Iron-Catalyzed Amination of Strong Aliphatic C(sp3)-H Bonds

Intermolecular Enantioselective Amination Reactions Mediated by Visible Light and a Chiral Iron Porphyrin Complex

In the presence of 1 mol % of a chiral iron porphyrin catalyst, various 3-arylmethyl-substituted 2-quinolones and 2-pyridones underwent an enantioselective amination reaction (20 examples; 93-99 % ee). The substrates were used as the limiting reagents, and fluorinated aryl azides (1.5 equivalents) served as nitrene precursors. The reaction is triggered by visible light which allows a facile dediazotation at ambient temperature. The selectivity of the reaction is governed by a two-point hydrogen bond interaction between the ligand of the iron catalyst and the substrate. Hydrogen bonding directs the amination to a specific hydrogen atom within the substrate that is displaced by the nitrogen substituent either in a concerted fashion or by a rebound mechanism.

Convergent Paired Electrolysis for [3+2] Cycloaddition of Azidotrimethylsilane with N-Heterocycles

A widely used method to obtain tetrazoles is through the azide and nitrile [3+2] cycloaddition. However, this process often involves using non-recyclable transition metals or Lewis acid catalysts and stoichiometric amounts of oxidants and additives, which reduces atom efficiency. We have discovered a convergent paired electrochemical reaction to perform this cycloaddition reaction, without the need for metal catalysts or oxidants. This tetrazolation strategy uses azidotrimethylsilane (TMSN3) and N-heterocycles in an undivided cell at a constant current. We use a mixture of CH3CN and equivalent amounts of H2O as co-solvent at room temperature. It is crucial to produce a stoichiometric amount of active hydroxyl ions through the cathodic reduction of water. Cyclic voltammetry (CV) studies and control experiments confirm that the cycloaddition reaction is specific to the electrode electron transfer process, eliminating the need for a mediator to shuttle electrons. This metal- and oxidant-free strategy is highly compatible with different functional groups and produces products with moderate to good yields. We have successfully tetrazolated bioactive compounds at a late stage, scaled up batches efficiently, and synthesized free amino-containing N-heterocycles via denitrogenation of tetrazoles.

Iron(II) Phthalocyanine-Catalyzed Homodimerization and Tandem Diamination of Diazo Compounds with Primary Amines: Access to Construct Substituted 2,3-Diaminosuccinonitriles in One-Pot

We herein first report the homodimerization and tandem diamination of diazo compounds with primary amines catalyzed by the iron(II) phthalocyanine (PcFe(II)), which can construct one C-C bond and two C-N bonds within 20 min in one-pot. Compared to the traditional metal-catalyzed N-H insertion reaction between amines with diazo reagents, the developed reaction almost does not generate the N-H insertion product, but the homodimerization/tandem diamination product. The proposed mechanism studies indicate that primary amines play a crucial role in the homocoupling of diazo compounds via dimerization of iron(III)-acetonitrile radical generated from the reaction between diazoacetonitrile with PcFe(II) coordinated by bis(amines); the β-hydride elimination is involved, and then, the attack of primary amines toward the carbon atoms on the formed C-C bond is followed. Moreover, this novel reaction can be used to effectively prepare substituted 2,3-diaminosuccinonitriles with high yields and even up to >99:1 d.r., encouragingly these products contain both 1,2-diamines and succinonitrile motifs, which are two classes of important organic compounds with significant applications in many yields. This reaction is also suitable for the gram-scale preparation of 2,3-bis(phenylamino)succinonitrile (2a) with a yield of 84%. Therefore, the developed reaction represents a new type of transformation.

Metalloradical Catalysis: General Approach for Controlling Reactivity and Selectivity of Homolytic Radical Reactions

Since Friedrich Wöhler's groundbreaking synthesis of urea in 1828, organic synthesis over the past two centuries has predominantly relied on the exploration and utilization of chemical reactions rooted in two-electron heterolytic ionic chemistry. While one-electron homolytic radical chemistry is both rich in fundamental reactivities and attractive with practical advantages, the synthetic application of radical reactions has been long hampered by the formidable challenges associated with the control over reactivity and selectivity of high-energy radical intermediates. To fully harness the untapped potential of radical chemistry for organic synthesis, there is a pressing need to formulate radically different concepts and broadly applicable strategies to address these outstanding issues. In pursuit of this objective, researchers have been actively developing metalloradical catalysis (MRC) as a comprehensive framework to guide the design of general approaches for controlling over reactivity and stereoselectivity of homolytic radical reactions. Essentially, MRC exploits the metal-centered radicals present in open-shell metal complexes as one-electron catalysts for homolytic activation of substrates to generate metal-entangled organic radicals as the key intermediates to govern the reaction pathway and stereochemical course of subsequent catalytic radical processes. Different from the conventional two-electron catalysis by transition metal complexes, MRC operates through one-electron chemistry utilizing stepwise radical mechanisms.

Direct synthesis of branched amines enabled by dual-catalyzed allylic C─H amination of alkenes with amines

Direct conversion of hydrocarbons into amines represents an important and atom-economic goal in chemistry for decades. However, intermolecular cross-coupling of terminal alkenes with amines to form branched amines remains extremely challenging. Here, a visible-light and Co-dual catalyzed direct allylic C─H amination of alkenes with free amines to afford branched amines has been developed. Notably, challenging aliphatic amines with strong coordinating effect can be directly used as C─N coupling partner to couple with allylic C─H bond to form advanced amines with molecular complexity. Moreover, the reaction proceeds with exclusive regio- and chemoselectivity at more steric hinder position to deliver primary, secondary, and tertiary aliphatic amines with diverse substitution patterns that are difficult to access otherwise.

Efficient and metal-free synthesis of 2-aroyl 7-azaindoles via thermally induced denitrogenative intramolecular annulation of 1,2,3,4-tetrazolopyridines

A facile and metal-free intramolecular denitrogenative annulation strategy for the preparation of novel 2-aroyl 7-azaindoles has been developed from 3-(tetrazolo[1,5-a]pyridin-8-yl)prop-2-en-1-one in the presence of the deep eutectic solvent Dowtherm A. The valuable features of the protocol include a short reaction time, absence of any metal catalyst, utilization of a eutectic solvent, easy product isolation, and very good yields of novel 2-aroyl 7-azaindoles.

Iodine-Mediated δ-Amination of sp3 C-H Bonds

We present a direct δ-amination reaction of sp3 C-H bonds, employing molecular iodine (I2) as the sole oxidant under transition-metal-free conditions. This remote C-H functionalization approach is operationally simple and provides facile, efficient access to pyrrolidines and related heterocyclic derivatives from readily accessible substrates.

Visible-Light-Driven Iron-Catalyzed Intermolecular Benzylic C(sp3)-H Amination with 1,2,3,4-Tetrazoles

A visible-light-driven iron-catalyzed C(sp3)-H amination of diphenylmethane derivatives with 1,2,3,4-tetrazoles under mild conditions has been developed. The reaction proceeds with photosensitizer-free conditions and features satisfactory to good yields. Mechanistic studies revealed that the reaction proceeded via an iron-nitrene intermediate, and H atom abstraction was the rate-determining step. Computational studies showed that the denitrogenation of 1,2,3,4-tetrazole depends on the conversion of the sextet ground state of 1,2,3,4-tetrazole-bounding iron species to the quartet spin state under visible-light irradiation.

One-Pot Manganese (I)-Catalyzed Oxidant-Controlled Divergent Functionalization of 2-Arylindazoles

The oxidant-controlled divergent synthesis of C-2' formyl 2H-indazoles and indazoloindazolediones has been developed through Mn(I)- catalyzed ortho C-H functionalization of 2H-indazoles with para-formaldehyde to afford C-2' hydroxymethylated 2H-indazoles and subsequently oxidation with varying the amount of DDQ in one-pot. By employing selectfluor as the oxidant instead of DDQ, this reaction exclusively provided indazolebenzoxazine derivatives. This strategy delivered unsymmetrical indazoloindazoledione and indazolobenzoxazine with varied functional group tolerance in moderate to good yields.

New Mode of Asymmetric Induction for Enantioselective Radical N-Heterobicyclization via Kinetically Stable Chiral Radical Center

Enantioselective radical N-heterobicyclization of N-allylsulfamoyl azides have been developed via metalloradical catalysis (MRC). The Co(II)-based catalytic system can homolytically activate the organic azides with varied electronic and steric properties for asymmetric radical N-heterobicyclization under mild conditions without the need of oxidants, allowing for stereoselective construction of chiral [3.1.0]-bicyclic sulfamoyl aziridines in excellent yields with high diastereoselectivities and enantioselectivities. The key to achieving the enantioselective radical process relies on catalyst development through ligand design. We demonstrate that the use of new-generation D2-symmetric chiral bridged amidoporphyrin ligand HuPhyrin with judicious variation of the alkyl bridge length can dictate both reactivity and selectivity of Co(II)-based MRC. We present both experimental and computational studies that shed light on the working details of the unprecedented mode of asymmetric induction consisting of enantioface-selective radical addition and stereospecific radical substitution. We showcase the synthetic applications of the resulting enantioenriched bicyclic aziridines through a number of stereospecific transformations.

The Recent Advances in Iron-Catalyzed C(sp3 )-H Functionalization

The use of iron as a core metal in catalysis has become a research topic of interest over the last few decades. The reasons are clear. Iron is the most abundant transition metal on Earth's crust and it is widely distributed across the world. It has been extracted and processed since the dawn of civilization. All these features render iron a noncontaminant, biocompatible, nontoxic, and inexpensive metal and therefore it constitutes the perfect candidate to replace noble metals (rhodium, palladium, platinum, iridium, etc.). Moreover, direct C-H functionalization is one of the most efficient strategies by which to introduce new functional groups into small organic molecules. The majority of organic compounds contain C(sp3 )-H bonds. Given the enormous importance of organic molecules in so many aspects of existence, the utilization and bioactivity of C(sp3 )-H bonds are of the utmost importance. This review sheds light on the substrate scope, selectivity, benefits, and limitations of iron catalysts for direct C(sp3 )-H bond activations. An overview of the use of iron catalysis in C(sp3 )-H activation protocols is summarized herein up to 2022.

Iron(III)-based metalloradical catalysis for asymmetric cyclopropanation via a stepwise radical mechanism

Metalloradical catalysis (MRC) exploits the metal-centred radicals present in open-shell metal complexes as one-electron catalysts for the generation of metal-stabilized organic radicals-key intermediates that control subsequent one-electron homolytic reactions. Cobalt(II) complexes of porphyrins, as stable 15e-metalloradicals with a well-defined low-spin d7 configuration, have dominated the ongoing development of MRC. Here, to broaden MRC beyond the use of Co(II)-based metalloradical catalysts, we describe systematic studies that establish the operation of Fe(III)-based MRC and demonstrate an initial application for asymmetric radical transformations. Specifically, we report that five-coordinate iron(III) complexes of porphyrins with an axial ligand, which represent another family of stable 15e-metalloradicals with a d5 configuration, are potent metalloradical catalysts for olefin cyclopropanation with different classes of diazo compounds via a stepwise radical mechanism. This work lays a foundation and mechanistic blueprint for future exploration of Fe(III)-based MRC towards the discovery of diverse stereoselective radical reactions.

Chemical insights into the synthetic chemistry of five-membered saturated heterocycles-a transition metal-catalyzed approach

Drug design and delivery is primarily based on the hunt for new potent drug candidates and novel synthetic techniques. Recently, saturated heterocycles have gained enormous attention in medicinal chemistry as evidenced by the medicinal drugs listed in the FDA Orange Book. Therefore, the demand for novel saturated heterocyclic syntheses has increased tremendously. Transition metal (TM)-catalyzed reactions have remained the prime priority in heterocyclic syntheses for the last three decades. Nowadays, TM catalysis is well adorned by combining it with other techniques such as bio- and/or enzyme-catalyzed reactions, organocatalysis, or using two different metals in a single catalysis. This review highlights the recent developments of the transition metal-catalyzed synthesis of five-membered saturated heterocycles.

Iron-Catalyzed Intermolecular C-N Cross-Coupling Reactions via Radical Activation Mechanism

A concept for intermolecular C-N cross-coupling amination has been discovered using tetrazoles and aromatic and aliphatic azides with boronic acids under iron-catalyzed conditions. The amination follows an unprecedented metalloradical activation mechanism that is different from traditional metal-catalyzed C-N cross-coupling reactions. The scope of the reaction has been demonstrated by the employment of a large number of tetrazoles, azides, and boronic acids. Moreover, several late-stage aminations and a short synthesis of a drug candidate have been showcased for further synthetic utility. Collectively, this iron-catalyzed C-N cross-coupling should have wide applications in the context of medicinal chemistry, drug discovery, and pharmaceutical industries.

Asymmetric Radical Bicyclization for Stereoselective Construction of Tricyclic Chromanones and Chromanes with Fused Cyclopropanes

Asymmetric radical bicyclization processes have been developed via metalloradical catalysis (MRC) to stereoselectively construct chiral chromanones and chromanes bearing fused cyclopropanes. Through optimization of a versatile D2-symmetric chiral amidoporphyrin ligand platform, a Co(II)-metalloradical system can homolytically activate both diazomalonates and α-aryldiazomethanes containing different alkene functionalities under mild conditions for effective radical bicyclization, delivering cyclopropane-fused tricyclic chromanones and chromanes, respectively, in high yields with excellent control of both diastereoselectivities and enantioselectivities. Combined computational and experimental studies, including the electron paramagnetic resonance (EPR) detection and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) trapping of key radical intermediates, shed light on the working details of the underlying stepwise radical mechanisms of the Co(II)-catalyzed bicyclization processes. The two catalytic radical processes provide effective synthetic tools for stereoselective construction of valuable cyclopropane-fused chromanones and chromanes with newly generated contiguous stereogenic centers. As a specific demonstration of synthetic application, the Co(II)-catalyzed radical bicyclization has been employed as a key step for the first asymmetric total synthesis of the natural product (+)-Radulanin J.

Origin of the Distinctive Electronic Structure of Co- and Fe-Porphyrin-Nitrene and Its Effect on Their Nitrene Transfer Reactivity

Metal-bound nitrene species are the crucial intermediate in catalytic nitrene transfer reactions exhibited by engineered enzymes and molecular catalysts. The electronic structure of such species and its correlation with nitrene transfer reactivity have not been fully understood yet. This work presents an in-depth electronic structure analysis and nitrene transfer reactivity of two prototypical metal-nitrene species derived from Co^II(TPP) and Fe^II(TPP) (TPP = meso-tetraphenylporphyrin) complexes and tosyl azide nitrene precursor. Parallel to the well-known "cobalt(III)-imidyl" electronic structure of the Co-porphyrin-nitrene species, the formation mechanism and electronic structure of the elusive Fe-porphyrin-nitrene have been established using density functional theory (DFT) and multiconfigurational complete active-space self-consistent field (CASSCF) calculations. Electronic structure evolution analysis for the metal-nitrene formation step and CASSCF-derived natural orbitals advocates that the electronic nature of the metal-nitrene (M-N) core of Fe(TPP) is strikingly different from that of the Co(TPP). Specifically, the "imidyl" nature of the Co-porphyrin-nitrene [(TPP)Co^III-^•NTos] (Tos = tosyl) (I1_Co) is contrasted by the "imido-like" character of the Fe-porphyrin-nitrene [(TPP)Fe^IV[Formula: see text]NTos] (I1_Fe). This difference between Co- and Fe-nitrene has been attributed to the additional interactions between Fe-d_π and N-p_π orbitals in Fe-nitrene, which is further complemented by the shortened Fe-N bond length of 1.71 Å. This stronger M-N bond in Fe-nitrene compared to the Co-nitrene is also reflected in the higher exothermicity (ΔΔH = 16 kcal/mol) of the Fe-nitrene formation step. The "imido-like" character renders a relatively lower spin population on the nitrene nitrogen (+0.42) in the Fe-nitrene complex I1_Fe, which undergoes the nitrene transfer to the C═C bond of styrene with a considerably higher enthalpy barrier (ΔH^‡ = 10.0 kcal/mol) compared to the Co congener I1_Co (ΔH^‡ = 5.6 kcal/mol) possessing a higher nitrogen spin population (+0.88) and a relatively weaker M-N bond (Co-N = 1.80 Å).

Advancements in Gold-Catalyzed Cascade Reactions to Access Carbocycles and Heterocycles: An Overview

This review summarizes recent developments (from 2006 to 2022) in numerous important and efficient carbo- and heterocycle generations using gold-catalyzed cascade protocols. Herein, methodologies involve selectivity, cost-effectiveness, and ease of product formation being controlled by the ligand as well as the counter anion, catalyst, substrate, and reaction conditions. Gold-catalyzed cascade reactions covered different strategies through the compilation of various approaches such as cyclization, hydroarylation, intermolecular and intramolecular cascade reactions, etc. This entitled reaction is also useful for the synthesis of spiro, fused, bridged carbo- and heterocycles.

Chiral Iron Porphyrins Catalyze Enantioselective Intramolecular C(sp3 )-H Bond Amination Upon Visible-Light Irradiation

Iron-catalyzed asymmetric amination of C(sp³ )-H bonds is appealing for synthetic applications due to the biocompatibility and high earth abundance of iron, but examples of such reactions are sparse. Herein we describe chiral iron complexes of meso- and β-substituted-porphyrins that can catalyze asymmetric intramolecular C(sp³ )-H amination of aryl and arylsulfonyl azides to afford chiral indolines (29 examples) and benzofused cyclic sulfonamides (17 examples), respectively, with up to 93 % ee (yield: up to 99 %) using 410 nm light under mild conditions. Mechanistic studies, including DFT calculations, for the reactions of arylsulfonyl azides reveal that the Fe(NSO₂ Ar) intermediate generated in situ under photochemical conditions reacts with the C(sp³ )-H bond through a stepwise hydrogen atom transfer/radical rebound mechanism, with enantioselectivity arising from cooperative noncovalent interactions between the Fe(NSO₂ Ar) unit and the peripheral substituents of the chiral porphyrin scaffold.

Transition-Metal-Catalyzed Denitrogenative Annulation to Access High-Valued N-Heterocycles

Over the past few years, the development of efficient methods to construct high-valued N-heterocyclic molecules have received massive attention owing to their extensive application in the areas of medicinal chemistry, drug discovery, natural product synthesis and so on. To access those high-valued N-heterocycles, many methods have been developed. In this context, transition-metal-catalyzed denitrogenative annulation of 1,2,3-triazoles and 1,2,3,4-tetrazoles has appeared as a powerful synthetic tool because it offers a step- and atom-economical route for the preparation of the nitrogen-rich molecules. Compared with the denitrogenative annulation of various 1,2,3-triazole frameworks, annulation of 1,2,3,4-tetrazole remains more challenging due to the inertness of the tetrazole moiety. This Review summarizes the significant achievements made in the field of denitrogenative annulation of various 1,2,3-triazoles and 1,2,3,4-tetrazoles including some pioneering examples in this area of research. We anticipate that this denitrogenative annulation reaction will find broad applications in the pharmaceutical industry, drug discovery and other fields of medicinal chemistry.

Iron-Catalyzed Intermolecular Amination of Benzylic C(sp3)-H Bonds

A catalytic system for intermolecular benzylic C(sp³)-H amination is developed utilizing 1,2,3,4-tetrazole as a nitrene precursor via iron catalysis. This method enables direct installation of 2-aminopyridine into the benzylic and heterobenzylic position. The method selectively aminates 2° benzylic C(sp³)-H bond over the 3° and 1° benzylic C(sp³)-H bonds. Experimental studies reveal that the C(sp³)-H amination undergoes via the formation of a benzylic radical intermediate. This study reports the discovery of new method for 2-pyridine substituted benzylamine synthesis using inexpensive, biocompatible base metal catalysis that should have wide application in the context of medicinal chemistry and drug discovery.

An iron(ii)-based metalloradical system for intramolecular amination of C(sp2)-H and C(sp3)-H bonds: synthetic applications and mechanistic studies

A catalytic system for intramolecular C(sp2)-H and C(sp3)-H amination of substituted tetrazolopyridines has been successfully developed. The amination reactions are developed using an iron-porphyrin based catalytic system. It has been demonstrated that the same iron-porphyrin based catalytic system efficiently activates both the C(sp2)-H and C(sp3)-H bonds of the tetrazole as well as azide-featuring substrates with a high level of regioselectivity. The method exhibited an excellent functional group tolerance. The method affords three different classes of high-value N-heterocyclic scaffolds. A number of important late-stage C-H aminations have been performed to access important classes of molecules. Detailed studies (experimental and computational) showed that both the C(sp2)-H and C(sp3)-H amination reactions involve a metalloradical activation mechanism, which is different from the previously reported electro-cyclization mechanism. Collectively, this study reports the discovery of a new class of metalloradical activation modes using a base metal catalyst that should find wide application in the context of medicinal chemistry, drug discovery and industrial applications.

Intermolecular C-H Aminocyanation of Indoles via Copper-iodine Cocatalyzed Tandem C-N/C-C Bond Formation

An efficient copper-iodine cocatalyzed intermolecular C-H aminocyanation of indoles with a broad substrate scope has been developed for the first time. This method enables highly step-economic access to 2-amino-3-cyanoindoles in moderate to good yields and provides a complementary strategy for the regioselective difunctionalization of carbon═carbon double bonds of interest in organic synthesis and related areas. Mechanistic studies suggest that these transformations are initiated by iodine-mediated C2-H amination with azoles, followed by copper-catalyzed C3-H cyanation with ethyl cyanoformate.

PdII-Catalyzed γ-C(sp3)-H (Hetero)Arylation of Ketones Enabled by Transient Directing Groups

Pd(II)-catalyzed γ-C(sp3)-H (hetero)arylation of aliphatic ketones is developed using α-amino acid as transient directing groups (TDG). A variety of aliphatic ketones were (hetero)arylated at the γ-position via a 5,6-membered fused cyclopalladation intermediate to afford the remotely arylated products in up to 88% yield. The crucial ligand effect of 2-pyridone is further enhanced by reducing the loading of acid additives. Consequentially, the improved reactivity of this catalytic system has also made possible the cyclic γ-methylene C(sp3)-H arylation of ketones. Mechanistic investigtigation and comparison to the γ-C-H arylation of aldehydes revealed a structural insight for designing site selective TDG.

Recent advances in 3-aminoindazoles as versatile synthons for the synthesis of nitrogen heterocycles

Nitrogen-based heterocycles are an important class of structural scaffolds distributed in biologically active natural products, medicinal chemistry, and agrochemicals. Hence, there is increasing interest in the development of novel synthetic strategies for the construction of these privileged structural motifs. Recently, 3-aminoindazoles have emerged as versatile synthons participating in a variety of condensation annulation, denitrogenative transannulation and rearrangement ring expansion reactions, which provide efficient synthetic routes for the formation of nitrogen heterocycles. This review systematically highlights for the first time the most recent advances in 3-aminoindazoles to provide a deep understanding of using 3-aminoindazoles as versatile synthons in organic transformations for synthetic and medicinal chemists.

Combined radical and ionic approach for the enantioselective synthesis of β-functionalized amines from alcohols

Chiral amines are among the most important organic compounds and have widespread applications. Enantioselective construction of chiral amines is a major aim in organic synthesis. Among synthetic methods, direct functionalization of omnipresent C-H bonds with common organic nitrogen compounds represents one of the most attractive strategies. However, C-H amination strategies are largely limited to constructing a specific type of N-heterocycles or amine derivatives. To maximize the synthetic potential of asymmetric C-H amination, we report here an approach that unites the complementary reactivities of radical and ionic chemistry for streamlined synthesis of functionalized chiral amines. This synthesis merges the development of an enantioselective radical process for 1,5-C(sp ³)-H amination of alkoxysulfonyl azides via Co(II)-based metalloradical catalysis with an enantiospecific ionic process for ring-opening of the resulting five-membered chiral sulfamidates by nucleophiles. Given that alkoxysulfonyl azides are derived from the corresponding alcohols, this approach offers a powerful synthetic tool for enantioselective β-C-H amination of common alcohols while converting the hydroxy group to other functionalities through formal nucleophilic substitution.

Radical differentiation of two ester groups in unsymmetrical diazomalonates for highly asymmetric olefin cyclopropanation

Diazomalonates have been demonstrated as effective metalloradicophiles for asymmetric radical olefin cyclopropanation via Co(II)-metalloradical catalysis (MRC). Supported by D ₂-symmetric chiral amidoporphyrin ligand, Co(II)-based metalloradical system can efficiently activate unsymmetrical methyl phenyl diazomalonate (MPDM) with effective differentiation of the two ester groups for asymmetric cyclopropanation, enabling stereoselective construction of 1,1-cyclopropanediesters bearing two contiguous chiral centers, including all-carbon quaternary stereogenic center. The Co(II)-catalyzed asymmetric cyclopropanation, which operates at room temperature without slow addition of the diazo compound, is generally applicable to broad-ranging olefins and tolerates various functionalities, providing a streamlined synthesis of chiral 1,1-cyclopropanediesters in high yields with both high diastereoselectivity and enantioselectivity. Combined computational and experimental studies support the underlying stepwise radical mechanism for Co(II)-catalyzed cyclopropanation. In addition to functioning as 1,3-dipoles for forming five-membered structures, enantioenriched (E)-1,1-cyclopropanediesters serve as useful building blocks for stereoselective synthesis of different cyclopropane derivatives. In addition, the enantioenriched (E)-1,1-cyclopropanediesters can be stereoselectively converted to (Z)-diastereomers.

Metalloradical Activation of In Situ-Generated α-Alkynyldiazomethanes for Asymmetric Radical Cyclopropanation of Alkenes

α-Alkynyldiazomethanes, generated in situ from the corresponding sulfonyl hydrazones in the presence of a base, can serve as effective metalloradicophiles in Co(II)-based metalloradical catalysis (MRC) for asymmetric cyclopropanation of alkenes. With D₂-symmetric chiral amidoporphyrin 2,6-DiMeO-QingPhyrin as the optimal supporting ligand, the Co(II)-based metalloradical system can efficiently activate different α-alkynyldiazomethanes at room temperature for highly asymmetric cyclopropanation of a broad range of alkenes. This catalytic radical process provides a general synthetic tool for stereoselective construction of alkynyl cyclopropanes in high yields with high both diastereoselectivity and enantioselectivity. Combined computational and experimental studies offer several lines of evidence in support of the underlying stepwise radical mechanism for the Co(II)-catalyzed olefin cyclopropanation involving a unique α-metalloradical intermediate that is associated with two resonance forms of α-Co(III)-propargyl radical and γ-Co(III)-allenyl radical. The resulting enantioenriched alkynyl cyclopropanes, as showcased with several stereospecific transformations, may serve as valuable chiral building blocks for stereoselective organic synthesis.

Cu(II)-Catalyzed C-H Amidation/Cyclization of Azomethine Imines with Dioxazolones via Acyl Nitrenes: A Direct Access to Diverse 1,2,4-Triazole Derivatives

We report a Cu(II)-catalyzed C-H amidation/cyclization of azomethine imines with dioxazolones as acyl nitrene transfer reagents under additive- and ligand-free conditions. An array of 1,2,4-triazolo[1,5-a]pyridine derivatives were afforded in moderate to good yields with excellent functional group tolerance. In addition, scale-up reaction and photoluminescence properties were discussed.

Alkali-Amide Controlled Selective Synthesis of 7-Azaindole and 7-Azaindoline through Domino Reactions of 2-Fluoro-3-methylpyridine and Aldehydes

Azaindoles and azaindolines are important core structures in pharmaceuticals and natural products, which have found wide applications in the field of medicinal chemistrty. In this study, we developed a novel...

A comparative study of Rh2-catalyzed intermolecular nitrene transfer reactions: mechanism and chemoselectivity

The intermolecular catalytic mechanisms using Rh2(esp)2 and Rh2(OAc)4 are analogous and their large difference in aziridination-to-amination chemoselectivity stems from the steric effect.

Road Map for the Construction of High-Valued N-Heterocycles via Denitrogenative Annulation

The pursuit for the discovery of new and powerful synthetic methods to access high-value N-heterocycles has been at the forefront of organic chemistry research for more than a century. Considering the importance of N-scaffolds in modern science, over the past few decades, great research efforts have been made to develop efficient synthetic methods for the construction of nitrogen-rich molecules. Among many efforts, transition metal catalyzed denitrogenative annulation reaction has emerged as a cornerstone due to its innate versatility and wider scope of application.The denitrogenative annulation approach offers clear advantages over many existing methods, as it enables effective, single-step interconversion of easily available feedstocks into a variety of other important N-containing heterocyclic frameworks. Recently, transition metal catalyzed denitrogenative annulation reaction of the 1,2,3-triazole via a metal carbene intermediate sparked significant interest in the application of various important heterocycle syntheses. Denitrogenative annulation reaction of 1,2,3-triazoles proceeds via an ionic mechanism. Recently, we demonstrated a new concept for the denitrogenative reaction of triazoles with alkenes and alkynes via in situ generated 2-(diazomethyl)pyridines. The method takes advantage of the inherent properties of a Co(III)-carbene radical intermediate and is the first report of the denitrogenative annulation/cyclopropanation by a radical-activation mechanism.On the other hand, in contrast to the denitrogenative annulation of 1,2,3-triazole, annulation reaction of 1,2,3,4-tetrazole (a surrogate of azide having an important pyridyl unit) via metal nitrene remains a big challenge. Previously, flash vacuum pyrolysis studies had been used for nitrene-nitrene rearrangement of 1,2,3,4-tetrazole at high temperature. This Account summarizes our recent efforts in developing transition metal catalyzed denitrogenative annulation of 1,2,3-triazoles via a radical mechanism and 1,2,3,4-tetrazoles via metal nitrene to access important nitrogen-rich molecules. We demonstrated that the 1,2,3,4-tetrazole under Ir-catalyzed reaction conditions can produce a productive Ir-nitrene intermediate that can successfully be employed for the construction of a wide number of α-carbolines and 7-azaindoles. Moreover, we developed an iron-based unique strategy for the intermolecular denitrogenative annulation reaction between tetrazoles and alkynes. The reaction overcomes the traditional click reaction and proceeds via an unprecedented metalloradical activation mechanism. Furthermore, we used our understanding of tetrazole reactivity to design an iron-catalyzed intramolecular denitrogenative C(sp³)-H amination reaction of primary, secondary, and tertiary centers by using a metalloradical activation concept. At the same time, we also developed a general catalytic method to enable two distinct reactions (1,3-cycloaddition and denitrogenative annulation) using Mn(TPP)Cl that afforded two different classes of nitrogen heterocycles. Mechanistic studies showed that although the click reaction likely proceeds through an ionic mechanism and the denitrogenative annulation reaction likely proceeds via an electrophilic metallonitrene intermediate rather than a metallonitrene radical intermediate. Finally, we report an iron-catalyzed rearrangement reaction (ring expansion/migration) that proceeded with an unprecedented level of selectivity, reactivity, and functional group tolerance offering rapid access to numerous complex N-heterocycles. We believe that our continuous efforts in this field would be beneficial for pharmaceutical industries, drug discovery, and other fields of medicinal chemistry.

Theoretical studies on iron-catalyzed azaindoline formation: mechanism and site-selectivity

The mechanism and site-selectivity for Fe-catalyzed azaindoline formation from 1,2,3,4-tetrazole were examined computationally. The H-atom abstraction/radical rebound stepwise mechanism is proposed. The aliphatic H-atom abstraction (HAA) vs. electrophilic aromatic substitution (EAS) steps are responsible for the sp³vs. sp² C-H amination site-selectivity and a larger steric congestion disfavors sp² EAS, thus resulting in Fe-catalyzed site-selectivity toward sp³ C-H amination.

Visible-Light-Catalyzed in Situ Denitrogenative Sulfonylation of Sulfonylhydrazones

A photocatalyzed in situ denitrogenative sulfonylation of N-arylsulfonyl hydrazones has been developed. This transformation provides a low-carbon strategy to assemble arylalkyl sulfones in a stepwise denitrogenation/sulfonylation manner.

On the application of 3d metals for C-H activation toward bioactive compounds: The key step for the synthesis of silver bullets

Several valuable biologically active molecules can be obtained through C-H activation processes. However, the use of expensive and not readily accessible catalysts complicates the process of pharmacological application of these compounds. A plausible way to overcome this issue is developing and using cheaper, more accessible, and equally effective catalysts. First-row transition (3d) metals have shown to be important catalysts in this matter. This review summarizes the use of 3d metal catalysts in C-H activation processes to obtain potentially (or proved) biologically active compounds.

Controlling Enantioselectivity and Diastereoselectivity in Radical Cascade Cyclization for Construction of Bicyclic Structures

Radical cascade cyclization reactions are highly attractive synthetic tools for the construction of polycyclic molecules in organic synthesis. While it has been successfully implemented in diastereoselective synthesis of natural products and other complex compounds, radical cascade cyclization faces a major challenge of controlling enantioselectivity. As the first application of metalloradical catalysis (MRC) for controlling enantioselectivity as well as diastereoselectivity in radical cascade cyclization, we herein report the development of a Co(II)-based catalytic system for asymmetric radical bicyclization of 1,6-enynes with diazo compounds. Through the fine-tuning of D2-symmetric chiral amidoporphyrins as the supporting ligands, the Co(II)-catalyzed radical cascade process, which proceeds in a single operation under mild conditions, enables asymmetric construction of multisubstituted cyclopropane-fused tetrahydrofurans bearing three contiguous stereogenic centers, including two all-carbon quaternary centers, in high yields with excellent stereoselectivities. Combined computational and experimental studies have shed light on the underlying stepwise radical mechanism for this new Co(II)-based cascade bicyclization that involves the relay of several Co-supported C-centered radical intermediates, including α-, β-, γ-, and ε-metalloalkyl radicals. The resulting enantioenriched cyclopropane-fused tetrahydrofurans that contain a trisubstituted vinyl group at the bridgehead, as showcased in several stereospecific transformations, may serve as useful intermediates for stereoselective organic synthesis. The successful demonstration of this new asymmetric radical process via Co(II)-MRC points out a potentially general approach for controlling enantioselectivity as well as diastereoselectivity in synthetically attractive radical cascade reactions.

Asymmetric Radical Process for General Synthesis of Chiral Heteroaryl Cyclopropanes

A highly efficient catalytic method has been developed for asymmetric radical cyclopropanation of alkenes with in situ-generated α-heteroaryldiazomethanes via Co(II)-based metalloradical catalysis (MRC). Through fine-tuning the cavity-like environments of newly-synthesized D2-symmetric chiral amidoporphyrins as the supporting ligand, the optimized Co(II)-based metalloradical system is broadly applicable to α-pyridyl and other α-heteroaryldiazomethanes for asymmetric cyclopropanation of wide-ranging alkenes, including several types of challenging substrates. This new catalytic methodology provides a general access to valuable chiral heteroaryl cyclopropanes in high yields with excellent both diastereoselectivities and enantioselectivities. Combined computational and experimental studies further support the underlying stepwise radical mechanism of the Co(II)-based olefin cyclopropanation involving α- and γ-metalloalkyl radicals as the key intermediates.