Recent Advances in Transition‐Metal‐Catalyzed Reactions of N‐Tosylhydrazones

Understanding the Ligand Influence in the Multistep Reaction of Diazoalkanes with Palladium Complexes Leading to Carbene-Aryl Coupling

The reaction of diphosphino aryl complexes [Pd(C6F5)(L-L)(NCMe)](BF4) (L-L = dppe, dppp, dppb) with diazoalkanes N2CHR (R = -CH=CHPh, Ph) leads to η3-allyl or η3-benzyl palladium derivatives that are the organometallic products resulting from carbene-aryl coupling. The experimental trend shows that the reaction is favored for dppe > dppp > dppb. It involves several consecutive steps, i.e., diazoalkane coordination, nitrogen extrusion to give a Pd-carbene, and migratory insertion, which are experimentally inseparable, but they can be studied with the help of DFT calculations. The bulkiness and bite angle of the ligand exert a large influence in the relative rate of the steps involved in the reaction, and we have found that carbene formation by N2 extrusion is the step with the largest barrier for dppe. In contrast, the coordination of the diazoalkane is the most energy-demanding step for the larger dppp and dppb diphosphines. Thus, ligand substitution controls the rate, an important elemental step rarely considered in mechanistic studies of carbene cross coupling reactions. Since diazoalkanes are the most common carbene precursors, either directly or generated from hydrazones, the choice of ligand can be very important to facilitate the entrance of the carbene precursor in the catalytic cycle.

The Carbene Chemistry of N-Sulfonyl Hydrazones: The Past, Present, and Future

N-Sulfonyl hydrazones have been extensively used as operationally safe carbene precursors in modern organic synthesis due to their ready availability, facile functionalization, and environmental benignity. Over the past two decades, there has been tremendous progress in the carbene chemistry of N-sulfonyl hydrazones in the presence of transition metal catalysts, under metal-free conditions, or using photocatalysts under photoirradiation conditions. Many carbene transfer reactions of N-sulfonyl hydrazones are unique and cannot be achieved by any alternative methods. The discovery of novel N-sulfonyl hydrazones and the development of highly enantioselective new reactions and skeletal editing reactions represent the notable recent achievements in the carbene chemistry of N-sulfonyl hydrazones. This review describes the overall progress made in the carbene chemistry of N-sulfonyl hydrazones, organized based on reaction types, spotlighting the current state-of-the-art and remaining challenges to be addressed in the future. Special emphasis is devoted to identifying, describing, and comparing the scope and limitations of current methodologies, key mechanistic scenarios, and potential applications in the synthesis of complex molecules.

Photochemical carboborylation and three-component difunctionalization of α,β-unsaturated ketones with boronic acids via tosylhydrazones

The reactions of cyclic α,β-unsaturated N-tosylhydrazones and alkylboronic acids promoted by 370-390 nm light in the presence of a base give rise to allylic boronic acids that can be trapped as the corresponding pinacolboronates by treatment with pinacol. This reaction features wide scope regarding both coupling partners and functional group tolerance, allowing for the incorporation of a variety of natural product-derived fragments. The allylic boronic acids can be also reacted in a one-pot process with aldehydes, to produce homoallylic alcohols with very high diastereoselectivity. A three-component one-pot procedure has been developed revealing that the methodology is a powerful tool for the generation of structural diversity that is accomplished by incorporation of an ample variety of each of the three elements. Moreover, from a synthetic perspective, in the reaction, the formation of two C-C bonds, at the carbonyl and the β positions of a α,β-unsaturated carbonyl, has been achieved in the three-component reaction.

N-H Insertion of Anilines on N-Tosylhydrazones Induced by Visible Light Irradiation

Diazo compounds and their precursors represent an interesting chemical category for organic synthesis. Particularly, N-tosylhydrazones have attracted attention for their easy accessibility and diverse reactivity, including carbene transfer reactions. We described a visible light-induced N-H insertion reaction of anilines on in situ-generated diazo compounds. Optimal conditions using DBU in toluene efficiently yielded the desired products. Mechanistic studies enabled us to trap a carbene intermediate that plays a key role in the transformation.

Acetone Serving as a Solvent and Interaction Partner Promotes the Direct Olefination of N-Tosylhydrazones under Visible Light

The photochemistry of noncovalent interactions to promote organic transformations is an emerging approach to providing fresh opportunities in synthetic chemistry. Generally, the external substance is necessary to add as an interaction partner, thereby sacrificing the atom economy of the reaction. Herein, we describe a catalyst-free and noncovalent interaction-mediated strategy to access the olefination of N-tosylhydrazones using acetone as a solvent and an interaction partner. This protocol also features broad substrate scope, excellent functional group compatibility, and mild reaction conditions without transition metals. Moreover, the gram-scale synthesis of olefins and the generation of pharmaceutical intermediates highlighted its practical applicability. Lastly, mechanistic studies indicate that the reaction was initiated via noncovalent interactions between acetone and N-tosylhydrazone anion, which is also supported by density functional theory calculations.

Transition-Metal Catalyzed Synthesis of Pyrimidines: Recent Advances, Mechanism, Scope and Future Perspectives

Pyrimidine is a pharmacologically important moiety that exhibits diverse biological activities. This review reflects the growing significance of transition metal-catalyzed reactions for the synthesis of pyrimidines (with no discussion being made on the transition metal-catalyzed functionalization of pyrimidines). The effect of different catalysts on the selectivity/yields of pyrimidines and catalyst recyclability (wherever applicable) are described, together with attempts to illustrate the role of the catalyst through mechanisms. Although several methods have been researched for synthesizing this privileged scaffold, there has been a considerable push to expand transition metal-catalyzed, sustainable, efficient and selective synthetic strategies leading to pyrimidines. The aim of the authors with this update (2017-2023) is to drive the designing of new transition metal-mediated protocols for pyrimidine synthesis.

The Mizoroki-Heck reaction between in situ generated alkenes and aryl halides: cross-coupling route to substituted olefins

This review covers palladium-catalyzed typical Mizoroki-Heck cross-coupling reactions of aryl halides with in situ generated alkenes, by following a typical Heck coupling mechanism to form substituted olefins unlike direct cross-coupling of alkenes with aryl halides in Heck olefination. These reactions solve the issue of alkenes undergoing polymerization at high temperatures and increase reaction efficiency by reducing the reaction time and purification steps.

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.

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.

Diazo compounds and palladium-aryl complexes: trapping the elusive carbene migratory insertion organometallic products

The reactions of Pd-aryl complexes with diazo compounds N₂CH-CHCHPh and N₂CHPh allowed us to isolate the organometallic products formed right after the migratory insertion of a non-stabilized CHR carbene into the Pd-aryl bond. η³-Allylic and η³-benzylic palladium complexes were formed respectively. This is compelling experimental evidence for the key step in the palladium-catalyzed cascade transformations of diazo derivatives leading to multiple C-C or C-X bond formation.

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.

Palladium-Catalyzed Para-C-H Bond Amination of 2-Aryl Chloromethylbenzenes

Palladium-catalyzed para-C-H bond amination of 2-aryl chloromethylbenzenes is described for the first time. The reactions of 2-aryl chloromethylbenzenes with cyclic amines proceeded smoothly in the presence of Pd(acac)₂, tri(2-furyl)phosphine, and NaH in tetrahydrofuran at 40 °C to provide para-C-H bond aminated products in satisfactory to high yields with acceptable regioselectivity in most cases. The electronic property of the substituents linked to the benzene rings did not significantly influence the reactivity of the 2-aryl chloromethylbenzene substrates and the reaction regioselectivity.

Insights into the Activation Mode of α-Carbonyl Sulfoxonium Ylides in Rhodium-Catalyzed C-H Activation: A Theoretical Study

A computational study has been performed to investigate the mechanism of RhIII -catalyzed C-H bond activation using sulfoxonium ylides as a carbene precursor. The stepwise and concerted activation modes for sulfoxonium ylides were investigated. Detailed theoretical results showed that the favored stepwise pathway involves C-H bond activation, carbonization, carbene insertion, and protonation. The free energy profiles for dialkylation of 2-phenylpyridine were also calculated to account for the low yield of this reaction. Furthermore, the substituent effect was elucidated by comparing the energy barriers for the protonation of meta- and para-substituted sulfoxonium ylides calculated by density functional theory.

Accessing Illusive E Isomers of α-Ester Hydrazones via Visible-Light-Induced Pd-Catalyzed Heck-Type Alkylation

A visible-light-induced Pd-catalyzed stereoselective synthesis of alkylated ester hydrazones has been developed. This method operates via generation of a nucleophilic carbon-centered radical from alkyl bromide, iodide, or redox-active ester, followed by its addition to hydrazone, and a subsequent desaturation by palladium. The majority of products have E configuration, which are inaccessible by conventional condensation methods. In addition, a sequential C,N-alkylation protocol has been developed: a reaction between 1,3-dihalides and glyoxylate-derived hydrazone, delivering tetrahydropyridazines.

Pd-Catalyzed Coupling of N-Tosylhydrazones with Benzylic Phosphates: Toward the Synthesis of Di- or Tri-Substituted Alkenes

This study shows that various di- and tri-substituted alkenes with high chemoselectivity were obtained in good to high yields by coupling N-tosylhydrazones (NTHs) with benzylic phosphates as electrophilic partners. The obtained new catalytic system consisted of PdCl₂(CH₃CN)₂/dppp, LiOtBu as a base, and cyclopentyl methyl ether as a green solvent. In addition, we performed a gram-scale transformation using NTH derivatives and benzylic phosphates having a C sp²-Cl bond. The latter was used as a starting point for further postfunctionalization of the key intermediates.

Synthesis of Polyallenoates through Copper-Mediated Cross-Coupling of Dialkynes and Bis-α-Diazoesters

The copper-catalyzed cross-coupling of alkynes and α-diazoesters have been applied in the synthesis of polyallenoates for the first time. The polymerization tolerated various functional groups and afforded the polyallenoates with...

Palladium-catalyzed allylic alkylation of hydrazones with hydroxy-tethered allyl carbonates: synthesis of functionalized hydrazones

Pd-catalyzed allylic alkylation of hydroxy-tethered allyl carbonates and hydrazones worked well without an external base to afford various E configurations of functionalized hydrazones, which were successfully transformed into pyridazines.

Palladium-Catalyzed Carbene Coupling Reactions of Cyclobutanone N-Sulfonylhydrazones

Described herein are the palladium-catalyzed cross-coupling reactions of cyclobutanone-derived N-sulfonylhydrazones with aryl or benzyl halides, suggesting that the metal carbene process and β-hydride elimination can smoothly occur in strained ring systems. Structurally diversified products including cyclobutenes, methylenecyclobutanes, and conjugated dienes are selectively afforded in good to excellent yields. Preliminary success in asymmetric carbene coupling reactions in strained ring systems has been achieved, providing a promising route for the synthesis of enantioenriched four-membered-ring molecules.

Pd/GF-Phos-Catalyzed Asymmetric Three-Component Coupling Reaction to Access Chiral Diarylmethyl Alkynes

Significant attention has been given in the past few years to the selective transformations of N-tosylhydrazones to various useful compounds. However, the development of enantioselective versions poses considerable challenges. Herein we report a Pd-catalyzed enantioselective three-component coupling of N-tosylhydrazone, aryl halide, and terminal alkyne under mild conditions utilizing a novel chiral sulfinamide phosphine ligand (GF-Phos), which provides a facile access to chiral diarylmethyl alkynes, which are useful synthons in organic synthesis as well as exist as the skeleton in many bioactive molecules. A pair of enantiomers of the product could be easily prepared using the same chiral ligand by simply changing the aryl substituents of the N-tosylhydrazone and aryl halide. The salient features of this reaction include the readily available starting materials, general substrate scope, high enantioselectivity, ease of scale-up, mild reaction conditions, and versatile transformations.