Silver-Catalyzed [2+1] Cyclopropenation of Alkynes with Unstable Diazoalkanes: N-Nosylhydrazones as Room-Temperature Decomposable Diazo Surrogates

Accessing elusive σ-type cyclopropenium cation equivalents through redox gold catalysis

Cyclopropenes are the smallest unsaturated carbocycles. Removing one substituent from cyclopropenes leads to cyclopropenium cations (C3+ systems, CPCs). Stable aromatic π-type CPCs were discovered by Breslow in 1957 by removing a substituent on the aliphatic position. In contrast, σ-type CPCs-formally accessed by removing one substituent on the alkene-are unstable and relatively unexplored. Here we introduce electrophilic cyclopropenyl-gold(III) species as equivalents of σ-type CPCs, which can then react with terminal alkynes and vinylboronic acids. With catalyst loadings as low as 2 mol%, the synthesis of highly functionalized alkynyl- or alkenyl-cyclopropenes proceeded under mild conditions. A class of hypervalent iodine reagents-the cyclopropenyl benziodoxoles (CpBXs)-enabled the direct oxidation of gold(I) to gold(III) with concomitant transfer of a cyclopropenyl group. This protocol was general, tolerant to numerous functional groups and could be used for the late-stage modification of complex natural products, bioactive molecules and pharmaceuticals.

Synthesis of Alkynyl Hydrazones from Unprotected Hydrazine and Their Reactivity as Diazo Precursors

Alkynyl hydrazones are synthesized conveniently from 2-oxo-3-butynoates and hydrazine by suppressing the susceptible formation of pyrazoles. The resultant hydrazones are transformed into alkynyl diazoacetates under metal-free and mild oxidative conditions in excellent yields. Further, the alkynyl cyclopropane and propargyl silane carboxylates are synthesized in good yields by developing an unprecedented copper-catalyzed alkynyl carbene transfer reaction.

Tunable Key [3 + 2] and [2 + 1] Cycloaddition of Enaminones and α-Diazo Compounds for the Synthesis of Isomeric Isoxazoles: Metal-Controlled Selectivity

The three-component reactions of enaminones, α-diazo esters/ketones, and t-butyl nitrite (TBN) for the switchable synthesis of isomeric isoxazoles have been realized. The catalysis with Cu(II) salt provides 3,4-disubsituted isoxazoles via [3 + 2] cycloaddition. On the other hand, the catalysis of Ag(I) with identical substrates leads to isomeric isoxazoles with reversed C3 and C4 substitution based on a key [2 + 1] cycloaddition.

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.

N-Triftosylhydrazones: A New Chapter for Diazo-Based Carbene Chemistry

ConspectusOver recent decades, N-sulfonylhydrazones have attracted significant attention in academic and industrial contexts owing to their ease of preparation, versatile reactivity, high stability, and practicality. In particular, the use of N-sulfonylhydrazones as precursors for diazo compounds has paved the way for innovative and original organic reactions that are otherwise difficult to achieve. Three key developments are noteworthy in the history of N-sulfonylhydrazone chemistry: (1) Bamford and Stevens initially disclosed the application of N-tosylhydrazones as a diazo source in 1952; (2) Aggarwal and co-workers investigated N-tosylhydrazone salts as diazo precursors for sulfur ylide-mediated asymmetric epoxidation and aziridination in 2001; and (3) Barluenga, Valdés and co-workers first reported Pd-catalyzed cross-coupling reactions with N-tosylhydrazones in 2007, thus introducing the direct use of N-tosylhydrazones in carbene transfer reactions. In the past 2 decades, the synthetic exploration of N-sulfonylhydrazones in carbene chemistry has increased remarkably. N-Tosylhydrazones are the most commonly used N-sulfonylhydrazones, but they are not easy to decompose and normally need relatively high temperatures (e.g., 90-110 °C). Temperature, as a key reaction parameter, has a significant influence on the selectivity and scope of organic reactions, especially the enantioselectivity. Aggarwal and co-workers have addressed this issue by using N-tosylhydrazone salts and achieved a limited number of asymmetric organic reactions, but the method is greatly limited because the salts must be freshly prepared or stored in the dark at -20 °C prior to use. Hence, easily decomposable N-sulfonylhydrazones, especially those capable of decomposing at low temperature, should open up new opportunities for the development of N-sulfonylhydrazone chemistry. Since 2014, our group has worked toward this goal and eventually identified N-2-(trifluoromethyl)benzenesulfonylhydrazone (i.e., N-triftosylhydrazone) as an efficient diazo surrogate that can decompose at temperatures as low as -40 °C. This allowed us to carry out a range of challenging synthetic transformations and to broaden the applications of some known reactions of great relevance.In this Account, we report our achievements in the application of N-triftosylhydrazones in carbene chemistry. On the basis of the reaction types, such applications can be categorized as (i) C(sp³)-H insertion reactions, (ii) defluorinative reactions of fluoroalkyl N-triftosylhydrazones, (iii) cycloaddition reactions with alkenes and alkynes, and (iv) asymmetric reactions. Additional applications in Doyle-Kirmse rearrangements and cross-coupling with isocyanides (ours) and benzyl chlorides (from the group of Xia) are also summarized in this Account concerning miscellaneous reactions. In terms of reaction efficiency, selectivity, and functional group tolerance, N-triftosylhydrazones are generally superior to traditional N-tosylhydrazones because of their easy decomposition. Mechanistic investigations by theoretical calculations provide insights into both the reaction mechanisms and the origin of selectivity. We hope that this Account will inspire broad interest and promote new progress in the synthetic exploration of easily decomposable N-sulfonylhydrazones.

Recent Advances in Catalytic Alkyne Transformation via Copper Carbene Intermediates

As one of the abundant and inexpensive metals on the earth, copper has demonstrated broad applications in synthetic chemistry and catalysis. Among these copper-catalyzed advances, copper carbenes are versatile and reactive intermediates that can mediate a variety of transformations, which have attracted much attention in the past decades. The present review summarizes two different reaction models that take place between a copper carbene intermediate and alkyne species, including the cross-coupling reaction of copper carbene intermediate with terminal alkyne, and the addition of copper carbene intermediate onto the C–C triple bond. This article will cover the profile from 2010 to 2021 by placing emphasis on the detailed catalytic models and highlighting the synthetic applications offered by these practical and mild methods.

Direct gem-Difluoroalkenylation of X-H Bonds with Trifluoromethyl Ketone N-Triftosylhydrazones for Synthesis of Tetrasubstituted Heteroatomic gem-Difluoroalkenes

The direct gem-difluoroalkenylation of X-H bonds represents the most straightforward approach to access heteroatomic gem-difluoroalkenes that, as the isostere of the carbonyl group, have great potency in drug discovery. However, the construction of tetrasubstituted heteroatomic gem-difluoroalkenes by this strategy is still an unsolved problem. Here, we report the first direct X-H bond gem-difluoroalkenylation of amines and alcohols with trifluoromethyl ketone N-triftosylhydrazones under silver (for (hetero)aryl hydrazones) or rhodium (for alkyl hydrazones), thereby providing a most powerful method for the synthesis of tetrasubstituted heteroatomic gem-difluoroalkenes. This method features a broad substrate scope, high product yield, excellent functional group tolerance, and operational simplicity (open air conditions). Moreover, the site-specific replacement of the carbonyl group with a gem-difluorovinyl ether bioisostere in drug Trimebutine and the post-modification of bioactive molecules demonstrates potential use in medicinal research. Finally, the reaction mechanism was investigated by combining experiments and DFT calculations, and disclosed that the key step of HF elimination occurred via five-membered ring transition state, and the difference in the electrophilicity of Ag- and Rh-carbenes as well as the multiple intermolecular interactions rendered the effectiveness of Rh catalyst selectively for alkyl hydrazones.

Silver-catalyzed cyclopropanation of alkenes with alkynyl N-nosylhydrazones leading to alkynyl cyclopropanes

Here, a novel method for the stereoselective synthesis of alkynyl cyclopropanes, by the silver-catalyzed alkynylcyclopropanation of alkenes using alkynyl N-nosylhydrazones as alkynyl carbene precursors, is reported. This method provides a straightforward and powerful approach for preparing various alkynyl cyclopropanes in high yield with excellent stereoselectivities. In addition, the practicality of this method was demonstrated by gram-scale synthesis and late-stage modification of bioactive molecules.

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.

Hydrazones as Substrates in the Synthesis of Isoxazolidines via a KOH-Promoted One-Pot Three-Component Cycloaddition with Nitroso Compounds and Olefins

Hydrazones have been employed as the starting materials in a KOH-mediated one-pot three-component cycloaddition with readily accessible nitroso compounds and olefins to construct various isoxazolidines. Compared with diazo compounds as starting materials, this methodology could afford a wider range of products in good to excellent yields and diastereoselectivities for most substrates, and hydrazones are cheaper, more accessible, and safer substrates. The experimental study shows that the choice of suitable hydrazones is crucial.

Nucleophilic Migratory Cyclopropenation of Activated Alkynes: A Nonmetal Approach to Unbiased Cyclopropenes

An unprecedented reductive [2 + 1] annulation of α-keto esters with alkynones mediated by P(NMe₂)₃ is described. Although this nonmetal cyclopropenation is a nucleophilic process, attributed to the ester migration via a formal [2 + 2] cycloaddition reaction of Kukhtin-Ramirez adducts and alkynones followed by a fragmentation, cyclopropenes with an unbiased alkene scaffold are formed in good to excellent yields, thus providing a promising complementarity to electrophilic metal-catalyzed cyclopropenation.

Gold-Catalyzed Synthesis of Small Rings

Three- and four-membered rings, widespread motifs in nature and medicinal chemistry, have fascinated chemists ever since their discovery. However, due to energetic considerations, small rings are often difficult to assemble. In this regard, homogeneous gold catalysis has emerged as a powerful tool to construct these highly strained carbocycles. This review aims to provide a comprehensive summary of all the major advances and discoveries made in the gold-catalyzed synthesis of cyclopropanes, cyclopropenes, cyclobutanes, cyclobutenes, and their corresponding heterocyclic or heterosubstituted analogs.

Diastereoselective Synthesis of Pyrazolines by Metal-Free Rearrangement of Bicyclic Triazolines

The metal-free preparation of diazoalkanes through the ring rearrangement of bicyclic triazolines is reported here. Their use in 1,3-dipolar cycloaddition reactions with electron-withdrawing alkenes was investigated. This synthetic procedure allows differently substituted pyrazolines to be obtained in few steps and with high atom economy.

Blue Light-promoted Carbene Transfer Reactions of Tosylhydrazones

Metal-free photochemical carbene-transfer reactions of tosylhydrazones were developed under blue light irradiation at room temperature. This reaction constructs C-X (X=C, N, O, S) bonds and cyclopropanes from readily available and stable starting materials.

Recent progress on donor and donor-donor carbenes

Donor and donor-donor carbenes are two important kinds of carbenes, which have experienced tremendous growth in the past two decades. This review provides a comprehensive overview of the recent development of donor and donor-donor carbene chemistry. The development of this chemistry offers efficient protocols to construct a wide variety of C-C and C-X bonds in organic synthesis. This review is organized based on the different types of carbene precursors, including diazo compounds, hydrazones, enynones, cycloheptatrienes and cyclopropenes. The typical transformations, the reaction mechanisms, as well as their subsequent applications in the synthesis of complex natural products and bioactive molecules are discussed. Due to the rapidly increasing interest in this area, we believe that this review will provide a timely and comprehensive discussion of recent progress in donor and donor-donor carbene chemistry.

Difluoroacetaldehyde N-Triftosylhydrazone (DFHZ-Tfs) as a Bench-Stable Crystalline Diazo Surrogate for Diazoacetaldehyde and Difluorodiazoethane

Despite the growing importance of volatile functionalized diazoalkanes in organic synthesis, their safe generation and utilization remain a formidable challenge because of their difficult handling along with storage and security issues. In this study, we developed a bench-stable difluoroacetaldehyde N-triftosylhydrazone (DFHZ-Tfs) as an operationally safe diazo surrogate that can release in situ two low-molecular-weight diazoalkanes, diazoacetaldehyde (CHOCHN₂ ) or difluorodiazoethane (CF₂ HCHN₂ ), in a controlled fashion under specific conditions. DFHZ-Tfs has been successfully employed in the Fe-catalyzed cyclopropanation and Doyle-Kirmse reactions, thus highlighting the synthetic utility of DFHZ-Tfs in the efficient construction of molecule frameworks containing CHO or CF₂ H groups. Moreover, the reaction mechanism for the generation of CHOCHN₂ from CF₂ HCHN₂ was elucidated by density functional theory (DFT) calculations.

Recent developments in cyclopropene chemistry

As readily accessible strained carbocycles, cyclopropenes show a diverse range of reactivities, and a lot of novel and useful transformations have been developed.

Thermal Stability and Explosive Hazard Assessment of Diazo Compounds and Diazo Transfer Reagents

Despite their wide use in academia as metal-carbene precursors, diazo compounds are often avoided in industry owing to concerns over their instability, exothermic decomposition, and potential explosive behavior. The stability of sulfonyl azides and other diazo transfer reagents is relatively well understood, but there is little reliable data available for diazo compounds. This work first collates available sensitivity and thermal analysis data for diazo transfer reagents and diazo compounds to act as an accessible reference resource. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and accelerating rate calorimetry (ARC) data for the model donor/acceptor diazo compound ethyl (phenyl)diazoacetate are presented. We also present a rigorous DSC dataset with 43 other diazo compounds, enabling direct comparison to other energetic materials to provide a clear reference work to the academic and industrial chemistry communities. Interestingly, there is a wide range of onset temperatures (T _onset) for this series of compounds, which varied between 75 and 160 °C. The thermal stability variation depends on the electronic effect of substituents and the amount of charge delocalization. A statistical model is demonstrated to predict the thermal stability of differently substituted phenyl diazoacetates. A maximum recommended process temperature (T _D24) to avoid decomposition is estimated for selected diazo compounds. The average enthalpy of decomposition (ΔH _D) for diazo compounds without other energetic functional groups is -102 kJ mol^-1. Several diazo transfer reagents are analyzed using the same DSC protocol and found to have higher thermal stability, which is in general agreement with the reported values. For sulfonyl azide reagents, an average ΔH _D of -201 kJ mol^-1 is observed. High-quality thermal data from ARC experiments shows the initiation of decomposition for ethyl (phenyl)diazoacetate to be 60 °C, compared to that of 100 °C for the common diazo transfer reagent p-acetamidobenzenesulfonyl azide (p-ABSA). The Yoshida correlation is applied to DSC data for each diazo compound to provide an indication of both their impact sensitivity (IS) and explosivity. As a neat substance, none of the diazo compounds tested are predicted to be explosive, but many (particularly donor/acceptor diazo compounds) are predicted to be impact-sensitive. It is therefore recommended that manipulation, agitation, and other processing of neat diazo compounds are conducted with due care to avoid impacts, particularly in large quantities. The full dataset is presented to inform chemists of the nature and magnitude of hazards when using diazo compounds and diazo transfer reagents. Given the demonstrated potential for rapid heat generation and gas evolution, adequate temperature control and cautious addition of reagents that begin a reaction are strongly recommended when conducting reactions with diazo compounds.

Visible light mediated, metal-free carbene transfer reactions of diazoalkanes with propargylic alcohols

The photolysis of donor–acceptor diazoalkanes in the presence of propargylic alcohols furnishes valuable, sterically demanding tetra-substituted cyclopropenes in high yield under metal-free conditions.

Influence of sulfonyl substituents on the decomposition of N-sulfonylhydrazones at room temperature

A systematic investigation on the influence of sulfonyl substituents on the decomposition of N-sulfonylhydrazones at room temperature is described.

Iron-catalyzed synthesis of cyclopropanes by in situ generation and decomposition of electronically diversified diazo compounds

The modular synthesis of a variety of trans 1,2-disubstituted cyclopropanes in a safe and user-friendly one-pot iron-catalyzed cyclopropanation reaction is described. Easily synthesized N-nosylhydrazones are used as diazo precursors, allowing the in situ generation of electron-rich diazo compounds under mild reaction conditions and their direct participation in the cyclopropanation reaction.

Silver-Catalyzed Regio- and Stereoselective Formal Carbene Insertion into Unstrained C-C σ-Bonds of 1,3-Dicarbonyls

A regio- and stereoselective silver-catalyzed formal carbene insertion into 1,3-dicarbonyls has been developed, using N-nosylhydrazones as diazo surrogates. Two new C-C bonds are constructed at the carbenic carbon center through the selective cleavage of the C-C(=O) σ-bond of acyclic 1,3-dicarbonyls, enabling the preparation of various synthetically useful polysubstituted γ-diketones, γ-ketoesters, and γ-ketoamides in high yields. The in situ formation of a donor-acceptor cyclopropane, via reaction of the enolate of the 1,3-dicarbonyl with an electrophilic silver carbenoid, is proposed as a key process in the catalytic cycle.

Desymmetrization of Cyclopropenes via the Potassium-Templated Diastereoselective 7- exo- trig Cycloaddition of Tethered Amino Alcohols toward Enantiopure Cyclopropane-Fused Oxazepanones with Antimycobacterial Activity

A strain-release-driven, cation-templated intramolecular nucleophilic addition of tethered alkoxides to prochiral cyclopropenes is described. Employment of chiral β- and γ-amino alkoxides allowed for highly diastereoselective assembly of a small series of enantiopure cyclopropane-fused oxazepanones. It was shown that the chiral center at C-4 plays a crucial role in controlling desymmetrization of the cyclopropenyl moiety, instigated by a profound potassium-templated effect. The preliminary biological activities of the new cyclopropane-fused medium heterocycles against Gram-positive bacteria, Gram-negative bacteria, mycobacteria, cancer cells, and fungus were evaluated.

Gold-catalyzed (4 + 2)-annulations between α-alkyl alkenylgold carbenes and benzisoxazoles with reactive alkyl groups

This work reports new (4 + 2)-annulations of α-alkyl vinylgold carbenes with benzisoxazoles to afford 3,4-dihydroquinoline derivatives with high anti-stereoselectivity.

This work reports new (4 + 2)-annulations of α-alkyl vinylgold carbenes with benzisoxazoles to afford 3,4-dihydroquinoline derivatives with high anti-stereoselectivity. The annulations are operable with carbenes in both acyclic and cyclic forms. This reaction sequence involves an initial formation of imines from α-alkylgold carbenes and benzisoxazoles, followed by a novel carbonyl-enamine reaction to yield 3,4-dihydroquinoline derivatives. This system presents the first alkyl C–H reactivity of α-alkyl gold carbenes with an external substrate.

Silver-Catalyzed Cyclopropanation of Alkenes Using N-Nosylhydrazones as Diazo Surrogates

An efficient silver-catalyzed [2 + 1] cyclopropanation of sterically hindered internal alkenes with diazo compounds in which room-temperature-decomposable N-nosylhydrazones are used as diazo surrogates is reported. The unexpected unique catalytic activity of silver was ascribed to its dual role as a Lewis acid activating alkene substrates and as a transition metal forming silver carbenoids. A wide range of internal alkenes, including challenging diarylethenes, were suitable for this protocol, thereby affording a variety of cyclopropanes with high efficiency in a stereoselective manner under mild conditions.