Harnessing the versatility of hydrazones through electrosynthetic oxidative transformations

Hydrazones are important structural motifs in organic synthesis, providing a useful molecular platform for the construction of valuable compounds. Electrooxidative transformations of hydrazones constitute an attractive opportunity to take advantage of the versatility of these reagents. By directly harnessing the electrical current to perform the oxidative process, a large panel of organic molecules can be accessed from readily available hydrazones under mild, safe and oxidant-free reaction conditions. This review presents a comprehensive overview of oxidative electrosynthetic transformations of hydrazones. It includes the construction of azacycles, the C(sp2)-H functionalization of aldehyde-derived hydrazones and the access to diazo compounds as either synthetic intermediates or products. A special attention is paid to the reaction mechanism with the aim to encourage further development in this field.

Transition-Metal-Free Sulfoximidation of Sulfonylhydrazones for the Synthesis of N-Sulfanylidenehydrazonamides

Herein, we report a transition-metal-free C-H bond sulfoximidation protocol of sulfonyl hydrazones with hypervalent iodine(III) reagents. A library of novel N-sulfaneylidenehydrazonamides was constructed via chemoselective C-N bond formation reactions at aldehyde C-H bonds of sulfonyl hydrazones in the presence of a base. The reaction demonstrated broad substrate group diversity under exceedingly mild reaction conditions, and excellent yields were achieved at room temperature.

A Cascade Synthesis of Indoles

We report herein an expedient method for the regioselective synthesis of indoles from o-haloanilines and α-ketol-derived N-tosylhydrazones. This two-step, modular synthesis of N-H indoles can be carried out conveniently without purification of intermediates.

Copper-Catalyzed Sulfur Alkylation of Sulfenamides with N-Sulfonylhydrazones

Sulfilimines are valuable compounds in both organic synthesis and pharmaceuticals. In this study, we present a copper-catalyzed sulfur alkylation of sulfenamides with N-sulfonylhydrazones. In contrast to prior findings, hydrazones derived from aldehydes act as donor-type carbene precursors, effectively engaging in coupling with sulfenamides via a copper catalyst, demonstrating exclusive S selectivity. The utility of the protocol was highlighted in the rapid access to a wide range of sulfoximine derivatives.

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.

Palladium-catalyzed asymmetric carbene coupling en route to inherently chiral heptagon-containing polyarenes

Developing facile and direct synthesis routes for enantioselective construction of cyclic π-conjugated molecules is crucial. However, originate chirality from the distorted structure around heptagon-containing polyarenes is largely overlooked, the enantioselective construction of all-carbon heptagon-containing polyarenes remains a challenge. Herein, we present a highly enantioselective synthesis route for fabricating all carbon heptagon-containing polyarenes via palladium-catalyzed carbene-based cross-coupling of benzyl bromides and N-arylsulfonylhydrazones. A wide range of nonplanar, saddle-shaped tribenzocycloheptene derivatives are efficiently prepared in high yields with excellent enantioselectivities using this approach. In addition, stereochemical stability experiments show that these saddle-shaped tribenzocycloheptene derivatives have high inversion barriers.

N-Heterocyclic Carbene-Catalyzed Facile Synthesis of Phthalidyl Sulfonohydrazones: Density Functional Theory Mechanistic Insights and Docking Interactions

N-heterocyclic carbene catalysis reaction protocol is disclosed for the synthesis of phthalidyl sulfonohydrazones. A broad range of N-tosyl hydrazones react effectively with phthalaldehyde derivatives under open-air conditions, enabling the formation of a new C-N bond via an oxidative path. The reaction proceeds under mild reaction conditions with broad substrate scopes, wide functional group tolerance, and good to excellent yields. The mechanistic pathway is studied successfully using control experiments, competitive reactions, ESI-MS spectral analyses of the reaction mixture, and computational study by density functional theory. The potential use of one of the phthalidyl sulfonohydrazone derivatives as the inhibitor of β-ketoacyl acyl carrier protein synthase I of Escherichia coli is investigated using molecular docking.

Construction of Trisubstituted Hydrazones via Base-Mediated Cascade Condensation N-Alkylation

A practical base-promoted tandem condensation N-alkylation reaction for the formation of trisubstituted hydrazones has been developed employing aldehydes and hydrazines with alkyl halides. Crucially, this reaction successfully overcomes chemoselectivity problems, allowing for the reaction of multiple components in a one-pot manner. Halo- and heterofunctional groups, as well as free hydroxyl and amino groups, are tolerated in this transformation to produce a wide range of trisubstituted hydrazones in good to excellent yields.

Polycyclic Pyrazoles from Alkynyl Cyclohexadienones and Nonstabilized Diazoalkanes via [3 + 2]-Cycloaddition/[1,5]-Sigmatropic Rearrangement/Aza-Michael Reaction Cascade

An efficient method for the stereoselective synthesis of "all center substituted" polycyclic pyrazoles from alkynyl cyclohexa-2,5-dienones and nonstabilized diazoalkanes via sequential [3 + 2]-cycloaddition/[1,5]-sigmatropic rearrangement and aza-Michael reactions is reported. The developed process is highly regioselective and stereoselective. It employs a wide substrate scope to furnish structurally diverse linear and bridged [4.4.n.0] ring-fused pyrazoles in moderate to good yields. One-pot and gram-scale syntheses and synthetic transformations have also been showcased.

Dearomatization of Pyridines: Stereoselective Synthesis of Functionalized [1,2,4]Triazolo[4,3-a]pyridines

The stereoselective preparation of functionalized [1,2,4]triazolo[4,3-a]pyridines from N-tosylhydrazones and pyridines was developed through the dearomatization of pyridines. The current transformation features good step- and atom-economy, high diastereoselectivity, and the efficient formation of four new carbon-heteroatom bonds in the corresponding product tetrahydro pyridines.

Ruthenium-Catalyzed Chemo-Selective Carbene Insertion into C-H Bond of Styrene over Cyclopropanation: C-C Bond Formation

The C-C bond formation reactions are important in organic synthesis. Heck reaction is known to arylate the terminal carbon of olefins; however, direct alkylation of the terminal carbon of olefin is limited. Herein, we report a novel ruthenium-catalyzed selective cross-coupling reaction of styrene and α-diazoesters to form a new C-C bond over cyclopropanation via the C-H insertion process for the first time. Using this novel methodology, a wide variety of substrates have been utilized and a variety of α-vinylated benzylic esters and densely functionalized olefins have been synthesized with good stereoselectivity under mild reaction conditions. The overall reaction process proceeds through the carbene insertion into styrene to form the desired products in good to excellent yields with proper stereoselectivity. The selective C-H inserted product, wide substrate scope, and excellent functional group tolerance are the best features of this work.

A New Pathway for the Synthesis of Ketones from Aldehydes and Sulfonylhydrazones: Is Diazo the Key Intermediate?

A new pathway via a cyclic intermediate for the synthesis of ketones from aldehydes and sulfonylhydrazone derivatives under basic conditions is proposed. Several control experiments were performed along with analysis of the mass spectra and in-situ IR spectra of the reaction mixture. Inspired by the new mechanism, an efficient and scalable method for homologation of aldehydes to ketones was developed. A wide variety of target ketones were obtained in yields of 42-95 % by simply heating the 3-(trifluoromethyl)benzene sulfonylhydrazones (3-(Tfsyl)hydrazone) for 2 h at 110 °C with aldehydes and with K2 CO3 and DMSO as base and solvent, respectively.

Synthesis of Indoles from o-Haloanilines

A convenient method for the synthesis of indoles has been developed by the sequential orchestration of the cross-coupling reaction of o-haloaniline and PIFA oxidation of the resulting 2-alkenylanilines. A highlight of this two-step indole synthesis is a modular strategy which is applicable to both acyclic and cyclic starting materials. Particularly noteworthy is the regiochemistry that is complementary to the Fischer indole synthesis and related variants. Direct preparation of N-H indoles with no N-protecting group is also advantageous.

Catalyst-Free Synthesis of Functionalized 4-Substituted-4H-Benzo[d][1,3]oxazines via Intramolecular Cyclization of ortho-Amide-N-tosylhydrazones

Functionalized 4-aryl-4H-benzo[d][1,3]oxazines are synthesized under transition-metal-free conditions using ortho-amide-N-tosylhydrazones. This synthetic method uses readily available N-tosylhydrazones as the diazo compound precursors and involves an intramolecular ring closure reaction mediated by a protic polar additive (iPrOH). A wide range of functionalized oxazines are obtained by this straightforward method in good to excellent yields. Furthermore, the viability of our strategy is illustrated by the gram-scale elaboration of a bromo-substituted 4H-benzo[d][1,3]oxazine and its post-functionalization by palladium-catalyzed cross-couplings.

Stereoselective Synthesis of trans-Stilbenes through Silver-Catalyzed Self-Coupling of N-Triftosylhydrazones: An Experimental and Theoretical Study

A silver-mediated homocoupling of N-triftosylhydrazones for the construction of trans-stilbenes has been presented. This protocol is characterized by its suitability for both inter- and intramolecular reactions, operational simplicity, high efficiency with excellent stereoselectivity, broad substrate scope, and good functional group tolerance. A plausible mechanism involving nucleophilic attack of in situ generated sulfonium ylides on silver carbene was proposed on the basis of experimental results and DFT calculations, which further indicates that π-π stacking interactions play a dominant role in stereoselectivity control.

Copper-Catalyzed Direct Carbonylation of Carbenes toward the Synthesis of Propanedioic Acid Derivatives

Carbenes are highly active reaction intermediates, which can be used as reaction precursors to modify organisms, drugs, and material molecules. In this work, we realized a new cheap metal-catalyzed carbonylation of carbene to give propanedioic acid derivatives. With copper salt as the catalyst, synthetically important malonates and related compounds were produced in good yields under mild reaction conditions.

Copper iodide nanoparticles (CuI NPs): an efficient catalyst for the synthesis of alkynyl esters

An environmentally benign protocol for the synthesis of alkynyl esters, by the cross-coupling of diazoacetate with various substituted alkynes under neat reaction conditions, has been described. Copper iodide nanoparticles (CuI NPs) were found to promote the Sonogashira-type coupling to afford the corresponding alkynyl esters in good yields. The CuI nanoparticles were characterized by PXRD, FESEM, EDAX, and Raman techniques. The developed methodology has several advantages such as a broad substrate scope, less reaction time, atom economy, avoidance of an additive/base/solvent, and enhanced values of green chemistry. The catalyst was recycled up to threefold without the loss of its catalytic activity.

Recyclable Copper(I)-Catalyzed Cross-Coupling of Trialkylsilylethynes and N-Tosylhydrazones Leading to the Formation of C(sp)-C(sp3) Bonds

A highly efficient heterogeneous copper(I)-catalyzed cross-coupling of trialkylsilylethynes with N-tosylhydrazones has been achieved in dioxane at 90-110 °C via the Cu carbene migratory insertion with an SBA-15-immobilized l-proline-Cu(I) complex [SBA-15-l-Proline-CuI] as the catalyst and LiO^tBu as the base, leading to the formation of C(sp)-C(sp³) bonds. The reaction generates a wide variety of alkyltrialkylsilylalkynes in moderate to high yields. This new heterogenized copper(I) complex exhibits a comparable catalytic efficiency to homogeneous CuI and can be easily recovered through a simple centrifugation process and is recyclable up to 12 times without a remarkable loss of activity.

Palladium-Catalyzed Cyclizative Borylation of Allenyl Ketones through Carbene Boryl Migratory Insertion: Access to Densely Substituted Furyl Boronates

Herein the palladium-catalyzed cyclizative borylation of allenyl ketones with diboron compounds is reported which involves the carbene boryl migratory insertion as the key step. This reaction features mild conditions, good functional group tolerance and broad substrate scope. Thus, it represents an efficient methodology for the assembly of diverse tri-substituted furyl boronates. In addition, a series of transformations of the resultant multi-substituted furyl boronates were conducted to provide various densely substituted furan derivatives in good yields, further illustrating the potential synthetic utility of this methodology.

Transition-Metal-Free Multicomponent Approach to the Regioselective Synthesis of Highly Substituted N-Alkylpyrazoles

An efficient transition-metal-free multicomponent approach to the regioselective synthesis of highly substituted N-alkylpyrazoles through 1,6-addition of pyrazole (in situ generated from α,β-unsaturated aldehyde and hydrazide) to para-Quinone Methides has been developed. The N-alkylpyrazole containing triarylmethanes having several heteroaryl rings (quinoline, pyridine, thiophene) at the central methine carbon atom was developed. This chemical process may be used for large-scale synthesis and provides a novel way to produce triarylmethanes with diverse functional groups.

α-Carbonyl sulfoxonium ylides in transition metal-catalyzed C-H activation: a safe carbene precursor and a weak directing group

Transition metal-catalyzed cross-coupling of sp² C-H bonds with diazo compounds via carbene migratory insertion represents an efficient strategy for the construction of C-C and C-heteroatom bonds in organic synthesis. Despite the popularity of diazo compounds as coupling partners in C-H activation, they pose serious safety and stability issues due to potential exothermic reactions linked with the release of N₂ gas. However, compared with diazo compounds, sulfoxonium ylides are generally crystalline solids, more stable, widely used in industrial scales, and easier/safer to prepare. Therefore, recent years have witnessed an upsurge in employing α-carbonyl sulfoxonium ylides as an alternative carbene surrogate in transition metal-catalyzed C-H activation. Unlike diazo compounds, α-carbonyl sulfoxonium ylides contain inherent potential to serve as a coupling partner as well as a weak directing group. This review will summarize the progress made in both categories of reactions.

Cu/SaBox-Catalyzed Photoinduced Coupling of Acylsilanes with Alkynes

The transition metal-catalyzed cross-coupling reaction with Fischer metal carbene intermediates bearing an electron-rich alkoxyl or siloxyl group remains a big challenge due to the lack of readily available corresponding carbene precursors. Herein, we report the coupling of alkynes with the Fischer-type copper carbene species bearing a α-siloxyl group, which could be in situ generated from acylsilanes catalytically under photoirradiation and redox-neutral conditions. The side-arm modified bisoxazoline (SaBox) ligands prove to be crucial for this coupling reaction, which provides the corresponding alkynyl alcohol in high yields with remarkable heterocycle tolerance and broad substrate scope.

Highly electrophilic silver carbenes

Catalytic carbene transfer reactions are fundamental transformations in modern organic synthesis, which enable direct access to diverse structurally complex molecules. Despite diazo precursors playing a crucial role in catalytic carbene transfer reactions, most reported methodologies take into account only diazoacetates or related compounds. This is primarily because diazoalkanes, unless they contain a resonance stabilizing group, are more susceptible to violent exothermic decomposition. In this feature article, we present an alternative approach to carbene-transfer reactions based on the formation of highly electrophilic silver carbenes from N-sulfonylhydrazones, where the high electrophilicity of silver carbenes stems from the weak interaction between silver and the carbenic carbon. These precursors are readily accessible, stable, and environmentally sustainable. Using the strategy that employs highly electrophilic silver carbenes, it is possible to develop novel intermolecular transformations involving non-stabilized carbenes, including C(sp³)-H insertion, C(sp³)-C(O) insertion, cycloaddition, and defluorinative functionalization. The silver-catalyzed carbene transfer reactions described here have high efficiency, unusual reactivity, exceptional selectivity, and a reaction pathway that differs from typical transition metal-catalyzed reactions. Our research provided fundamental insight into silver carbene chemistry, and we hope to apply this mode of catalysis to other more general transformations, including asymmetric transformations.

Transition-metal-free azide insertion of N-triftosylhydrazones using a non-metallic azide source

Benzylic azides, an important class of active organic synthons, were synthesized in high yields from the easily accessible N-triftosylhydrazones with stable TMSN₃ under mild conditions. The reaction features high efficiency and excellent functional group tolerance, as illustrated by gram-scale synthesis and the synthesis of drug-like molecules. Mechanistic studies reveal that azidation occurs at the electron-deficient diazo-carbon via the elimination of N₂ by an azide ion.

Photo‐Induced Homologation of Carbonyl Compounds for Iterative Syntheses

We describe a photo-induced reaction for the in situ generation of highly reactive alkyl diazo species from carbonyl precursors via photo-excitation of N-tosylhydrazone anions. The diazo intermediates undergo efficient C-H insertion of aldehydes, leading to the productive synthesis of aldehydes and ketones. The method is applicable to the iterative synthesis of densely functionalized carbonyl compounds through sequential trapping of the diazo species with various aldehydes. The reaction proceeds without the need of any catalyst by light irradiation and features high functional group tolerance. More than 70 examples, some performed on a gram-scale, demonstrate the broad applicability of this reaction sequence in synthesis.

Palladium/GF-Phos-catalyzed asymmetric carbenylative amination to access chiral pyrrolidines and piperidines

The cross-coupling of N-tosylhydrazones has emerged as a powerful method for the construction of structurally diverse molecules, but the development of catalytic enantioselective versions still poses considerable challenges and only very limited examples have been reported. We herein report an asymmetric palladium/GF-Phos-catalyzed carbenylative amination reaction of N-tosylhydrazones and (E)-vinyl iodides pendent with amine, which allows facile access to a range of chiral pyrrolidines and piperidines in good yields (45-93%) with up to 96.5 : 3.5 er. Moreover, mild conditions, general substrate scope, scaled-up preparation, as well as the efficient synthesis of natural product (-)-norruspoline are practical features of this method.

Copper-catalyzed C-3 benzylation of quinoxalin-2(1H)-ones with benzylsulfonyl hydrazides

An unprecedented use of benzylsulfonyl hydrazides as benzylating agents has been demonstrated in the direct C-3 benzylation of quinoxalin-2(1H)-ones. A range of benzylsulfonyl hydrazides participated in the C-3 benzylation of quinoxalin-2(1H)-ones with CuCN as the catalyst and DTBP as the oxidant, delivering structurally diverse 3-benzylquinoxalin-2(1H)-ones in moderate to good yields.

Method for Preparing N-Tosylhydrazonyl Chlorides

A facile and efficient method for preparing N-tosylhydrazonyl chlorides was developed. The reaction was general for a variety of substrates and displayed wonderful compatibility to diverse substituents. A range of N-tosylhydrazonyl chlorides was prepared for the first time.

Coupling Reactions of Anhydro-Aldose Tosylhydrazones with Boronic Acids

A catalyst-free coupling reaction between O-peracetylated, O-perbenzoylated, O-permethylated, and O-permethoxymethylated 2,6-anhydro-aldose tosylhydrazones (C-(β-d-glycopyranosyl)formaldehyde tosylhydrazones) and aromatic boronic acids is reported. The base-promoted reaction is operationally simple and exhibits a broad substrate scope. The main products in most of the transformations were open-chain 1-C-aryl-hept-1-enitol type compounds while the expected β-d-glycopyranosylmethyl arenes (benzyl C-glycosides) were formed in subordinate yields only. A mechanistic rationale is provided to explain how a complex substrate may change the well-established course of the reaction.

Synthesis of Hydrazides from N-Tosylhydrazones and Its Application in the Preparation of Indazolones

We have developed a facile synthesis of hydrazides from N-tosylhydrazones under metal-free conditions. The reactions were suitable for different substrates and had excellent tolerance to various substituents. Meanwhile, a series of indazolones could be prepared from corresponding o-bromobenzohydrazides under mild conditions.

Cu(i)- and Pd(ii)-catalyzed decarboxylative cross-couplings of alkynyl carboxylic acids with N-tosylhydrazones: access to trisubstituted allenes and conjugated enynes

This paper reports the copper- and palladium-catalyzed decarboxylative cross-coupling reactions of alkynyl carboxylic acids and N-tosylhydrazones, affording trisubstituted allenes and conjugated enynes, respectively.

Elucidating mechanism and reactivity of reaction between donor-acceptor-acceptor 1,3-bisdiazo compound and cinnamyl alcohol catalyzed by Rh2(OAc)4: a DFT study

It is important for diazo compound to take part in organic synthesis. More theoretical and experimental studies focus on mono-diazo compound but fewer on bi-diazo compounds. Here, the mechanism and...

1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU)-promoted reduction of azides to amines under metal-free conditions

A simple and novel metal-free reduction of azides to amines in the presence of DBU is reported. This DBU-promoted transformation features good functional group tolerance and high chemo-selectivity.

Palladium-catalyzed dearomative 1,4-arylmethylenation of naphthalenes

An efficient palladium-catalyzed construction of E-exocyclic-double-bond-containing spirooxindoles through 1,4-arylmethylenation of naphthalenes has been developed. Aryl aldehyde-derived N‑tosylhydrazones were successfully applied as carbene precursors to capture the endocyclic π-allylpalladium intermediate, which...

Dearomative [4 + 3] cycloaddition of furans with vinyl-N-triftosylhydrazones by silver catalysis: stereoselective access to oxa-bridged seven-membered bicycles

A practical dearomative [4 + 3] cycloaddition of furans with vinylcarbenes to access oxa-bridged seven-membered carbocycles, with complete and predictable stereoselectivity, is achieved by merging silver catalysis and vinyl-N-triftosylhydrazones.

Silver-catalyzed [4 + 3] cycloaddition of 1,3-dienes with alkenyl-N-triftosylhydrazones: a practical approach to 1,4-cycloheptadienes

A formal [4 + 3] cycloaddition of 1,3-dienes with alkenyl-N-triftosylhydrazones was developed using silver catalysis, producing a broad spectrum of complex 1,4-cycloheptadienes with high yields and predictable stereochemistry.

Ligand-free access to benzisothiazolones and benzisoselenazolones through NiFe2O4 catalyzed concomitant annulation of 2-halobenzanilides with chalcogens and their late-stage transformations

A practical non-hazardous one-pot protocol for the synthesis of benzisothiazolones and benzisoselenazolones, involving magnetically retrievable nano-nickel ferrite catalyzed tandem annulation between 2-halobenzanilides and elemental S8 or Se.

Pd-Catalyzed coupling of benzyl bromides with BMIDA-substituted N-tosylhydrazones: synthesis of trans-alkenyl MIDA boronates

A palladium-catalyzed stereoselective synthesis of alkenyl boronates from N-methyliminodiacetyl boronate (BMIDA)-substituted N-tosylhydrazone and benzyl bromides is developed. A range of trans-alkenyl MIDA boronates as single stereoisomers were obtained in moderate yields with good functional group compatibility. The resultant boronate products may be transformed to other boron-containing compounds and may also be directly used in cross-coupling reactions.

Divergent, C-C Bond Forming Macrocyclizations Using Modular Sulfonylhydrazone and Derived Substrates

A divergent approach to C-C bond forming macrocycle construction is described. Modular sulfonylhydrazone and derived pyridotriazole substrates with three key building blocks have been constructed and cyclized to afford diverse macrocyclic frameworks. Broad substrate scope and functional group tolerance have been demonstrated. In addition, site-selective postfunctionalization allowed for further diversification of macrocyclic cores.

A modular strategy for the synthesis of marine originated meroterpenoid-type natural products

A modular strategy for meroterpenoid-type marine natural products has been developed from commercially available (+)-sclareolide using a palladium-catalyzed tandem carbene migratory insertion as one of the key steps. Its applicability is showcased by the formal synthesis of (-)-pelorol and 9-epi-pelorol and the concise total synthesis of (+)-yahazunone and (+)-yahazunol. It is worth noting that the formal synthesis of (-)-pelorol and 9-epi-pelorol was achieved by controlling the reaction sequence of hydrogenation and cyclization.