Synthesis of Dihydropyrazoles via Ligand-Free Pd-Catalyzed Alkene Aminoarylation of Unsaturated Hydrazones with Diaryliodonium Salts

Palladium-catalysed alleneamination of γ,δ-unsaturated hydrazones with propargylic acetates: access to tetrahydropyridazines bearing allenes

A highly efficient palladium-catalysed alleneamination of γ,δ-unsaturated hydrazones with propargylic acetates has been developed. This novel method was used to synthesize tetrahydropyridazines bearing allene derivatives, and did so in moderate to good yields. This protocol features a simple catalytic system, broad substrate scope, easy scale-up, and versatile transformations.

Iron(II)-Catalyzed Synthesis of Nitrated Dihydropyrazole via a Nitrification and C-H Activation/Cyclization Cascade Reaction

A route for nitrification and C-H activation/cyclization of (E)-N-allyl-N'-benzylidenebenzenesulfonohydrazide and cobalt nitrate via an iron(II)-catalyzed cascade reaction to synthesize nitrated dihydropyrazoles has been developed. The method features convenient operation and good functional group tolerance. In addition, it employs insensitive and inexpensive FeSO4 as the catalyst and provides a direct approach for the preparation of dihydropyrazoles.

Structural Reassignment of Covalent Organic Framework-Supported Palladium Species: Heterogenized Palladacycles as Efficient Catalysts for Sustainable C-H Activation

Recent decades have witnessed remarkable progress in ligand-promoted C-H activation with palladium catalysts. While a number of transformations have been achieved with a fairly broad substrate scope, the general requirements for high palladium loadings and enormous challenges in catalyst recycling severely limit the practical applications of C-H activation methodologies in organic synthesis. Herein, we incorporate N,C-ligand-chelated palladacycles into rigid, porous, and crystalline covalent organic frameworks for the C-H arylation of indole and pyrrole derivatives. These heterogeneous palladium catalysts exhibit superior stability and recyclability compared to their homogeneous counterparts. We not only produce several highly reactive palladacycles embedded on new framework supports to facilitate C-H activation/C-C bond-forming reactions but also reassign heterogenized palladium species on frameworks containing a benzaldehyde-derived imine moiety as imine-based palladacycles via comprehensive characterization. Our findings provide guidance for the rational design of framework-supported metallacycles in the development of heterogeneous transition-metal catalysis.

Recent Progress in Synthetic Applications of Hypervalent Iodine(III) Reagents

Hypervalent iodine(III) compounds have found wide application in modern organic chemistry as environmentally friendly reagents and catalysts. Hypervalent iodine reagents are commonly used in synthetically important halogenations, oxidations, aminations, heterocyclizations, and various oxidative functionalizations of organic substrates. Iodonium salts are important arylating reagents, while iodonium ylides and imides are excellent carbene and nitrene precursors. Various derivatives of benziodoxoles, such as azidobenziodoxoles, trifluoromethylbenziodoxoles, alkynylbenziodoxoles, and alkenylbenziodoxoles have found wide application as group transfer reagents in the presence of transition metal catalysts, under metal-free conditions, or using photocatalysts under photoirradiation conditions. Development of hypervalent iodine catalytic systems and discovery of highly enantioselective reactions using chiral hypervalent iodine compounds represent a particularly important recent achievement in the field of hypervalent iodine chemistry. Chemical transformations promoted by hypervalent iodine in many cases are unique and cannot be performed by using any other common, non-iodine-based reagent. This review covers literature published mainly in the last 7-8 years, between 2016 and 2024.

Electrocatalytic dehydrogenative and defluorinative coupling between aldehyde-derived N,N-dialkylhydrazones and fluoromalonates: synthesis of 2-pyrazolines

An electrocatalytic synthesis of 2-pyrazolines via dehydrogenative and defluorinative cross-coupling reactions between (hetero)arylaldehyde-derived N,N-dialkylhydrazones and fluoromalonates is disclosed. Salient features of this work include (i) readily available starting materials, (ii) practical reaction conditions, and (ii) a formal oxidative (4 + 1)-cycloaddition via triple C-H bond functionalization. Cyclic voltammetry analyses support the electrocatalytic formation of an α-fluoromalonyl radical.

Integrating Olefin Carboamination and Hofmann-Löffler-Freytag Reaction by Radical Deconstruction of Hydrazonyl N-N Bond

As a type of elementary organic compounds containing N-N single bond, hydrazone involved chemical conversions are extremely extensive, but they are mainly limited to N2-retention and N2-removal modes. We report herein an unprecedented protocol for the realization of division utilization of the N2-moiety of hydrazone by a radical facilitated N-N bond deconstruction strategy. This new conversion mode enables the successful combination of alkene carboamination and Hofmann-Löffler-Freytag reaction by the reaction of N-homoallyl mesitylenesulfonyl hydrazones with ethyl difluoroiodoacetate under photocatalytic redox neutral conditions. Mechanism studies reveal that the reaction undergoes a radical relay involving addition, crucial remote imino-N migration and H-atom transfer. Consequently, a series of structurally significant ϵ-N-sulphonamide-α,α-difluoro-γ-amino acid esters are efficiently produced via continuous C-C bond and dual C-N bonds forging.

Synthesis of Bis-Heterocycles Bearing Methyleneindole Motifs by Pd-Catalyzed Domino Reaction

A highly robust, general, and practically simple palladium-catalyzed domino bicyclization strategy is presented to synthesize nitrogen-containing bis-heterocycles bearing methylene indole motifs from alkyne-tethered carbamoyl chlorides and β,γ- or γ,δ-unsaturated hydrazones. The salient features of this transformation include broad substrate scope, good functional group tolerance, ease for scale-up, and convenient conversion.

Cu(II)-Catalyzed Aminocyclization of N-Propargyl Hydrazones to Substituted Pyrazolines

An efficient route for the copper(II)-catalyzed synthesis of substituted pyrazolines from readily accessible N-propargyl hydrazones has been reported under open flask conditions via intramolecular C-N bond formation. N-acyl and N-tosyl-substituted pyrazolines have been prepared in moderate to excellent yields. Mechanistic investigations using NMR, high-resolution mass spectrometry (HRMS), and Hammett analyses suggest that the Cu(II) catalyst generally acts as a Lewis acid to form an iminium-ion intermediate via cyclization, which afforded the desired pyrazolines upon hydrolysis. One progesterone receptor antagonist has also been synthesized utilizing this reaction methodology.

SOCl2-Catalyzed Meyer-Schuster Rearrangement of 3°-Propargylic Alcohols: Synthesis of Densely Arene-Substituted Pyrazolines Bearing Quaternary Centers from α,β-Unsaturated Carbonyl Compounds and Arylhydrazines

Meyer-Schuster rearrangement of 3°-propargyl alcohol to the corresponding α,β-unsaturated carbonyl compounds under SOCl2 catalysis has been reported. Terminal and internal propargyl alcohols efficiently participated in the reaction. Furthermore, we have demonstrated the synthetic utility of conjugated carbonyl compounds to access densely arene-substituted pyrazolines bearing quaternary centers by reacting with arylhydrazine hydrochloride. Isotopic labeling studies were carried out to gain insights into the reaction mechanism.

Annulative Aminosulfonylation of Unactivated Alkenes for Accessing Pyrazolines via Multicomponent SO2 Insertion

A direct arylsulfonylation of β,γ-unsaturated hydrazones method, in which sulfonated pyrazolines are accessed by a three-component reaction of β,γ-unsaturated hydrazones, DABSO, and aryldiazonium tetrafluoroborates, has been developed without external oxidants or catalysts. This transformation is triggered by the formation of arylsulfonyl radicals in situ from the reaction of aryldiazonium tetrafluoroborates and DABSO, and is enabled by controllable generation of C center radical, in which DABSO was utilized as the sulfone source and an oxidant in this radical-mediated cascaded reaction. A wide range of substrates can be applied in this process to afford pyrazolines in good yield, and it is amenable for gram-scale synthesis.

Synthesis of Tetrahydrofurans and Pyrrolidines by Copper-Catalyzed Oxy/Aminoarylation of Alkenes

An efficient copper-catalyzed inter/intramolecular oxy/aminoarylation of γ-hydroxy/aminoalkenes with diaryliodonium triflates is reported. Simple activation of these arylating agents with copper(II) triflate in dichloromethane triggers a smooth activation of the alkene, which is simultaneously trapped by the internal nucleophile, yielding, depending upon its nature, a range of highly substituted tetrahydrofurans and pyrrolidines. The cyclization was moreover found to be stereospecific, with diastereoisomeric alkenes yielding diastereoisomers of the cyclized product, and could be extended to oxyalkynylation.

Synthetic Approaches, Biological Activities, and Structure-Activity Relationship of Pyrazolines and Related Derivatives

It has been established that pyrazolines and their analogs are pharmacologically active scaffolds. The pyrazoline moiety is present in several marketed molecules with a wide range of uses, which has established its importance in pharmaceutical and agricultural sectors, as well as in industry. Due to its broad-spectrum utility, scientists are continuously captivated by pyrazolines and their derivatives to study their chemistry. Pyrazolines or their analogs can be prepared by several synthesis strategies, and the focus will always be on new greener and more economical ways for their synthesis. Among these methods, chalcones, hydrazines, diazo compounds, and hydrazones are most commonly applied under different reaction conditions for the synthesis of pyrazoline and its analogs. However, there is scope for other molecules such as Huisgen zwitterions, different metal catalysts, and nitrile imine to be used as starting reagents. The present article consists of recently reported synthetic protocols, pharmacological activities, and the structure-activity relationship of pyrazoline and its derivatives, which will be very useful to researchers.

Palladium/PC-Phos-Catalyzed Asymmetric Heck/Tsuji-Trost Reactions of Amino-Tethered 1,3-Cyclohexadiene with Aryl and Alkenyl Halides

Chiral perhydroindoles are found in a number of natural products and biologically active compounds. Therefore, the development of new asymmetric methodology for rapid access to this core is of high importance. Herein, we reported a highly regio- and diastereo-selective palladium/PC-Phos-catalyzed asymmetric Heck/Tsuji-Trost reactions of readily available amino tethered 1,3-cyclohexadienes with aryl and alkenyl halides, delivering various functionalized chiral hexahydroindoles in good yields with high enantioselectivity. The application of this reaction to the concise synthesis of (-)-α-Lycorane was demonstrated. DFT computation results indicate that the difference in ΔE_dis of two migration insertion transition states determines the enantioselectivity of the reaction.

Chemodivergent Synthesis of Aza-Heterocycles with a Quarternary Carbon Center via [4 + 1] Annulation between Azoalkenes and α-Bromo Carbonyl Compounds

An efficient [4 + 1] annulation reaction between in situ generated azoalkene intermediates and α-bromocarbonyls has been established. A series of skeletally diverse aza-heterocycles with a functionalized quaternary center were obtained in up to 89% yield under mild conditions.

Synthesis of dihydropyrazoles enabled by Pd-catalyzed carboamination of alkenyl hydrazones with alkenyl and aryl halides

The synthesis of dihydropyrazoles enabled by Pd-catalyzed carboamination of alkenyl hydrazones with alkenyl or aryl halides is described. This method provides a practical and efficient route towards the synthesis of functionalized dihydropyrazoles.

A benzene-bridged divanadium complex-early transition metal catalyst for alkene alkylarylation with PhI(O2CR)2via decarboxylation

The synthesis, structure and catalytic activity of a benzene-bridged divanadium complex were comprehensively studied. The reduction of (Nacnac)VCl2 (1) (Nacnac = (2,6-iPr2C6H3NCMe)2HC) supported by β-diketiminate with potassium graphite (KC8) by employing benzene as the solvent allows access to the benzene-bridged inverted-sandwich divanadium complex (μ-η6:η6-C6H6)[V(Nacnac)]2 (2a), which can catalyze alkene alkylarylation with hypervalent iodine(iii) reagents (HIRs) via decarboxylation to generate regioselectively diverse indolinones. Furthermore, the mild nature of this reaction was amenable to a wide range of functionalities on alkenes and HIRs. Mechanistic studies revealed a relay sequence of decarboxylative radical alkylation/radical arylation/oxidative re-aromatization.

Brønsted acid-promoted hydroamination of unsaturated hydrazones: access to biologically important 5-arylpyrazolines

A novel and efficient Brønsted acid-promoted hydroamination of hydrazone-tethered olefins has been developed. A variety of pyrazolines have been easily obtained in good to excellent yields with high chemo- and regioselectivity under simple and mild conditions. This method represents a straightforward, facile, and practical approach toward biologically important 5-arylpyrazolines, which are difficult to access by previously reported radical hydroamination of β,γ-unsaturated hydrazones.

An efficient, chemo- and regioselective Brønsted acid-promoted hydroamination reaction of hydrazone-tethered olefins towards 5-arylpyrazolines was developed.

Access to cyano-substituted pyrazolines through copper-catalyzed cascade cyanation/cyclization of unactivated olefins

A mild copper-catalyzed cascade cyanation/cyclization of hydrazone-tethered unactivated olefins was developed for the efficient and practical synthesis of cyano-containing pyrazolines.

Rhenium-catalyzed alkylarylation of alkenes with PhI(O2CR)2via decarboxylation to access indolinones and dihydroquinolinones

Rhenium-catalyzed alkylarylation of alkenes with hypervalent iodine(iii) reagents (HIRs) via decarboxylation to access various 3,3-disubstituted indolinones and trans-3,4-dihydroquinolinones is described.

Visible-Light-Driven Chlorotrifluoromethylative and Chlorotrichloromethylative Cyclizations of Enynes

Described herein is a visible-light-driven chlorotrifluoromethylative and chlorotrichloromethylative cyclization reaction to synthesize chlorotrifluoromethylated and chlorotrichloromethylated cyclic compounds. Visible-light photochemistry was utilized to generate trifluoromethyl and trichloromethyl radicals and trigger radical addition/cyclization/chlorination sequences. The use of terminal alkene-derived enynes enables the regioselective and stereoselective synthesis of chlorotrifluoromethylated and chlorotrichloromethylated pyrrolidines, piperidines, and cyclopentanes.

Copper-Catalyzed Aerobic 6-Endo-Trig Cyclization of β,γ-Unsaturated Hydrazones for the Divergent Synthesis of Dihydropyridazines and Pyridazines

A divergent synthetic strategy to 1,6-dihydropyridazines and pyridazines through Cu(II)-catalyzed controllable aerobic 6-endo-trig cyclization was developed. The selectivity can be rationally tuned via the judicious choice of reaction solvent. It was found that the 1,6-dihydropyridazines were obtained in moderate to high yields with CH₃CN as the reaction solvent, whereas employment of AcOH directly afforded pyridazines in up to 92% yields, probably arising from the oxidation of the in situ generated 1,6-dihydropyridazines.

Formal [4 + 1] cycloaddition of in situ generated 1,2-diaza-1,3-dienes with diazo esters: facile approaches to dihydropyrazoles containing a quaternary center

A Cu(ii)/bisoxazoline ligand-promoted formal [4 + 1] cycloaddition of diazo esters with azoalkenes formed in situ has been developed. This strategy provides a potential protocol for the construction of dihydropyrazoles containing a quaternary center with good to excellent yields.

Metal-free cyclization of unsaturated hydrazones for the divergent assembly of pyrazolones and pyrazolines

An oxidative cyclization of β,γ-unsaturated hydrazones for the divergent assembly of pyrazolones and pyrazolines under metal-free conditions is presented.

Transition-metal-free N-difluoromethylation of hydrazones with TMSCF2Br as the difluoromethylation reagent

A transition-metal-free N-difluoromethylation protocol is developed to synthesize a series of N-difluoromethyl hydrazones with TMSCF₂Br as the difluoromethylation reagent.

Palladium-Catalyzed Intermolecular Arylcarbonylation of Unactivated Alkenes: Incorporation of Bulky Aryl Groups at Room Temperature

A palladium-catalyzed intermolecular arylcarbonylation of unactivated alkenes has been developed. Unsymmetrical diaryliodonium salts (DAISs) were used as arylation reagents, the bulky aryl group (Ar_L ) of which was exclusively incorporated into the arylcarbonylated products, which contained the Ar_L group and a carboxylic ester group at the α- and β-carbon position, respectively, of the original terminal C-C double bond. The reaction features excellent chemo- and regioselectivity, high functional-group tolerance, and very mild reaction conditions.

Copper-Mediated Aminoazidation, Aminohalogenation, and Aminothiocyanation of β,γ-Unsaturated Hydrazones: Synthesis of Versatile Functionalized Pyrazolines

A versatile method for the rapid synthesis of diverse functionalized pyrazolines has been developed based on copper-mediated aminofunctionalization of β,γ-unsaturated hydrazones. The scope of this strategy encompasses a range of difunctionalization reactions: aminoazidation, aminohalogenation, and aminothiocyanation. These reactions provide straightforward access to a series of useful pyrazoline building blocks containing various functional groups that are hard to access traditionally.

Tandem cyclization of o-alkynylanilines with isocyanides triggered by intramolecular nucleopalladation: access to heterocyclic fused 2-aminoquinolines

Herein, a novel strategy for the synthesis of various heterocyclic fused 2-aminoquinolines via palladium-catalyzed tandem cyclization of o-alkynylanilines with isocyanides has been developed. This process includes trans-oxy/aminopalladation, isocyanide insertion, elimination and 1,3-hydrogen migration. Besides high atom and step economy, this method shows good functional group compatibility with excellent chemo- and regioselectivities under mild reaction conditions.

Palladium-Catalyzed Ring-Forming Alkene Aminoaroylation of Unsaturated Hydrazones and Sulfonamides

The first example of a Pd(OAc)₂-catalyzed ring-forming alkene aminoaroylation of unsaturated hydrazones and sulfonamides is described. This protocol features the use of diaryliodonium salts as both oxidants and aryl sources, thus enabling mild reaction conditions, good chemoselectivity, a broad substrate scope, and high functional group tolerance. A wide range of synthetically and biologically important functionalized dihydropyrazoles and isoxazolidines have been obtained in good yields.

Copper-Promoted 6- endo-trig Cyclization of β,γ-Unsaturated Hydrazones for the Synthesis of 1,6-Dihydropyridazines

A novel and efficient strategy for the synthesis of 1,6-dihydropyridazines via copper-promoted 6- endo-trig cyclization of readily available β,γ-unsaturated hydrazones have been developed. A series of 1,6-dihydropyridazines have been synthesized by this method with good yields, high functional group tolerance, and remarkable regioselectivity under mild conditions. Importantly, the 1,6-dihydropyridazines can be efficiently converted to biologically important pyridazines in the presence of NaOH.

Iodine(iii)-mediated intramolecular sulfeno- and selenofunctionalization of β,γ-unsaturated tosyl hydrazones and oximes

A cascade radical cyclization/sulfenylation or selenylation of β,γ-unsaturated hydrazones and oximes was realized under mild conditions with phenyliodine(iii) diacetate (PIDA) as the sole oxidant, leading to the construction of diversely functionalized heteroatom-containing pyrazoline and isoxazoline derivatives.