Synthesis of Heterocycles through Denitrogenative Cyclization of Triazoles and Benzotriazoles

Copper-Catalyzed Synthesis of Furan-Tethered Benzocyclobutenes via Carbene-Mediated 1,4-Sulfinate Migration-Annulation

A copper-catalyzed intramolecular cascade reaction of conjugated enynones has been achieved via a pivotal 1,4-sulfinate migration step. This process leverages a cost-effective and ecofriendly copper salt as catalyst, enabling the efficient construction of five- and four-membered rings in a rapid, sequential manner, producing furan-tethered benzocyclobutenes in good to excellent yields under mild conditions. The reaction is characterized by 100% atom economy, outstanding efficiency, and excellent diastereoselectivity in the cases studied. The robustness of this method is evidenced by its compatibility with air exposure and the use of undistilled, commercially available solvents, further enhancing its practicality.

Enantioselective Nickel-Catalyzed Denitrogenative Transannulation En Route to N-N Atropisomers

Nickel-catalyzed transannulation reactions triggered by the extrusion of small gaseous molecules have emerged as a powerful strategy for the efficient construction of heterocyclic compounds. However, their use in asymmetric synthesis remains challenging because of the difficulty in controlling stereo- and regioselectivity. Herein, we report the first nickel-catalyzed asymmetric synthesis of N-N atropisomers by the denitrogenative transannulation of benzotriazones with alkynes. A broad range of N-N atropisomers was obtained with excellent regio- and enantioselectivity under mild conditions. Moreover, density functional theory (DFT) calculations provided insights into the nickel-catalyzed reaction mechanism and enantioselectivity control.

Structural Design of Single-Atom Catalysts for Enhancing Petrochemical Catalytic Reaction Process

Petroleum, as the "lifeblood" of industrial development, is the important energy source and raw material. The selective transformation of petroleum into high-end chemicals is of great significance, but still exists enormous challenges. Single-atom catalysts (SACs) with 100% atom utilization and homogeneous active sites, promise a broad application in petrochemical processes. Herein, the research systematically summarizes the recent research progress of SACs in petrochemical catalytic reaction, proposes the role of structural design of SACs in enhancing catalytic performance, elucidates the catalytic reaction mechanisms of SACs in the conversion of petrochemical processes, and reveals the high activity origins of SACs at the atomic scale. Finally, the key challenges are summarized and an outlook on the design, identification of active sites, and the appropriate application of artificial intelligence technology is provided for achieving scale-up application of SACs in petrochemical process.

Masters of Mediation: MN(SiMe3)2 in Functionalization of C(sp3)-H Latent Nucleophiles

Organoalkali compounds have undergone a far-reaching transformation being a coupling partner to a mediator in unusual organic conversions which finds its spot in the field of sustainable synthesis. Transition-metal catalysis has always been the priority in C(sp3)-H bond functionalization, however alternatively, in recent times this has been seriously challenged by earth-abundant alkali metals and their complexes arriving at new sustainable organometallic reagents. In this line, the importance of MN(SiMe3)2 (M=Li, Na, K & Cs) reagent revived in C(sp3)-H bond functionalization over recent years in organic synthesis is showcased in this minireview. MN(SiMe3)2 reagent with higher reactivity, enhanced stability, and bespoke cation-π interaction have shown eye-opening mediated processes such as C(sp3)-C(sp3) cross-coupling, radical-radical cross-coupling, aminobenzylation, annulation, aroylation, and other transformations to utilize readily available petrochemical feedstocks. This article also emphasizes the unusual reactivity of MN(SiMe3)2 reagent in unreactive and robust C-X (X=O, N, F, C) bond cleavage reactions that occurred alongside the C(sp3)-H bond functionalization. Overall, this review encourages the community to exploit the untapped potential of MN(SiMe3)2 reagent and also inspires them to take up this subject to even greater heights.

Divergent Reactivity of 1,2,3-Benzotriazin-4(3H)-ones: Photocatalytic Synthesis of 3-Substituted Isoindolinones Achieved through a Nitrogen-Mediated Hydrogen Atom Shift

A regioselective visible-light-mediated denitrogenative alkene insertion of 1,2,3-benzotriazin-4(3H)-ones was developed to access 3-substituted isoindolinones, an important structural motif present in many biologically active molecules and natural products. Notably, divergent reactivity was achieved by switching from reported nickel catalysis (where C3-substituted 3,4-dihydroisoquinolin-1(2H)-ones form) to photocatalysis, where photocatalytic denitrogenation and a subsequent nitrogen-mediated hydrogen atom shift lead to exclusive 3-substituted isoindolinone formation. The developed photocatalytic reaction is compatible with activated terminal alkenes and cyclic α,β-unsaturated esters and ketones, with wide functional group tolerance for N-substitution of the 1,2,3-benzotriazin-4(3H)-ones. The utility of this procedure is highlighted by a gram-scale synthesis and postsynthetic amidation. To understand the origin of this unique product selectivity, experimental and computational mechanistic studies were performed.

Synthesis of 2,3-Dihydropyrroles by Rhodium(II)-Catalyzed Transannulation of N-Sulfonyl-1,2,3-triazoles: Diversity Generation by One-Pot Methodologies

A versatile one-pot strategy for the generation of compounds of synthetic interest has been presented, promoting the development of practical processes. First, the transannulation of N-sulfonyltriazoles through alkenes and rhodium catalysis was described, giving 2,3-dihydropyrroles in 13-76% yield. As contributions of the strategy, the evaluation of alkenes with different properties, and the use of only drops of solvent (0.40 M) was highlighted. In addition, we described a methodology for the modulation of the N-sulfonyltriazoles, to obtain selectively cyclopropyl tosylimines or 2,3-dihydropyrroles. For the latter products, neat conditions were also included. Finally, the potential of the methodology was demonstrated by the synthesis of six structurally different analogues starting from the same substrates and late-stage transformation of bioactive molecules. These compounds were generated in 38-63% yield, after two or more conversion steps carried out in the same reaction vessel.

NH-1,2,3-triazoles as versatile building blocks in denitrogenative transformations

The utilization of NH-1,2,3-triazoles as easily accessible building blocks in denitrogenative ring cleavage transformations with electrophiles to provide multifunctionalized nitrogen heterocycles and N-alkenyl compounds is reviewed. Leveraging the ready availability of NH-1,2,3-triazoles, these processes provide a convenient route to a range of pharmaceutically relevant heterocyclic cores and N-alkenyl compounds. The synthetic usefulness of in situ acylated NH-1,2,3-triazoles as viable alternatives to widely explored N-sulfonyl-1,2,3-triazoles in ring cleavage processes is highlighted.

Multicomponent cyclization with azides to synthesize N-heterocycles

Heterocyclic compounds, both naturally derived and synthetically produced, constitute a wide variety of biologically active and industrially important compounds. The synthesis and application of heterocyclic compounds have garnered significant attention and experienced rapid growth in recent decades. Organic azides, due to their unique properties and distinctive reactivity, have become a convenient chemical tool for achieving a wide range of heterocycles such as triazoles and tetrazoles. Importantly, the field of multicomponent reaction (MCR) chemistry provides a convergent approach to access various N-heterocyclic scaffolds, offering novelty, diversity, and complexity. However, the exploration of MCR pathways to N-heterocyclic compounds remains incomplete. Here, we review the use of multicomponent reactions for the preparation of N-heterocycles. A wide range of reactions based on azides for the synthesis of various types of N-heterocyclic systems have been developed.

Rh(II)-Catalyzed Denitrogenative Reaction of N-Sulfonyl-1,2,3-triazoles with Quinolones and Isoquinolones

Herein, we developed an efficient approach to access biologically relevant 2-aminoquinolines and 1-aminoisoquinolines from readily available N-sulfonyl-1,2,3-triazoles and 2-quinolones or 1-isoquinolones. This transformation involves the selective O-H insertion of these derivatives onto the in situ generated Rh-azavinyl carbenes (Rh-AVC) followed by rearrangement. The reaction proceeds smoothly under operationally simple conditions and the protocol was found to be scalable.

Annulation-Induced Hidden Reactivity of the 1,2,4-Triazole Backbone

Triazoles are an important class of compounds with widespread applications. Functionalization of the triazole backbone is thus of significant interest. In comparison to 1,2,3-triazoles, C-H activation-functionalization of the congeners 1,2,4-triazoles is surprisingly underdeveloped. Indeed, no such C-H activation-functionalization has been reported for 4-substituted 1,2,4-triazole cores. Furthermore, although denitrogenative ring-opening of 1,2,3-triazoles is well-explored, 1,2,4-triazole/triazolium substrates have not been known to exhibit N-N bond-cleaving ring-opening reactivity so far. In this work, we unveiled an unusual hidden reactivity of the 1,2,4-triazole backbone involving the elusive N-N bond-cleaving ring-opening reaction. This new reactivity was induced by a Satoh-Miura-type C-H activation-annulation at the 1,2,4-triazole motif appended with a pyridine directing group. This unique reaction allowed ready access to a novel class of unsymmetrically substituted 2,2'-dipyridylamines, with one pyridine ring fully-substituted with alkyl groups. The unsymmetrical 2,2'-dipyridylamines were utilized to access unsymmetrical boron-aza-dipyridylmethene fluorescent dyes. Empowered with desirable optical/physical properties such as large Stokes shifts and suitable hydrophobicity arising from optimal alkyl chain length at the fully-substituted pyridine-ring, these dyes were used for intracellular lipid droplet-selective imaging studies, which provided useful information toward designing suitable lipid droplet-selective imaging probes for biomedical applications.

Rhodium Catalyzed [4 + 1]-Annulation of o-Acylanilines with 3-Diazoindoline-2-imines

An efficient rhodium catalyzed [4 + 1]-annulation of o-acylanilines with 3-diazoindoline-2-imines has been successfully accomplished for the synthesis of spiroindolines in good to excellent yield. The reaction occurs through formation of N-ylide followed by cyclization and showed good tolerance to various functional groups. Gram-scale synthesis, diastereoselective construction of tetrasubstituted indoline, synthesis of spirooxindole, and isolation of potential intermediates have also been demonstrated.

Catalyst-Free Synthesis of Polycyclic Spiroindolines by Cascade Reaction of 3-(2-Isocyanoethyl)indoles with 1-Sulfonyl-1,2,3-triazoles

A catalyst-free cascade reaction of 3-(2-isocyanoethyl)indoles and 1-sulfonyl-1,2,3-triazoles was realized. This dearomative spirocyclization provided an efficient protocol to synthesize a series of polycyclic indolines bearing spiro-α-carboline in moderate to high yields in one step under thermal reaction conditions.

Synthesis of Dihydro-3,1-benzoxazine Derivatives from 1,3-Amino Alcohols and N-Sulfonyl-1,2,3-triazole

An efficient rhodium-catalyzed synthesis of dihydro-3,1-benzoxazine derivatives has been accomplished from aniline-derived 1,3-amino alcohols and N-sulfonyl-1,2,3-triazole. The developed reaction demonstrates the new reactivity of azavinyl carbenes and allows access to diverse substituted dihydro-3,1-benzoxazines in good yields. Importantly, the reaction was readily extended to diols and could be used for selective protection of amino alcohols with N-sulfonyl-1,2,3-triazole as the protecting reagent.

Continuous Flow Synthesis of N-Sulfonyl-1,2,3-triazoles for Tandem Relay Cu/Rh Dual Catalysis

The continuous flow synthesis of N-sulfonyl-1,2,3-triazoles, which are convenient reactive azavinyl carbene precursors, for tandem relay Cu/Rh dual catalysis has been developed. Most reactions readily proceeded at 75 °C in a short residence time of 13.09 min in the presence of 2.5 mol % of CuTC. The scope of the reactions was explored by synthesizing diversely functionalized N-sulfonyl and sulfamoyl triazoles in yields ranging from 92 to 98%. To demonstrate the scalability of the process, the reaction was conducted on a 5.4 mmol scale with residence and collection times of 13.09 and 60 min, respectively. Furthermore, a series of controlled experiments were performed to investigate the compatibility of Cu and Rh in a batch or a continuous flow system. Finally, the first integrated flow system using the azavinyl carbene intermediate under the tandem relay Cu/Rh dual catalysis was developed for the synthesis of various cis-diamino enones from alkynes and sulfonyl azides.

Natural Product-Oriented Photo-Induced Denitrogenative Annulations of 1-Alkenylbenzotriazoles

The photo-induced denitrogenative annulations of a variety of 1-alkenylbenzotriazoles were investigated. By judiciously manipulating the structural variations of 1-alkenylbenzotriazoles, two characteristic polycyclic skeletons associated with monoterpene indole alkaloids were constructed through a diverted and controllable manner. The present work not only enriches the photochemistry of 1-alkenylbenzotriazoles, but also offers a unified approach to access skeletally diverse indole alkaloid scaffolds.

Visible Light-Promoted Radical-Mediated Ring-Opening/Cyclization of Vinyl Benzotriazoles: An Alternative Approach to Phenanthridines

A visible light-promoted radical-mediated ring-opening/cyclization of vinyl benzotriazoles has been developed. The method provides an efficient and practical approach to synthesize functionalized phenanthridines from vinyl benzotriazoles with alkyl bromides under mild conditions. Significantly, the readily available and bench-stable vinyl benzotriazoles can serve as valuable alternative radical acceptors during the synthesis of phenanthridines.

Metal-Free [5 + 1] Cycloaddition-Aromatization of Benzotriazoles and Sulfur Ylides to Construct 1,2,4-Benzotriazines

Reported herein is a novel [5 + 1] cycloaddition-aromatization of benzotriazoles and sulfur ylides to efficiently construct 1,2,4-benzotriazine derivates with good yield. This new protocol does not employ any transition metal reagent and enables the cycloaddition by cleavage of the N-N single bond. The use of inexpensive and readily available starting materials, a broad substrate scope, mild reaction conditions, metal-free, and versatile functionalization of the 1,2,4-benzotriazines make this strategy more attractive.

Synthesis of Azepane Derivatives via Formal 1,3-Migration of Hydroxy and Acyloxy Groups and Selective Annulation

Formal 1,3-migration of hydroxy and acyloxy groups initiated by α-imino rhodium carbene was achieved, and the following selective annulations of the corresponding zwitterions could efficiently afford azepane derivatives. Benefiting from a time-saving procedure as well as a good functional group tolerance, this unique migration-annulation protocol could provide an efficient tool for synthesizing seven-membered N-heterocycles. The plausible mechanism is discussed.

Synthesis of α-Amino Cyclobutanones via Formal 1,3-Hydroxy Migration Triggered by Formation of α-Imino Rhodium Carbene

Formal intramolecular 1,3-OH migration of α-imino carbene was achieved producing a unique zwitterion, and the subsequent selective annulation afforded α-amino cyclobutanone. Features such as readily available substrates, mild reaction conditions, a time-saving procedure, excellent functional group compatibility, and valuable transformations of the products qualified this unique protocol as an efficient tool for the synthesis of strained cyclic compounds. Density functional theory calculations were in good agreement with experimental observations, and a plausible mechanism is presented.

Access to tetrahydrocarbazoles and pyrrolo[3,4-b]carbazoles through sequential reactions of triazoles and indoles

Tetrahydrocarbazoles and pyrrolo[3,4-b]carbazoles could be synthesized conveniently through sequential reactions of ester-tethered 1-sulfonyl-1,2,3-triazoles and indoles. The reaction conditions were mild and the procedures were quite simple. Moreover, the key intermediate α,β-unsaturated imine acted as a [2C] synthon in the [4 + 2] cycloaddition reaction, and the imino group could be used as a nucleophile to construct the fourth ring.

Synthesis of dihydropyrroles from in situ-generated zwitterions via Rh2(adc)4/TBAI dual catalysis

A convenient rhodium/iodide dual catalysis strategy for the regioselective synthesis of dihydropyrroles was established.

Benzotriazoles and Triazoloquinones: Rearrangements to Carbazoles, Benzazirines, Azepinediones, and Fulvenimines

The denitrogenative rearrangements of several types of benzotriazoles were investigated by DFT (B3LYP/6-311G(d,p)) and CASPT2(10,10)sp/6-311G(d,p) calculations. The Graebe-Ullmann synthesis of carbazoles 18 by pyrolysis or photolysis of 1-arylbenzotriazoles 14 proceeds without the involvement of benzazirines and without Wolff-type ring contraction to fulvenimines. However, 1-aryltetrahydrobenzotriazoles undergo both cyclization to tetrahydrocarbazole and ring contraction. Triazoloquinones like 34 undergo predominant ring contraction to aminofulvenediones like 38 and also ring expansion to azepinediones like 40 and cyclization to N-arylbenzaziridinediones 39, whereas carbazolediones are not formed. Denitrogenation of 1-methylbenzotriazole 64 results in a facile 1,2-H shift with formation of N-phenylmethanimine 67. 1-Cyanobenzotriazole 71 undergoes destructive pyrolysis with charring, and the calculations predict the occurrence of several low-activation energy reaction pathways.

Catalytic Insertion Reactions of α-Imino Carbenoids

Over the past decade, α-imino carbenoids generated via transition metal (such as rhodium, nickel, copper, palladium, silver) catalyzed denitrogenative ring-opening of N-sulfonyl-1,2,3-triazoles have found an extensive account of applications in synthetic organic chemistry. Particularly, they have been widely utilized as a donor/acceptor carbene complex in a range of transformations leading to diverse nitrogen containing compounds and heterocycles. Along the same direction, 3-diazoindolin-2-imines were successfully applied as an alternative source of α-imino carbenoid precursors for the development of a number of methodologies to access diverse indole derivatives. This review summarizes the insertion reactions of α-imino metal carbenes derived from N-sulfonyl-1,2,3-triazoles and 3-diazoindolin-2-imines.

Visible-Light-Catalyzed in Situ Denitrogenative Sulfonylation of Sulfonylhydrazones

A photocatalyzed in situ denitrogenative sulfonylation of N-arylsulfonyl hydrazones has been developed. This transformation provides a low-carbon strategy to assemble arylalkyl sulfones in a stepwise denitrogenation/sulfonylation manner.

Rhodium-Catalyzed Synthesis of 2-Aroylpyrimidines via Cascade Heteropolyene Rearrangement

A one-step synthesis of cytotoxic 2-aroylpyrimidines by the denitrogenative reaction of 1-tosyl-1,2,3-triazoles with isoxazoles under rhodium catalysis has been developed. According to the density functional theory calculations and control experiments, the disclosed reaction proceeds via the rearrangement of an oxadiazatetraene intermediate involving a cascade of intramolecular aza-Diels-Alder and retro-aza-Diels-Alder reactions. The presence of a substituent at C4 of the isoxazole is a prerequisite for the formation of the pyrimidines.

Palladium-Catalyzed Asymmetric Tandem Denitrogenative Heck/Tsuji-Trost of Benzotriazoles with 1,3-Dienes

The asymmetric denitrogenative cycloaddition has emerged as a powerful tool to build chiral aza-heterocyles. However, only one example of asymmetric denitrogenative cycloaddition of benzotriazole with unsaturated hydrocarbons has been explored so far, because the ring-opening of benzotriazole to generate α-imino metal carbenoid species is a thermodynamically unfavorable process. We herein report an efficient asymmetric denitrogenative cycloaddition of benzotriazoles with cyclic and acyclic 1,3-dienes enabled by Pd and chiral sulfonamide phosphine ligand. A variety of substituted hexahydrocarbazoles and indolines were delivered in good yields with high ee values. Interestingly, a pair of enantiomers could be obtained with the use of Xu1 and PC2 with the same absolute configuration. The synthetic utilities of the optically active hexahydrocarbazoles were also showcased.

Synthesis of benzothiazonine by rhodium-catalyzed denitrogenative transannulation of 1-sulfonyl-1,2,3-triazole and thiochromone

A facile synthesis of multi-functionalized benzothiazonine was achieved by the rhodium-catalyzed denitrogenative annulation of 1-sulfonyl-1,2,3-triazole and thiochromone. In view of the excellent atom economy, broad substrate scope and easy availability of starting materials, the protocol provided an efficient strategy for the construction of medium N,S-heterocycles.

Stereoselective Synthesis of (Z)-β-Enamido Fluorides from N-Fluoroalkyl- and N-Sulfonyl-1,2,3-triazoles

A reaction of N-sulfonyl-1,2,3-triazole with boron trifluoride etherate afforded a (Z)-β-ensulfonylamido fluoride instead of the previously erroneously assigned E isomer. The correction of the stereochemistry was based on a ge-1D ROESY NMR experiment and X-ray crystal structure analyses. Application of the reaction to N-fluoroalkyl-1,2,3-triazoles afforded new (Z)-β-enamido fluorides in a stereoselective manner. A mechanism involving coordination of BF₃ with the triazole ring and vinyl diazonium and vinyl cation intermediates was proposed.

Rh(II)-catalyzed Denitrogenative Cascade of 1,2,3-Triazolyl Propiolates and Indoles: Access to Butenolide Tethered Homotryptamines

An efficient Rh(II)-catalyzed denitrogenative reaction of indoles with 1,2,3-triazolyl propiolates has been developed. This methodology provides facile access to butenolide tethered homotryptamines in good to excellent yields under operationally simple conditions and features a broad substrate scope. Overall, the reaction sequence involves the formation of three new bonds (two C-C and one C-O) in a nucleophilic cascade manner. Additionally, an intramolecular rearrangement of these derivatives to thermodynamically more stable butenolides is also demonstrated.

Domino Construction of Benzoxazole-Derived Sulfonamides via Metal-Free Denitrogenation of 5-Iodo-1,2,3-triazoles in the Presence of SO2 and Amines

A straightforward domino approach to assemble benzoxazole-derived sulfonamides has been developed. The method is based on annulation-induced in situ generation of diazo compounds from readily available 2-(5-iodo-1,2,3-triazolyl)phenols, followed by metal-free denitrogenative transformation upon the action of 1,4-diazabicyclo[2.2.2]octane bis(sulfur dioxide) (DABSO) and amines. The protocol is operationally simple and features a broad substrate scope, furnishing a library of target compounds in generally good yields.

Synthesis of Highly Conjugated Functionalized 2-Pyridones by Palladium-Catalyzed Aerobic Oxidative Dicarbonation Reactions of N-(Furan-2-ylmethyl) Alkyne Amides and Alkenes as Coupling Partners

A mild, step-economical method for the synthesis of highly conjugated functionalized 2-pyridones from N-(furan-2-ylmethyl) alkyne amides is reported. This method involves Pd-catalyzed aerobic oxidative dicarbonation reactions of alkynes with carbon nucleophiles of a furan ring and an acrylate or styrene as coupling partners. The UV-vis absorption spectra of some of the 2-pyridones indicated that they absorbed shortwave radiation, suggesting their potential utility for filtration of such radiation.

Two reaction modes of 1-sulfonyl-1,2,3-triazoles and pyridinium 1,4-zwitterionic thiolates: catalyst-free synthesis of pyrido[1,2-a]pyrazine derivatives and 1,4-thiazine derivatives

Efficient catalyst-free synthesis of pyrido[1,2-a]pyrazine and 1,4-thiazine derivatives from triazole and pyridinium 1,4-zwitterionic thiolate was realized.

Grob-type fragmentation of an oxonium ylide generated from α-imino rhodium carbene

A Grob-type fragmentation of an oxonium ylide generated from α-imino rhodium carbene and tethered tetrahydrofuran was realized, leading to functionalized acroleins.

Microwave-Assisted Synthesis of Quinazolines and Quinazolinones: An Overview

Microwave irradiation (MWI), as a unique, effective, sustainable, more economic, and greener source of energy compared to conventional heating, is applied in different organic transformations to result in the rapid formation of desired compounds due to thermal/kinetic effects. In this review, we try to underscore the applications of microwave irradiation (MWI) in the synthesis of quinazoline and quinazolinone derivatives that have been achieved and reported on in the last two decades.

Construction of fluorescence active MOFs with symmetrical and conformationally rigid N-2-aryl-triazole ligands

1,4-Bis-triazole-substituted arene (NAT) was designed and synthesized for the construction of metal organic frameworks. Unlike the tri-phenyl analogs, which give a twisted conformation between three benzene rings due to the A-1,3 repulsion, the NAT-ligand gave the energetically favored co-planar conformation with the strong fluorescence emission. With this ligand, two new MOFs, NAT-MOF-Cd (2,3,4-c) and NAT-MOF-Cu (4-c), were successfully obtained with the structure confirmed by X-ray. With the six-coordinated Cd(ii) cluster, an interesting metal–ligand coordination and H-bonding hybridized porous polymeric structures were observed. In contrast, a typical Cu(ii) paddle wheel coordination was obtained with NAT and Cu, giving a new MOF structure with moderate stability in aqueous solution from pH 1–11 for 24 hours, which suggests a promising future for applications in fluorescence sensing and photocatalysis.

1,4-Bis-triazole-substituted arene (NAT) was designed and synthesized for the construction of fluorescent metal organic frameworks.