioselectivity, and wide functional group tolerance under mild conditions. Moreover, the obtained 2-aminoindole products represent a multifunctional platform for the construction of various 2-aminoindolyl frameworks.

Reactivity of ynamides in catalytic intermolecular annulations

Ynamides are unique alkynes with a carbon-carbon triple bond directly attached to the nitrogen atom bearing an electron-withdrawing group. The alkyne is strongly polarized by the electron-donating nitrogen atom, but its high reactivity can be finely tempered by the electron-withdrawing group. Accordingly, ynamides are endowed with both nucleophilic and electrophilic properties and their chemistry has been an active research field. The catalytic intermolecular annulations of ynamides, featuring divergent assembly of structurally important amino-heterocycles in a regioselective manner, have gained much attention over the past decade. This review aims to provide a comprehensive summary of the advances achieved in this area involving transition metal and acid catalysis. Moreover, the intermolecular annulations of ynamide analogs including ynol ethers and thioalkynes are also discussed, which can provide insights into the reactivity difference caused by the heteroatoms.

Gold-catalyzed [5 + 2]-annulations of 1,3-diyn-1-amides with anthranils bearing no C(6)-substituents

This work reports the gold-catalyzed [5 + 2]-annulations of commonly used anthranils with 1,3-diynamides, yielding quiniline oxides as the main products.

Silver-catalyzed desulfurizative annulation of 1,2-benzisothiazoles with ynamides to construct multi-substituted isoquinolines

An unprecedented silver-catalyzed desulfurizative annulation of 1,2-benzisothiazoles with ynamides to access multi-substituted isoquinoline derivatives is disclosed.

Nitrene Transfer and Carbene Transfer in Gold Catalysis

Catalytic transformations involving metal carbenes are considered one of the most important aspects of homogeneous transition metal catalysis. Recently, gold-catalyzed generation of gold carbenes from readily available alkynes represents a significant advance in metal carbene chemistry. This Review summarizes the advances in the gold-catalyzed nitrene-transfer reactions of alkynes with nitrogen-transfer reagents, such as azides, nitrogen ylides, isoxazoles, and anthranils, and gold-catalyzed carbene-transfer reactions, involving oxygen atom-transfer reactions of alkynes with nitro compounds, nitrones, sulfoxides, and pyridine N-oxides, through the presumable α-imino gold carbene and α-oxo gold carbene intermediates, respectively. Gold-catalyzed processes are reviewed by highlighting their product diversity, selectivity, and applicability, and the mechanistic rationale is presented where possible.

Dibenzothiophenesulfilimines: A Convenient Approach to Intermolecular Rhodium-Catalysed C-H Amidation

A sulfilimine‐based Group 9 transition‐metal‐catalysed C−H amidation procedure is reported. Dibenzothiophene‐based sulfilimines were shown to constitute a class of novel amidation reagents which enable the transfer of a wide range of N‐sulfonyl and N‐acyl moieties. It was demonstrated that sulfilimines, which are easily accessible from cheap reagents, are safe‐to‐handle and represent broadly applicable amidation reagents. The dibenzothiophene can be recycled after use. The C−H amidation was shown to proceed with high selectivity and gave the mono‐amidated products, mostly in good to excellent yields.

Distinct Roles of Ag(I) and Cu(II) as Cocatalysts in Achieving Positional-Selective C-H Alkenylation of Isoxazoles: A Theoretical Investigation

For C-H alkenylation of aryl-substituted diarylisoxazoles, one mode is N-directed C-H alkenylation and the other is C-H alkenylation in the isoxazole ring. In this study, selective C-H alkenylations of 3,5-diarylisoxazoles have been investigated theoretically with the aid of density functional theory (DFT) calculations. With Cp*Rh^III as the catalyst, the N-directed C-H alkenylation is preferred as a result of the stronger interaction energy caused by the nitrogen-directing effect. With Pd(OAc)₂ as the catalyst and Ag₂CO₃ as the cocatalyst, their combination switches the regioselectivity to the C-H alkenylation in the isoxazole ring. The strong structural distortion involved in the competing N-directed olefin insertion transition state was found to suppress N-directed C-H alkenylation. With Pd(OAc)₂ as the catalyst and Cu(OTf)₂ as the cocatalyst, the N-directed C-H alkenylation becomes preferred due to the strong coordination of the nitrogen atom to the copper center. In particular, the structural and mechanistic information involved in the above two heterodimetallic Pd/Ag and Pd/Cu catalytic systems will help toward understanding and designing novel relevant heterodimetallic-catalyzed reactions.

α-Imino Gold Carbene Intermediates from Readily Accessible Sulfilimines: Intermolecular Access to Structural Diversity

Catalytic approaches to pharmaceutically important bioactive skeletons through gold carbene intermediates have experienced a dramatic development in the last decade. Although various carbene precursors continue to play an important role in heterocyclic syntheses, these reagents are associated with some drawbacks in terms of functional group tolerance, synthetic methods and safety limitations. A new generation of nitrene transfer reagents was established in 2019: the sulfilimines. These are safe, inexpensive and readily available. They can conveniently be stored and handled, and thus represent ideal reagents for the fast and modular modification of scaffolds and the preparation of libraries by intermolecular reactions of two components. Both the practical methods for synthesizing sulfilimines and the versatility of these ylidic species in gold-catalyzed preparation of structural diversity, for both heterocycles and carbocycles, will be outlined in this Concept article.

Acyl Migration versus Epoxidation in Gold Catalysis: Facile, Switchable, and Atom-Economic Synthesis of Acylindoles and Quinoline Derivatives

We report a switchable synthesis of acylindoles and quinoline derivatives via gold-catalyzed annulations of anthranils and ynamides. α-Imino gold carbenes, generated in situ from anthranils and an N,O-coordinated gold(III) catalyst, undergo electrophilic attack to the aryl π-bond, followed by unexpected and highly selective 1,4- or 1,3-acyl migrations to form 6-acylindoles or 5-acylindoles. With the (2-biphenyl)di-tert-butylphosphine (JohnPhos) ligand, gold(I) carbenes experienced carbene/carbonyl additions to deliver quinoline oxides. Some of these epoxides are valuable substrates for the preparation of 3-hydroxylquinolines, quinolin-3(4H)-ones, and polycyclic compounds via facile in situ rearrangements. The reaction can be efficiently conducted on a gram scale and the obtained products are valuable substrates for preparing other potentially useful compounds. A computational study explained the unexpected selectivities and the dependency of the reaction pathway on the oxidation state and ligands of gold. With gold(III) the barrier for the formation of the strained oxirane ring is too high; whereas with gold(I) this transition state becomes accessible. Furthermore, energetic barriers to migration of the substituents on the intermediate sigma-complexes support the observed substitution pattern in the final product.

Insight into the reaction mechanism and chemoselectivity in the cycloaddition of ynamides and isoxazoles with H2O

The mechanism and chemoselectivity in the cycloaddition of ynamides and isoxazoles have been explored by the density functional theory (DFT) in model systems composed of a Brønsted acid (HNTf₂), gold(i) [IPrAuNTf₂] or platinum(ii) (PtCl₂/CO) catalyst.

Zinc-Catalyzed Asymmetric Formal [4+3] Annulation of Isoxazoles with Enynol Ethers by 6π Electrocyclization: Stereoselective Access to 2H-Azepines

6π electrocyclization has attracted interest in organic synthesis because of its high stereospecificity and atom economy in the construction of versatile 5-7-membered cycles. However, examples of asymmetric 6π electrocyclization are quite scarce, and have to rely on the use of chiral organocatalysts, and been limited to pentadienyl-anion- and triene-type 6π electrocyclizations. Described herein is a zinc-catalyzed formal [4+3] annulation of isoxazoles with 3-en-1-ynol ethers via 6π electrocyclization, leading to the site-selective synthesis of functionalized 2H-azepines and 4H-azepines in good to excellent yields with broad substrate scope. Moreover, this strategy has also been used to produce chiral 2H-azepines with high enantioselectivities (up to 97:3 e.r.). This protocol not only is the first asymmetric heptatrienyl-cation-type 6π electrocyclization, but also is the first asymmetric reaction of isoxazoles with alkynes and the first asymmetric catalysis based on ynol ethers.

Gold(iii)-catalyzed chemoselective annulations of anthranils with N-allylynamides for the synthesis of 3-azabicyclo[3.1.0]hexan-2-imines

We herein report the gold(iii)-catalyzed selective annulation of anthranils with N-allylynamides under mild conditions. By trapping the in situ-generated α-imino gold carbenes, 3-azabicyclo[3.1.0]hexan-2-imines were obtained in high synthetic efficiency. The reaction, which can be conducted in the gram scale, tolerates electron-rich and electron-deficient anthranils as well as a diverse set of functionalized ynamides (aryl- and alkyl-substituted terminal).

Structure and Reactivity of Highly Twisted N-Acylimidazoles

A modular and efficient synthesis of highly twisted N-acylimidazoles is reported. These twist amides were characterized via X-ray crystallography, NMR spectroscopy, IR spectroscopy, and DFT calculations. Modification of the substituent proximal to the amide revealed a maximum torsional angle of 88.6° in the solid state, which may be the most twisted amide reported for a nonbicyclic system to date. Reactivity and stability studies indicate that these twisted N-acylimidazoles may be valuable, namely as acyl transfer reagents.

Synthesis of poly-functionalized benzofurans via one-pot domino oxidation/[3+2] cyclization reactions of a hydroquinone ester and ynamides

Densely substituted amino-functionalized benzofurans were concisely accessed via the first one-pot domino oxidation/[3+2] cyclization of a hydroquinone ester and easily accessible ynamides under mild conditions in a short time. The complex benzofurans were able to be efficiently synthesized all from simple and inexpensive starting materials in two steps.

Efficient method for the construction of poly-functionalized benzofurans has been developed via one-pot domino oxidation/[3+2] cyclization reactions.