PhI(OCOCF3)2-Mediated Intramolecular Oxidative N–N Bond Formation: Metal-Free Synthesis of 1,2,4-Triazolo[1,5-a]pyridines

Phenyliodine bis(trifluoroacetate) as a sustainable reagent: exploring its significance in organic synthesis

Iodine-containing molecules, especially hypervalent iodine compounds, have gained significant attention in organic synthesis. They are valuable and sustainable reagents, leading to a remarkable surge in their use for chemical transformations. One such hypervalent iodine compound, phenyliodine bis(trifluoroacetate)/bis(trifluoroacetoxy)iodobenzene, commonly referred to as PIFA, has emerged as a prominent candidate due to its attributes of facile manipulation, moderate reactivity, low toxicity, and ready availability. PIFA presents an auspicious prospect as a substitute for costly organometallic catalysts and environmentally hazardous oxidants containing heavy metals. PIFA exhibits remarkable catalytic activity, facilitating an array of consequential organic reactions, including sulfenylation, alkylarylation, oxidative coupling, cascade reactions, amination, amidation, ring-rearrangement, carboxylation, and numerous others. Over the past decade, the application of PIFA in synthetic chemistry has witnessed substantial growth, necessitating an updated exploration of this field. In this discourse, we present a concise overview of PIFA's applications as a 'green' reagent in the domain of synthetic organic chemistry. A primary objective of this article is to bring to the forefront the scientific community's awareness of the merits associated with adopting PIFA as an environmentally conscientious alternative to heavy metals.

Microwave-Mediated, Catalyst-Free Synthesis of 1,2,4-Triazolo[1,5-a]pyridines from Enaminonitriles

A catalyst-free, additive-free, and eco-friendly method for synthesizing 1,2,4-triazolo[1,5-a]pyridines under microwave conditions has been established. This tandem reaction involves the use of enaminonitriles and benzohydrazides, a transamidation mechanism followed by nucleophilic addition with nitrile, and subsequent condensation to yield the target compound in a short reaction time. The methodology demonstrates a broad substrate scope and good functional group tolerance, resulting in the formation of products in good-to-excellent yields. Furthermore, the scale-up reaction and late-stage functionalization of triazolo pyridine further demonstrate its synthetic utility. A plausible reaction pathway, based on our findings, has been proposed.

Emerging trends in the sustainable synthesis of N-N bond bearing organic scaffolds

N-N bond bearing organic frameworks such as azos, hydrazines, indazoles, triazoles and their structural moieties have piqued the interest of organic chemists due to the intrinsic nitrogen electronegativity. Recent methodologies with atom efficacy and a greener approach have overcome the synthetic obstacles of N-N bond construction from N-H. As a result, a wide range of amine oxidation methods have been reported early on. This review's vision emphasizes the emerging methods of N-N bond formation, particularly photo, electro, organo and transition metal free chemical methods.

Base-Promoted Tandem SNAr/Boulton-Katritzky Rearrangement: Access to [1,2,4]Triazolo[1,5-a]pyridines

A base-promoted tandem S_NAr/Boulton-Katritzky rearrangement is developed. It offers a simple and straightforward method for the formation of functionalized [1,2,4]triazolo[1,5-a]pyridines from 1,2,4-oxadiazol-3-amines or 3-aminoisoxazoles with 2-fluoropyridines.

Tandem C-N Coupling/Boulton-Katritzky Rearrangement Reactions of 3-Aminoisoxazoles or 1,2,4-Oxadiazol-3-amines with 2-Pyridyl Trifluoromethanesulfonate: A Rapid Access to [1,2,4]Triazolo[1,5-a]pyridines

An efficient and convenient synthesis of functionalized [1,2,4]triazolo[1,5-a]pyridines derivates is presented. The protocol is through a palladium catalyzed tandem C-N coupling/Boulton-Katritzky rearrangement process from 3-aminoisoxazoles or 1,2,4-oxadiazol-3-amines with 2-pyridyl trifluoromethanesulfonate....

A green polyol approach for the synthesis of Cu2O NPs adhered on graphene oxide: a robust and efficient catalyst for 1,2,4-triazole and imidazo[1,2-a]pyridine synthesis

Cu2O NPs immobilized on graphene oxide are used as a heterogeneous catalyst for the synthesis of a series of 1,2,4-triazoles and imidazo[1,2-a]pyridines under solvent-free conditions.

N-Iodosuccinimide-Promoted [3 + 2] Annulation Reaction of Aryldiazonium Salts with Guanidines To Construct Aminotetrazoles

A N-iodosuccinimide (NIS)-promoted [3 + 2] annulation reaction of aryldiazonium salts with guanidines has been developed for the construction of previously elusive 2-aryl-5-amino-2H-tetrazoles. This transformation takes advantage of readily available starting materials, proceeds under metal-free, mild, and robust conditions, and holds broad functional group compatibility. The utility of this protocol is further manifested via coupling, annulation, deamination, and denitrogenation derivatizations.

N-Triflination of pyrazolones: a new method for N-S bond formation

A simple method, which takes place quickly in 5 min, is developed for the N-triflination of pyrazolones using CF3SO2Na (Langlois reagent) and phenyliodine(iii)bis(trifluoroacetate) (PIFA). This reaction takes place at the imine nitrogen centre instead of the more reactive C4-position, forming a new N-S bond. A variety of pyrazolone derivatives were subjected to the reaction. Unlike the previous reports on sulfenylation or sulfonylation of pyrazolone, wherein the corresponding C-S bond is formed, this new method leads to the formation of the hetero-hetero atom bond (N-S bond) at room temperature.

Molecular Iodine-Promoted [3 + 2] Oxidative Cyclization for the Synthesis of Heteroarene-Fused [1,2,4] Thiadiazoles/Selenadiazoles

Two new classes of heteroarene-fused [1,2,4]thiadiazole and [1,2,4]selenadiazole are synthesized through the iodine-mediated [3 + 2] oxidative cyclization of 2-aminoheteroarenes and isothiocyanates/isoselenocyanates. This oxidative [3 + 2] annulation strategy is highly regiospecific to proceed a selective C-N bond formation at the endo-nitrogen of 2-aminoheteroarenes followed by an intramolecular oxidative N-S/N-Se bond formation. It is the first example of an I₂-mediated oxidative nitrogen-selenium (N-Se) bond formation.

Iodine(iii) promoted ring-rearrangement reaction of 1-arylamino-2-oxocyclopentane-1-carbonitriles to synthesize N-aryl-δ-valerolactams

Environmentally benign hypervalent iodine(iii) has been utilised selectively for carbocyclic to heterocyclic ring transformation under various electronic conditions with detailed structural and mechanistic investigation.

A facile, practical and metal-free microwave-assisted protocol for mono- and bis-[1,2,4]triazolo[1,5-a]pyridines synthesis utilizing 1-amino-2-imino-pyridine derivatives as versatile precursors

A facile and effective assembly of several substituted functionalized mono- and bis-[1,2,4]triazolo[1,5-a]pyridines from conveniently attainable 1-amino-2-imino-pyridines has been established. Using microwave irradiation speeds up the reaction efficiently, proceeding with a higher rate and yields than with conventional heating. In the presented protocol, a broad variety of carboxylic acids could be employed effectively to synthesize the respective derivatives via direct metal-free C–N bond construction. Interestingly, other substrates such as aldehydes (or their arylidene malononitriles), phenyl isothiocyanate, glyoxalic acid, and acrylonitriles could also provide the corresponding 1,2,4-triazolo[1,5-a]pyridines successfully. This versatile and convergent approach performs well with both deactivating and activating substrates in an environmentally benign manner compared with other already reported protocols. Other notable merits of the current strategy involve no need for column chromatography, no tedious work-up, and a direct pathway for the fast design of triazolopyridine frameworks. The identity of the newly synthesized compounds was established using several spectroscopic techniques, and X-ray single-crystal tools were employed to authenticate the suggested structures of some representative samples.

A novel and highly efficient, protocol for synthesizing mono- and bis-[1,2,4]triazolo[1,5-a]pyridines has been established utilizing the readily attainable 1-amino-2-imino-pyridines and microwave irradiation as green energy source.

Direct intramolecular double cross-dehydrogentive-coupling (CDC) cyclization of N-(2-pyridyl)amidines under metal-free conditions

A facile transition-metal-free protocol to form 2-iminoimidazo[1, 2-a]-pyridines bearing a -CHBr₂ group and an aza-quaternary carbon center at the 3 position from N-(2-pyridyl)amidines substrates, in which the new heterocyclic skeletons constructed from amidines via radical reactions or nucleophilic substitution reactions are promoted only by CBr₄ under mild conditions, is demonstrated. The reactions were realized by intramolecular CDC reaction involving C-N and C-C bond formation via the sequential C(sp³)-H bifunctionalization mode on the same carbon atom under mild conditions. Moreover, this work also provides an excellent and representative example for CBr₄ as an efficient reagent to initiate radical reactions under initiator-free conditions or to give rise to nucleophilic substitution reactions only by base.

C-N Coupling via Antiaromatic Endocyclic Nitrenium Ions

Herein, we report a C-N coupling reaction via antiaromatic endocyclic nitrenium ions. The nitrenium ion intermediate was generated by a combination of iodine(III) reagent PhI(OCOCF₃)₂ and the N-H center of benzimidazole units at ambient laboratory conditions. Metal-free synthesis of benzimidazole-fused phenanthridine derivatives was achieved in good to excellent yields.

Cyclization of Methyl-Coumalate-Derived Methyl 1-Benzamido-6-oxo-1,6-dihydropyridine-3-carboxylates: Assembly of the [1,2,4]Triazolo[1,5-a]pyridine Ring System

An efficient three-step synthesis of a series of fused bicyclic s-[1,2,4]triazolo[1,5-a]pyridines 1 was accomplished utilizing novel intermediates derived from inexpensive, commercially available hydrazides A and methyl coumalate B. A significant feature of this approach was the formation of a dihydrazide intermediate 2, bypassing the need for oxidative N-N bond formation in the 1,2,4-triazole synthesis. Further purification of the dihydrazides 2, beyond simple isolation, proved to be unnecessary owing to the impurity rejection afforded by the crystalline oxadiazolium salts 3. Additionally, the prepared oxadiazolium perchlorate salts showed excellent moisture stability, an unusual feature in compounds of this type.

A photo-auxiliary approach - enabling excited state classical phototransformations with metal free visible light irradiation

Most traditional photoreactions require UV light to yield the desired product. To address this issue, photoreaction of hydrazide based chromophores was evaluated with visible light using a metal free photocatalyst to afford photoproducts in high yields. This hydrazide functionality itself may be removed/modified after the photoreaction, highlighting its role as a "photo-auxiliary". A preliminary mechanistic model based on photophysical experiments is provided to highlight the generality of the strategy.

Bimetallic copper and zinc-catalyzed oxidative cycloaddition of 3-aminopyridazines and nitriles: a direct synthesis of 1,2,4-triazolo[1,5-b]pyridazines via C–N and N–N bond-forming process

One-pot formation of 1,2,4-triazolo[1,5-b]pyridazine derivatives is presented in this manuscript, in which the targets are offered via cooperative Cu(i)/Zn(ii)-catalyzed tandem C–N addition and I₂/KI-mediated intramolecular N–N bond formation.

PIFA-Mediated oxidative cyclization of 1-aroyl-N-arylcyclopropane-1-carboxamides and their application in the synthesis of pyrrolo[3,2-c]quinolinones

A PIFA-promoted ring-closing reaction for the synthesis of spirocyclopropane quinolinediones, and their application toward pyrrolo[3,2-c]quinolinones.

Organoiodine reagent-promoted intermolecular oxidative amination: synthesis of cyclopropyl spirooxindoles

An organoiodine-promoted intramolecular direct oxidative C(sp²)–N cross-coupling reaction was developed for the preparation of cyclopropyl spirooxindoles from readily available secondary cyclopropyl carboxamides.

Syntheses of [1,2,4]triazolo[1,5-a]benzazoles enabled by the transition-metal-free oxidative N-N bond formation

A transition-metal-free oxidative N-N bond formation strategy was developed to generate various structurally interesting [1,2,4]triazolo[1,5-a]benzazoles efficiently. The mechanism of the key oxidative N-N bond formation was investigated by using an intramolecular competition reaction. Notably, the first single crystal structure was also obtained to confirm the structure of 2-aryl[1,2,4]triazolo[1,5-a]benzimidazole.

Hypervalent iodine(iii)-promoted N-incorporation into N-aryl vinylogous carbamates to quinoxaline diesters: access to 1,4,5,8-tetraazaphenanthrene

An oxidative N-incorporation strategy for synthesis of quinoxaline diesters under metal-free and mild reaction conditions is described via the formation of two C(sp²)–N bonds utilizing NaN₃ as the cheap N-atom source.

I2/KI-Mediated Oxidative N-N Bond Formation for the Synthesis of 1,5-Fused 1,2,4-Triazoles from N-Aryl Amidines

An I2/KI-mediated oxidative N-N bond formation reaction is described. This new and environmentally benign approach allows for the convenient synthesis of a variety of 1,2,4-triazolo[1,5-a]pyridines and other 1,5-fused 1,2,4-triazoles from readily available N-aryl amidines in an efficient and scalable fashion.

Oxidative Heterocycle Formation Using Hypervalent Iodine(III) Reagents

Hypervalent iodine(III) reagents have been widely exploited in a diverse array of synthetic transformations. This chapter focuses on the general application of hypervalent iodine(III) reagents in the de novo synthesis and in the late stage functionalization of heterocyclic compounds.

Construction of N-containing heterocycles via oxidative intramolecular N–H/X–H coupling

The preparation of N-containing heterocycles using oxidative coupling has become a hot topic recently. This feature article summarized the methodology related to the construction of N-heterocycles through intramolecular oxidative N–H/X–H coupling.

Cu-catalyzed aerobic oxidative cyclization of guanidylpyridines and derivatives

A new method for the straightforward synthesis of 2-amino-[1,2,4]triazolo[1,5-a]pyridines and derivatives is presented. The target products are synthesized in high yields from guanidylpyridines and analogues via copper-catalyzed N-N coupling. The present methodology shows a wide scope, tolerating not only different substituents on the pyridine ring but also different heterocylic rings such as pyrazines, pyrimidines, and pyridazines.