Rhodium-Catalyzed Asymmetric Cyclopropanation of Indoles with N-Triftosylhydrazones.

Herein we report a general rhodium-catalyzed asymmetric intermolecular dearomative cyclopropanation of indoles using trifluoromethyl N-triftosylhydrazones as carbene precursors. The reaction enables the rapid construction of diverse cyclopropane-fused indolines bearing a trifluoromethylated quaternary stereocenter with high enantioselectivity (up to 99 % ee). This mild method exhibits broad substrate scope, tolerating various functional groups, and can even be utilized for the late-stage diversification of complex bioactive molecules. DFT calculations suggest that the formation of a key zwitterionic intermediate is responsible for the chiral induction. Overall, this approach opens up new possibilities for asymmetric cyclopropanation of challenging aromatic heterocyclic compounds.

Silver-Catalyzed Single-Carbon Insertion of Indoles with Acetophenone N-Triftosylhydrazones.

Here, we report a silver carbene-enabled single-carbon insertion reaction of indoles via a one-pot, two-step sequence to deliver a dearomative quaternary center quinoline scaffold in a modular fashion. Specifically, we used N-triftosylhydrazones as masked donor-donor carbene precursors that facilitate the insertion of carbon atoms bearing various functional groups to the library of functionalized quinoline. Experimental and DFT evidence support the transient presence of a cyclopropane species and removal of protecting groups.

Controllable Skeletal and Peripheral Editing of Pyrroles with Vinylcarbenes.

The skeletal editing of azaarenes through insertion, deletion, or swapping of single atoms has recently gained considerable momentum in chemical synthesis. Here, we describe a practical skeletal editing strategy using vinylcarbenes in situ generated from trifluoromethyl vinyl N-triftosylhydrazones, leading to the first dearomative skeletal editing of pyrroles through carbon-atom insertion. Furthermore, depending on the used catalyst and substrate, three types of peripheral editing reactions of pyrroles are also disclosed: α- or γ-selective C-H insertion, and [3+2] cycloaddition. These controllable molecular editing reactions provide a powerful platform for accessing medicinally relevant CF3-containing N-heterocyclic frameworks, such as 2,5-dihydropyridines, piperidines, azabicyclo[3.3.0]octadienes, and allylated pyrroles from readily available pyrroles. Mechanistic insights from experiments and density functional theory (DFT) calculations shed light on the origin of substrate- or catalyst-controlled chemo- and regioselectivity as well as the reaction mechanism.

Silver-catalyzed direct conversion of epoxides into cyclopropanes using N-triftosylhydrazones.

Epoxides, as a prominent small ring O-heterocyclic and the privileged pharmacophores for medicinal chemistry, have recently represented an ideal substrate for the development of single-atom replacements. The previous O-to-C replacement strategy for epoxides to date typically requires high temperatures to achieve low yields and lacks substrate range and functional group tolerance, so achieving this oxygen-carbon exchange remains a formidable challenge. Here, we report a silver-catalyzed direct conversion of epoxides into trifluoromethylcyclopropanes in a single step using trifluoromethyl N-triftosylhydrazones as carbene precursors, thereby achieving oxygen-carbon exchange via a tandem deoxygenation/[2 + 1] cycloaddition. The reaction shows broad tolerance of functional groups, allowing routine cheletropic olefin synthesis in a strategy for the net oxygen-carbon exchange reaction. The utility of this method is further showcased with the late-stage diversification of epoxides derived from bioactive natural products and drugs. Mechanistic experiments and DFT calculations elucidate the reaction mechanism and the origin of the chemo- and stereoselectivity.

Tunable molecular editing of indoles with fluoroalkyl carbenes.

Building molecular complexity from simple feedstocks through precise peripheral and skeletal modifications is central to modern organic synthesis. Nevertheless, a controllable strategy through which both the core skeleton and the periphery of an aromatic heterocycle can be modified with a common substrate remains elusive, despite its potential to maximize structural diversity and applications. Here we report a carbene-initiated chemodivergent molecular editing of indoles that allows both skeletal and peripheral editing by trapping an electrophilic fluoroalkyl carbene generated in situ from fluoroalkyl N-triftosylhydrazones. A variety of fluorine-containing N-heterocyclic scaffolds have been efficiently achieved through tunable chemoselective editing reactions at the skeleton or periphery of indoles, including one-carbon insertion, C3 gem-difluoroolefination, tandem cyclopropanation and N1 gem-difluoroolefination, and cyclopropanation. The power of this chemodivergent molecular editing strategy has been highlighted through the modification of the skeleton or periphery of natural products in a controllable and chemoselective manner. The reaction mechanism and origins of the chemo- and regioselectivity have been probed by both experimental and theoretical methods.

Rhodium-Catalyzed One-Carbon Ring Expansion of Aziridines with Vinyl-N-triftosylhydrazones for the Synthesis of 2-Vinyl Azetidines.

Azetidines, being four-membered N-heterocycles, possess significant potential in contemporary medicinal chemistry owing to their favorable pharmacokinetic properties. Regrettably, the incorporation of functionalized azetidines into pharmaceutical lead structures has been impeded by the absence of efficient synthetic methods for their synthesis. In this study, a Rh-catalyzed one-carbon ring expansion of aziridines with vinyl-N-triftosylhydrazones is presented, which facilitates the synthesis of high value-added 2-alkenyl azetidine products. This research represents the first example of ring expansion of aziridines enabled by vinyl carbenes. Additionally, a one-pot two-step protocol, initiated from cinnamaldehyde, was successfully achieved, offering a step-economical and facile approach for the synthesis of these compounds. The pivotal aspect of this successful transformation lies in the in situ formation of an alkenyl aziridinium ylide intermediate. Experimental investigations, coupled with computational studies, suggest that a diradical pathway is involved in the reaction mechanism.

Silver-Catalyzed Dearomative Skeletal Editing of Indazoles by Donor Carbene Insertion.

Given the prevalence of heterocyclic scaffolds in drug-related molecules, converting these highly modular heterocyclic scaffolds into structural diversified and dearomatized analogs is an ideal strategy for improving their physicochemical and pharmacokinetic properties. Here, we described an efficient method for silver carbene-mediated dearomative N-N bond cleavage leading to skeletal hopping between indazole and 1,2-dihydroquinazoline via a highly selective single-carbon insertion procedure. Using this methodology, a series of dihydroquinazoline analogues with diarylmethylene-substituted quaternary carbon centers were constructed with excellent yields and good functional group compatibility, which was further illustrated by the late-stage diversification of important pharmaceutically active ingredients. DFT calculations indicated that the silver catalyst not only induces the formation of the silver carbene, but also activates the diazahexatriene intermediate, which plays a crucial role in the formation of the C-N bond.

Dearomative Insertion of Fluoroalkyl Carbenes into Azoles Leading to Fluoroalkyl Heterocycles with a Quaternary Center.

The skeletal ring expansion of heteroarenes through carbene insertion is gaining popularity in synthetic chemistry. Efficient strategies for heterocyclic ring expansion to access heterocycles containing a fluoroalkyl quaternary carbon center through fluoroalkyl carbene insertion are highly desirable because of their broad applications in medicinal chemistry. Herein, we report a general strategy for the dearomative one-carbon insertion of azoles using fluoroalkyl N-triftosylhydrazones as fluoroalkyl carbene precursors, resulting in ring-expanded heterocycles in excellent yields with good functional-group compatibility. The broad generality of this methodology in the late-stage diversification of pharmaceutically interesting bioactive molecules and versatile transformations of the products has been demonstrated.

Defluorinative 1,3-Dienylation of Fluoroalkyl N-Triftosylhydrazones with Homoallenols.

A silver-catalyzed regioselective defluorinative 1,3-dienylation of trifluoromethyl phenyl N-triftosylhydrazones using homoallenols as 1,3-dienyl sources provides a variety of α-(di)fluoro-ß-vinyl allyl ketones with excellent functional group tolerance in moderate to good yields. The reaction proceeds through a silver carbene-initiated sequential etherification and Claisen type [3,3]-sigmatropic rearrangement cascade. The synthetic utility of this protocol was demonstrated through the downstream synthetic elaboration toward diverse synthetically useful building blocks.

Skeletal Editing of (Hetero)Arenes Using Carbenes.

(Hetero)arenes continue to prove their indispensability in pharmaceuticals, materials science, and synthetic chemistry. As such, the controllable modification of biologically significant (hetero)arenes towards diverse more-potent complex molecular scaffolds through peripheral and skeletal editing has been considered a challenging goal in synthetic organic chemistry. Despite many excellent reviews on peripheral editing (i. e., C-H functionalization) of (hetero)arenes, their skeletal editings via single atom insertion, deletion, or transmutations have received less attention in the review literature. In this review, we systematically summarize the state-of-the-art skeletal editing reactions of (hetero)arenes using carbenes, with a focus on general mechanistic considerations and their applications in natural product syntheses. The potential opportunities and inherent challenges encountered while developing these strategies are also highlighted.

Asymmetric [2+1] cycloaddition of difluoroalkyl-substituted carbenes with alkenes under rhodium catalysis: Synthesis of chiral difluoroalkyl-substituted cyclopropanes.

Herein, we report a novel strategy for the synthesis of chiral difluoroalkyl-substituted cyclopropanes through enantioselective [2 + 1] cyclopropanation of alkenes and difluoroalkyl-substituted carbenes under rhodium catalysis, wherein the newly designed α, α-difluoro-ß-carbonyl ketone N-triftosylhydrazones are used as the difluoroalkyl-substituted carbenes precursors. This approach represents the first asymmetric cyclopropanation of alkenes with difluoroalkyl carbenes, featuring high yield, high enantioselectivity, and broad substrate scope. Gram-scale synthesis and further interconversion of diverse functional groups demonstrate the usefulness of this protocol in the preparation of diverse functionalized chiral difluoroalkyl-substituted cyclopropanes.

Pyrrolidine-Derived Phenanthroline Diamides: An Influence of Fluorine Atoms on the Coordination of Lu(III) and Some Other f-Elements and Their Solvent Extraction.

Three pyrrolidine-derived phenanthroline diamides were studied as ligands for lutetium trinitrate. The structural features of the complexes have been studied using various spectral methods and X-ray. The presence of halogen atoms in the structure of phenanthroline ligands has a significant impact on both the coordination number of lutetium and the number of solvate water molecules in the internal coordination sphere. The stability constants of complexes with La(NO3)3, Nd(NO3)3, Eu(NO3)3, and Lu(NO3)3 were measured to demonstrate higher efficiency of fluorinated ligands. NMR titration was performed for this ligand, and it was found that complexation with lutetium leads to an approximately 13 ppm shift of the corresponding signal in the 19F NMR spectrum. The possibility of formation of a polymeric oxo-complex of this ligand with lutetium nitrate was demonstrated. Experiments on the liquid-liquid extraction of Am(III) and Ln(III) nitrates were carried out to demonstrate advantageous features of chlorinated and fluorinated pyrrolidine diamides.

Site-Selective C-H Allylation of Alkanes: Facile Access to Allylic Quaternary sp3 -Carbon Centers.

The construction of allylic quaternary sp3 -carbon centers has long been a formidable challenge in transition-metal-catalyzed alkyl-allyl coupling reactions due to the severe steric hindrance. Herein, we report an effective carbene strategy that employs well-defined vinyl-N-triftosylhydrazones as a versatile allylating reagent to enable direct assembly of these medicinally desirable structural elements from low-cost alkane feedstocks. The reaction exhibited excellent site selectivity for tertiary C-H bonds, broad scope (>60 examples and >20 : 1:0 r. r.) and good efficiency, even on a gram-scale, making it a convenient alternative to the well-known Trost-Tsuji allylation reaction for the formation of alkyl-allyl bonds. Combined experimental and computational studies were employed to unravel the mechanism and origin of site- and chemoselectivity of the reaction.

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.

Chemoselective carbene insertion into the N-H bonds of NH3·H2O.

The conversion of inexpensive aqueous ammonia (NH3·H2O) into value-added primary amines by N-H insertion persists as a longstanding challenge in chemistry because of the tendency of Lewis basic ammonia (NH3) to bind and inhibit metal catalysts. Herein, we report a chemoselective carbene N-H insertion of NH3·H2O using a TpBr3Ag-catalyzed two-phase system. Coordination by a homoscorpionate TpBr3 ligand renders silver compatible with NH3 and H2O and enables the generation of electrophilic silver carbene. Water promotes subsequent [1,2]-proton shift to generate N-H insertion products with high chemoselectivity. The result of the reaction is the coupling of an inorganic nitrogen source with either diazo compounds or N-triftosylhydrazones to produce useful primary amines. Further investigations elucidate the reaction mechanism and the origin of chemoselectivity.

Silver-Catalyzed Vinylcarbene Insertion into C-C Bonds of 1,3-Diketones with Vinyl-N-triftosylhydrazones.

We describe the silver-catalyzed formal insertion of a vinylcarbene into unstrained C(CO)-C bonds of 1,3-diketones using vinyl-N-triftosylhydrazones as vinylcarbene precursors. This method allows the rapid synthesis of otherwise inaccessible 2-vinyl-substituted 1,4-diketones from relatively simple substrates. This mild catalytic protocol exhibits a good functional group tolerance and substrate scope and allows for good chemoselectivity control. The preliminary theoretical calculations suggest that the reaction proceeds through a stepwise enol cyclopropanation/retro-aldol pathway.

Carbodefluorination of fluoroalkyl ketones via a carbene-initiated rearrangement strategy.

The C-F bond cleavage and C-C bond formation (i.e., carbodefluorination) of readily accessible (per)fluoroalkyl groups constitutes an atom-economical and efficient route to partially fluorinated compounds. However, the selective mono-carbodefluorination of trifluoromethyl (CF3) groups remains a challenge, due to the notorious inertness of C-F bond and the risk of over-defluorination arising from C-F bond strength decrease as the defluorination proceeds. Herein, we report a carbene-initiated rearrangement strategy for the carbodefluorination of fluoroalkyl ketones with ß,γ-unsaturated alcohols to provide skeletally and functionally diverse α-mono- and α,α-difluoro-γ,δ-unsaturated ketones. The reaction starts with the formation of silver carbenes from fluoroalkyl N-triftosylhydrazones, followed by nucleophilic attack of a ß,γ-unsaturated alcohol to form key silver-coordinated oxonium ylide intermediates, which triggers selective C-F bond cleavage by HF elimination and C-C bond formation through Claisen rearrangement of in situ generated difluorovinyl ether. The origin of chemoselectivity and the reaction mechanism are determined by experimental and DFT calculations. Collectively, this strategy by an intramolecular cascade process offers significant advances over existing stepwise strategies in terms of selectivity, efficiency, functional group tolerance, etc.

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

Over 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.

Silver-catalyzed site-selective C(sp3)-H benzylation of ethers with N-triftosylhydrazones.

The insertion of carbenes into the α-C-H bonds of ethers represents one of the most powerful approaches to access polysubstituted α-branched ethers. However, intermolecular carbene insertions remain challenging, since current approaches are generally limited to the use of toxic and potentially explosive α-diazocarbonyl compounds. We now report a silver-catalyzed α-C-H benzylation of ethers using bench-stable N-triftosylhydrazones as safe and convenient carbene precursors. This approach is well suited for both inter- and intramolecular insertions to deliver medicinally relevant homobenzylic ethers and 5-8-membered oxacycles in good yields. The synthetic utility of this strategy is demonstrated by its easy scalability, broad scope with valuable functional groups, high regioselectivity, and late-stage functionalization of complex oxygen-containing molecules. The relative reactivities of different types of silver carbenes and C-H bonds were also investigated by experments and DFT calculations.

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.