Intermolecular Reductive C-N Cross Coupling of Nitroarenes and Boronic Acids by PIII/PV═O Catalysis

Modular Assembly of Acridines by Integrating Photo-Excitation of o-Alkyl Nitroarenes with Copper-Promoted Cascade Annulation

Acridine frameworks stand as pivotal architectural elements in pharmaceuticals and photocatalytic applications, owing to their chemical adaptability, biological activity, and unique excited-state dynamics. Conventional synthetic routes often entail specialized starting materials, anaerobic or moisture-free conditions, and elaborate multi-stage manipulations for incorporating diverse functionalities. Herein, we present a convergent approach integrating photo-excitation of readily available ortho-alkyl nitroarenes with copper-promoted cascade annulation. This innovative system enables an aerobic, one-pot reaction of o-alkyl nitroarenes with arylboronic acids, thereby streamlining the modular construction of a wide array of acridine derivatives with various functional groups. This encompasses symmetrical, unsymmetrical and polysubstituted varieties, some of which are otherwise exceptionally difficult to synthesize. Furthermore, it significantly improves the production of structurally varied acridinium salts, featuring enhanced photophysical properties, high excited state potentials (E*red=2.08-3.15 V), and exhibiting superior performance in intricate photoredox transformations.

Electrophilic C(sp2)-H Cyanation with Inorganic Cyanate (OCN-) by PIII/PV=O-Catalyzed Phase Transfer Activation

An organophosphorus -catalyzed method for the direct electrophilic cyanation of C(sp2)-H nucleophiles with sodium cyanate (NaOCN) is reported. The catalytic deoxyfunctionalization of the OCN- anion is enabled by the use of a small-ring phosphacyclic (phosphetane) catalyst in combination with a terminal hydrosilane O-atom acceptor and a malonate-derived bromenium donor. In situ spectroscopy under single-turnover conditions demonstrate that insoluble inorganic cyanate anion is activated by bromide displacement on a bromophosphonium catalytic intermediate to give a reactive N-bound isocyanatophosphonium ion, which delivers electrophilic "CN+" equivalents to nucleophilic (hetero)arenes and alkenes with loss of a phosphine oxide. These results demonstrate the feasibility of deoxyfunctionalization of insoluble inorganic salts by PIII/PV=O catalyzed phase transfer activation.

Photocatalytic PPh3-Mediated Synthesis of C3-Functionalized Indoles via Radical Annulation of Nitroarenes and Alkenes

Oxidatively generated phosphine radical cations are reactive intermediates that can be used for the generation of carbon and heteroatom centered radicals via deoxygenation processes. Such P-radical cations can readily be generated via single electron transfer oxidation using a redox catalyst. Cheap and commercially available nitroarenes are ideal nitrogen sources for the construction of organic amines and N-containing heterocycles. Activation of nitroarenes with phosphines has been achieved in the ionic mode, which requires specially designed P-nucleophiles and high temperatures. Herein, we report an alternative mode of nitro activation that proceeds via a radical process. The radical strategy leads to open shell intermediates that show interesting unexplored reactivity. This is documented by the development of an economic and highly efficient synthesis of valuable indole derivatives through photocatalytic PPh3-mediated annulation of nitroarenes with alkenes showing large functional group tolerance. The method allows room-temperature activation of nitroarenes and a double C-H bond functionalization of alkenes is achieved to provide rapid access to C3-functionalized indoles, which are key structural components of diverse natural and drug molecules. Experimental mechanistic studies that are further supported by DFT calculations indicate that a nitrosoarene radical cation plays a key role in the annulation process.

Shining light on the nitro group: distinct reactivity and selectivity

The nitro moiety is an indispensable functional group in organic synthesis due to its facile introduction and reduction to the corresponding amines for a plethora of organic transformations. Owing to its distinct electronegative and conventional properties, it has been used for activated aromatic nucleophilic substitution (SNAr) reactions, Smiles reactions, Henry reactions, acyl anion equivalents, etc. Recently, the excellent photochemical properties of nitroarenes have been rediscovered by several groups, and their untapped potential in organic synthesis under UV or visible light irradiation has been exploited. Photoexcited nitroarenes can undergo facile reduction to amines, azo-coupling, metal-free reductive C-N coupling with boronic acids via a 1,2-boronate shift, hydrogen atom transfer (HAT), oxygen atom transfer for anaerobic oxidation of organic molecules, molecular editing via nitrene intermediates, denitrative coupling of β-nitrostyrene, radical α-alkylation of nitroalkanes, etc. They have also been used as a photolabile protecting group in medicinal chemistry and chemical biology applications. Here, we summarise the recent findings on visible-light-mediated transformations involving nitro-containing organic molecules.

A mild synthetic route to α-nitroso diaryl pyrroles

A new synthetic method to access α-nitroso pyrroles is presented. This method utilises the nitrosonium salt NOBF4, enabling short reaction times (<10 minutes) and avoiding the harsh acidic conditions usually associated with pyrrole nitrosation. Application of this procedure to diarylated pyrroles yielded several novel nitroso-pyrroles. Modifications to the method, through exclusion of air and inclusion of a mild base, allowed for the nitrosation of pyrroles bearing aryl groups substituted with electron-donating groups. Attempts to nitrosylate pyrroles bearing alkyl substituents resulted in the formation of a dimeric material composed of a pyrrolic unit and a 2-hydroxyimino-protected 1,5-dihydro-2H-pyrrol-2-one.

Bisgermylene-Stabilized Stannylone: Catalytic Reduction of Nitrous Oxide and Nitro Compounds via Element-Ligand Cooperativity

This study describes the synthesis, structural characterization, and catalytic application of a bis(germylene)-stabilized stannylone (2). The reduction of digermylated stannylene (1) with 2.2 equiv of potassium graphite (KC8) leads to the formation of stannylone 2 as a green solid in 78% yield. Computational studies showed that stannylone 2 possesses a formal Sn(0) center and a delocalized 3-c-2-e π-bond in the Ge2Sn core, which arises from back-donation of the p-type lone pair electrons on the Sn atom to the vacant orbitals of the Ge atoms. Stannylone 2 can serve as an efficient precatalyst for the selective reduction of nitrous oxide (N2O) and nitroarenes (ArNO2) with the formation of dinitrogen (N2) and hydrazines (ArNH-NHAr), respectively. Exposure of 2 with N2O (1 atm) resulted in the insertion of two oxygen atoms into the Ge-Ge and Ge-Sn bonds, yielding the germyl(oxyl)stannylene (3). Moreover, the stoichiometric reaction of 2 with 1-chloro-4-nitrobenzene afforded an amido(oxyl)stannylene (4) through the complete scission of the N-O bonds of the nitroarene. Stannylenes 3 and 4 serve as catalytically active species for the catalytic reduction of nitrous oxide and nitroarenes, respectively. Mechanistic studies reveal that the cooperation of the low-valent Ge and Sn centers allows for multiple electron transfers to cleave the N-O bonds of N2O and ArNO2. This approach presents a new strategy for catalyzing the deoxygenation of N2O and ArNO2 using a zerovalent tin compound.

CuH-Catalyzed Reductive Coupling of Nitroarenes with Phosphine Oxides for the Direct Synthesis of Phosphamides

A CuH-catalyzed reductive coupling of nitroarenes with phosphine oxides is developed, which produces a series of phosphamides in moderate to excellent yields with good functional group tolerance. Gram-scale synthesis and late-stage modification of nitro-aromatic functional molecule niclosamide are also successfully conducted. The mechanism study shows that the nitro group is transformed after being reduced to nitroso and a nucleophilic addition procedure is involved during the reaction.

Derivatization of 2,1,3-Benzothiadiazole via Regioselective C-H Functionalization and Aryne Reactivity

Despite growing interest in 2,1,3-benzothiadiazole (BTD) as an integral component of many functional molecules, methods for the functionalization of its benzenoid ring have remained limited, and many even simply decorated BTDs have required de novo synthesis. We show that regioselective Ir-catalyzed C-H borylation allows access to versatile 5-boryl or 4,6-diboryl BTD building blocks, which undergo functionalization at the C4, C5, C6, and C7 positions. The optimization and regioselectivity of C-H borylation are discussed. A broad reaction scope is presented, encompassing ipso substitution at the C-B bond, the first examples of ortho-directed C-H functionalization of BTD, ring closing reactions to generate fused ring systems, as well as the generation and capture reactions of novel BTD-based heteroarynes. The regioselectivity of the latter is discussed with reference to the Aryne Distortion Model.

Cobalt- or rhodium-catalyzed synthesis of 1,2-dihydrophosphete oxides via C-H activation and formal phosphoryl migration

A highly stereo- and chemoselective intermolecular coupling of diverse heterocycles with dialkynylphosphine oxides has been realized via cobalt/rhodium-catalyzed C-H bond activation. This protocol provides an efficient synthetic entry to functionalized 1,2-dihydrophosphete oxides in excellent yields via the merger of C-H bond activation and formal 1,2-migration of the phosphoryl group. Compared with traditional methods of synthesis of 1,2-dihydrophosphetes that predominantly relied on stoichiometric metal reagents, this catalytic system features high efficiency, a relatively short reaction time, atom-economy, and operational simplicity. Photophysical properties of selected 1,2-dihydrophosphete oxides are also disclosed.

Amino-λ3 -iodane-Enabled Electrophilic Amination of Arylboronic Acid Derivatives

In this report, we describe the use of amino-λ3 -iodanes in the electrophilic amination of arylboronic acids and boronates. Iodine(III) reagents with transferable amino groups, including one with an NH2 group, were synthesized and used in the amination, allowing the synthesis of a wide range of primary and secondary (hetero)arylamines. Mechanistic studies by DFT calculations indicate that the reaction proceeds through an electrophilic amination process from a tetravalent borate complex with a B-N dative bond.

Dual role of nitroarenes as electrophiles and arylamine surrogates in Buchwald-Hartwig-type coupling for C-N bond construction

One of the most widely utilized methods for the construction of C(sp2)-N bonds is the transition-metal-catalyzed cross-coupling of aryl halides/boronic acids with amines, known as Ullmann condensation, Buchwald-Hartwig amination, and Chan-Lam coupling. However, aryl halides/boronic acids often require multi-step preparation while generating a large amount of corrosive and toxic waste, making the reaction less attractive. Herein, we present an unprecedented method for the C(sp2)-N formation via Buchwald-Hartwig-type reactions using synthetically upstream nitroarenes as the sole starting materials, thus eliminating the need for arylhalides and pre-formed arylamines. A diverse range of symmetrical di- and triarylamines were obtained in a single step from nitroarenes, and more importantly, various unsymmetrical di- and triarylamines were also highly selectively synthesized in a one-pot/two-step process. Furthermore, the success of the scale-up experiments, the late-stage functionalization of a drug intermediate, and the rapid preparation of hole-transporting material TCTA showcased the utility and practicality of this protocol in synthetic chemistry. Mechanistic studies indicate that this transformation may proceed via an arylamine intermediate generated in situ from the reduction of nitroarenes, which is followed by a denitrative Buchwald-Hartwig-type reaction with another nitroarene to form a C-N bond.

Merging Excited-State Copper Catalysis and Triplet Nitro(hetero)arenes for Direct Synthesis of 2-Aminophenol Derivatives

Nitro(hetero)arene derivatives are essential commodity chemicals used in various products, such as drugs, polymers, and agrochemicals. In this study, we leverage the excited-state reactivities of copper catalysts and nitro(hetero)arenes, and the Umpolung reactivity of acyl radicals to convert readily available nitro(hetero)arenes directly to valuable 2-aminophenol derivatives, which are important scaffolds in many top-selling pharmaceuticals. This reaction is applicable to a variety of nitro(hetero)arenes, acyl chlorides, and late-stage modifications of complex molecules, making it a useful tool for the discovery of new functional molecules. Mechanistic studies, including radical trapping experiments, Stern Volmer quenching studies, light ON/OFF experiments, and 18O-labeling studies, suggest a reaction mechanism involving photoexcitation of a copper complex, diradical couplings, and an in-cage contact ion pair (CIP) migration. Our findings offer a streamlined protocol for synthesizing essential pharmacophores from nitro(hetero)arenes while simultaneously advancing knowledge in excited-state and radical chemistry and stimulating new reaction design and development.

Programmable late-stage functionalization of bridge-substituted bicyclo[1.1.1]pentane bis-boronates

Modular functionalization enables versatile exploration of chemical space and has been broadly applied in structure-activity relationship (SAR) studies of aromatic scaffolds during drug discovery. Recently, the bicyclo[1.1.1]pentane (BCP) motif has increasingly received attention as a bioisosteric replacement of benzene rings due to its ability to improve the physicochemical properties of prospective drug candidates, but studying the SARs of C2-substituted BCPs has been heavily restricted by the need for multistep de novo synthesis of each analogue of interest. Here we report a programmable bis-functionalization strategy to enable late-stage sequential derivatization of BCP bis-boronates, opening up opportunities to explore the SARs of drug candidates possessing multisubstituted BCP motifs. Our approach capitalizes on the inherent chemoselectivity exhibited by BCP bis-boronates, enabling highly selective activation and functionalization of bridgehead (C3)-boronic pinacol esters (Bpin), leaving the C2-Bpin intact and primed for subsequent derivatization. These selective transformations of both BCP bridgehead (C3) and bridge (C2) positions enable access to C1,C2-disubstituted and C1,C2,C3-trisubstituted BCPs that encompass previously unexplored chemical space.

Organocatalytic Stereospecific Appel Reaction

Herein we report a new method for the catalytic Appel reaction by P(III)/P(V) redox cycling at very low catalyst loadings of 1-2 mol % using low amounts of hexachloroacetone as the halogen source and phenylsilane as the terminal reductant. Twenty-six alcohols and nine epoxides containing a wide variety of functional groups were converted to the respective chlorides and dichlorides in yields of up to 97%, enantiospecificities of up to >99%, and enantiomeric ratios of up to >99:1.

Mechanistic Studies on the Bismuth-Catalyzed Transfer Hydrogenation of Azoarenes

Organobismuth-catalyzed transfer hydrogenation has recently been disclosed as an example of low-valent Bi redox catalysis. However, its mechanistic details have remained speculative. Herein, we report experimental and computational studies that provide mechanistic insights into a Bi-catalyzed transfer hydrogenation of azoarenes using p-trifluoromethylphenol (4) and pinacolborane (5) as hydrogen sources. A kinetic analysis elucidated the rate orders in all components in the catalytic reaction and determined that 1 a (2,6-bis[N-(tert-butyl)iminomethyl]phenylbismuth) is the resting state. In the transfer hydrogenation of azobenzene using 1 a and 4, an equilibrium between 1 a and 1 a ⋅ [OAr]2 (Ar=p-CF3 -C6 H4 ) is observed, and its thermodynamic parameters are established through variable-temperature NMR studies. Additionally, pKa -gated reactivity is observed, validating the proton-coupled nature of the transformation. The ensuing 1 a ⋅ [OAr]2 is crystallographically characterized, and shown to be rapidly reduced to 1 a in the presence of 5. DFT calculations indicate a rate-limiting transition state in which the initial N-H bond is formed via concerted proton transfer upon nucleophilic addition of 1 a to a hydrogen-bonded adduct of azobenzene and 4. These studies guided the discovery of a second-generation Bi catalyst, the rate-limiting transition state of which is lower in energy, leading to catalytic transfer hydrogenation at lower catalyst loadings and at cryogenic temperature.

The Development and Mechanistic Study of an Iron-Catalyzed Intramolecular Nitroso Ene Reaction of Nitroarenes

An intramolecular iron-catalyzed nitroso ene reaction was developed to afford six- or seven-membered N-heterocycles from nitroarenes using an earth abundant iron catalyst and phenylsilane as the terminal reductant. The reaction can be triggered using as little as 3 mol % of iron(II) acetate and 3 mol % of 4,7-dimethoxyphenanthroline as the ligand. The scope of the reaction is broad tolerating a range of electron-releasing or electron-withdrawing substituents on the nitroarene, and the ortho-substituent can be modified to diastereoselectively construct benzoxazines, dihydrobenzothiazines, tetrahydroquinolines, tetrahydroquinoxalines, or tetrahydrobenzooxazepines. Mechanistic investigations indicated that the reaction proceeds via a nitrosoarene intermediate, and kinetic analysis of the reaction revealed a first-order rate dependence in catalyst-, nitroarene-, and silane concentration, and an inverse kinetic order in acetate was observed. The difference in rates between PhSiH3 and PhSiD3 was found to be 1.50 ± 0.09, and investigation of the temperature dependence of the reaction rate revealed that the activation parameters to be ΔH‡ = 13.5 kcal•mol-1 and ΔS‡ = -39.1 cal•mol-1•K-1. These data were interpreted to indicate that the turnover-limiting step to be hydride transfer from iron to the coordinated nitroarene, which occurs through an ordered transition state with little Fe-H bond breaking.

Reductive coupling of nitro compounds with boronic acid derivatives: an overview

The purpose of this review is to summarize the current literature on reductive C-N coupling of nitro compounds and boronic acids, with special emphasis on the mechanistic features of the reactions. The metal-catalyzed reactions are discussed first. This is followed by electro-synthesis and organophosphorus-catalyzed reactions. Finally, the available examples of catalyst-free reactions will be covered at the end of this review.

Direct Synthesis of Unprotected Indolines Through Intramolecular sp3 C-H Amination Using Nitroarenes as Aryl Nitrene Precursors

Given the prevalence of molecules containing nitro groups in organic synthesis, innovative methods to expand the reactivity of this functional group are of interest in both industrial and academic settings. In this report, a metal-free intramolecular benzylic sp3 C-H amination is disclosed using aryl nitro compounds as aryl nitrene precursors. Organosilicon reagent N,N'-bis(trimethylsilyl)-4,4'-bipyridinylidene (Si-DHBP) served as an efficient reductant in the transformation, enabling the in situ generation of aryl nitrene species for the direct, metal-free synthesis of unprotected 2-arylindolines from the corresponding nitroarene compounds.

Reductive Arylation of Nitroarenes with Chloroarenes: Reducing Conditions Enable New Reactivity from Palladium Catalysts

Palladium-catalyzed C-N bond forming reactions are a key tool in modern synthetic organic chemistry. Despite advances in catalyst design enabling the use of a variety of aryl (pseudo)halides, the necessary aniline coupling partner is often synthesized in a discrete reduction step from a nitroarene. An ideal synthetic sequence would avoid the necessity of this step while maintaining the reliable reactivity of palladium catalysis. Herein, we describe how reducing conditions enable new chemical steps and reactivity from well-studied palladium catalysts, resulting in a new, useful transformation: the reductive arylation of nitroarenes with chloroarenes to form diarylamines. Mechanistic experiments suggest that under reducing conditions, BrettPhos-palladium complexes catalyze the dual N-arylation of typically inert azoarenes─generated via the in situ reduction of nitroarenes─via two distinct mechanisms. Initial N-arylation proceeds via a novel association-reductive palladation sequence followed by reductive elimination to yield an intermediate 1,1,2-triarylhydrazine. Arylation of this intermediate by the same catalyst via a traditional amine arylation sequence forms a transient tetraarylhydrazine, unlocking reductive N-N bond cleavage to liberate the desired product. The resulting reaction allows for the synthesis of diarylamines bearing a variety of synthetically valuable functionalities and heteroaryl cores in high yield.

P(III)-Promoted Reductive Coupling of Aromatic and Aliphatic Nitro Compounds with Grignard Reagents

A phosphine-promoted reductive coupling of nitro compounds with Grignard reagents is described. Polyfunctional and pharmaceutically relevant diarylamines were generated by this reaction in moderate to high yields. Aliphatic nitro compounds that are highly challenging substrates undergo a combination of α-arylation and reductive coupling to afford the α-arylated arylamines efficiently. A series of valuable biaryl compounds with polyfluorinated and heteroaryl rings are co-generated in 56-94% yields.

Denitrative Hydroxylation of Unactivated Nitroarenes

A one-step method for the conversion of nitroarenes into phenols under operationally simple, transition-metal-free conditions is described. This denitrative functionalization protocol provides a concise and economical alternative to conventional three-step synthetic sequences. Experimental and computational studies suggest that nitroarenes may be substituted by an electron-catalysed radical-nucleophilic substitution (S_RN 1) chain mechanism.

Tandem C/N-Difunctionalization of Nitroarenes: Reductive Amination and Annulation by a Ring Expansion/Contraction Sequence

A synthetic method for the reductive transformation of nitroarenes into ortho-aminated and -annulated products is reported. The method operates via the exhaustive deoxygenation of nitroarenes by an organophosphorus catalyst and a mild terminal reductant to access aryl nitrenes, which after ring expansion, are trapped by amine nucleophiles to give dearomatized 2-amino-3H-azepines. Treatment of these ring-expanded intermediates with acyl electrophiles triggers 6π electrocyclization to extrude the nitrogen atom and restore aromaticity of the phenyl ring, which delivers via C-H functionalization 2-aminoanilide and benzimidazole products.

Catalytic Vilsmeier-Haack Reactions for C1-Deuterated Formylation of Indoles

The Vilsmeier-Haack reaction is a powerful tool to introduce formyl groups into electron-rich arenes, but its wide application is significantly restricted by stoichiometric employment of caustic POCl₃. Herein, we reported a catalytic version of the Vilsmeier-Haack reaction enabled by a P(III)/P(V)═O cycle. This catalytic reaction provides a facile and efficient route for the direct construction of C1-deuterated indol-3-carboxaldehyde under mild conditions with stoichiometric DMF-d7 as the deuterium source. The products feature a remarkably higher deuteration level (>99%) than previously reported ones and are not contaminated by the likely unselective deuteration at other sites. The present transformation can also be used to transfer other carbonyl groups. Further downstream derivatizations of these deuterated products manifested their potential applications in the synthesis of deuterated bioactive molecules. Mechanistic insight was disclosed from studies of kinetics and intermediates.

Synthesis of trans-stilbenes via phosphine-catalyzed coupling reactions of benzylic halides

An efficient and practical phosphine-catalyzed homo-coupling reaction of benzyl chlorides is described. The reactions proceed smoothly in the presence of CsF/B(OMe)₃ and NaH as the base, respectively, to provide trans-stilbenes in good yields with a broad scope. Unsymmetrical stilbenes are also generated from the reactions of benzyl chlorides with phosphonium salts. Several P-based key intermediates have been detected by NMR and HRMS analyses, which shed light on the postulated catalytic cycle. In the presence of different bases, the transformations involve two different pathways, in which phenylcarbene and phosphonium alkoxide are considered as key intermediates, respectively. The two pathways are complementary in synthesis but different in mechanisms. The synthetic utility, including gram-scale reactions and straightforward access to π-conjugated molecules, has been demonstrated as well.

Copper-Catalyzed Stereospecific Transformations of Alkylboronic Esters

Copper-catalyzed stereospecific cross-couplings of boronic esters are reported. Boron "ate" complexes derived from pinacol boronic esters and tert-butyl lithium undergo stereospecific transmetalation to copper cyanide, followed by coupling with alkynyl bromides, allyl halides, propargylic halides, β-haloenones, hydroxylamine esters, and acyl chlorides. Through this simple transformation, commercially available inexpensive compounds can be employed to convert primary and secondary alkylboronic esters to a wide array of synthetically useful compounds.

Photoinduced Transition-Metal-Free Chan-Evans-Lam-Type Coupling: Dual Photoexcitation Mode with Halide Anion Effect

Herein, we report a photoinduced transition-metal-free C(aryl)-N bond formation between 2,4,6-tri(aryl)boroxines or arylboronic acids as an aryl source and 1,4,2-dioxazol-5-ones (dioxazolones) as an amide coupling partner. Chloride anion, either generated in situ by photodissociation of chlorinated solvent molecules or added separately as an additive, was found to play a critical cooperative role, thereby giving convenient access to a wide range of synthetically versatile N-arylamides under mild photo conditions. The synthetic virtue of this transition-metal-free Chan-Evans-Lam-type coupling was demonstrated by large-scale reactions, synthesis of 15N-labeled arylamides, and applicability toward biologically relevant compounds. On the basis of mechanistic investigations, two distinctive photoexcitations are proposed to function in the current process, in which the first excitation involving chloro-boron adduct facilitates the transition-metal-free activation of dioxazolones by single electron transfer (SET), and the second one enables the otherwise-inoperative 1,2-aryl migration of the thus-formed N-chloroamido-borate adduct.

Chemoselective Primary Amination of Aryl Boronic Acids by PIII/PV═O-Catalysis: Synthetic Capture of the Transient Nef Intermediate HNO

A catalytic approach to intercept the transient HNO for a chemoselective primary amination of arylboronic acids is reported. A phosphetane-based catalyst operating within P^III/P^V═O redox cycling is shown to capture HNO, generated in situ by Nef decomposition of 2-nitropropane, to selectively install the primary amino group at aryl C_sp2 centers. The method furnishes versatile primary arylamines from arylboronic acid substrates with the preservation of otherwise reactive functional groups.

Decarboxylative tandem C-N coupling with nitroarenes via SH2 mechanism

Aromatic tertiary amines are one of the most important classes of organic compounds in organic chemistry and drug discovery. It is difficult to efficiently construct tertiary amines from primary amines via classical nucleophilic substitution due to consecutive overalkylation. In this paper, we have developed a radical tandem C-N coupling strategy to efficiently construct aromatic tertiary amines from commercially available carboxylic acids and nitroarenes. A variety of aromatic tertiary amines can be furnished in good yields (up to 98%) with excellent functional group compatibility under mild reaction conditions. The use of two different carboxylic acids also allows for the concise synthesis of nonsymmetric aromatic tertiary amines in satisfactory yields. Mechanistic studies suggest the intermediacy of the arylamine–(TPP)Fe(III) species and might provide a possible evidence for an S_H2 (bimolecular homolytic substitution) pathway in the critical C-N bond formation step.

Aromatic tertiary amines are versatile building blocks in organic synthesis. In this article, the authors report on an iron-catalysed reaction for the decarboxylative C-N coupling from carboxylic acids and nitroarenes, leading to non-symmetric tertiary aromatic amines.

Enabling Reductive C-N Cross-Coupling of Nitroalkanes and Boronic Acids by Steric Design of P(III)/P(V)═O Catalysts

An organophosphorus-catalyzed C-N bond-forming reductive coupling of nitroalkanes with arylboronic acids and esters is reported. The method shows excellent chemoselectivity for the nitro/boronic acid substrate pair, allowing the synthesis of N-(hetero)arylamines rich in functionalization. The identification of a sterically reduced phosphetane catalyst capable of productive coupling in the P(III)/P(V)═O redox manifold is the key enabling development. Combined experimental kinetics and computational mechanistic studies show that the sterically reduced catalyst affects post-rate-limiting steps to enable the C-N coupling event in preference to deleterious side-paths.

Light-Promoted Low-Valent-Tungsten-Catalyzed Ambient Temperature Amination of Boronic Acids with Nitroaromatics

Triggering C-N bond formation with nitroaromatics and boronic acids at mild conditions is highly desirable, since most prior works were carried out under harsh conditions and sometimes suffered from poor chemo- or regioselectivity. Herein, a low-valent-tungsten-catalyzed reaction that enables the ambient temperature amination of boronic acids with nitroaromatics is disclosed. With readily available W(CO)₆ as a precatalyst under external-photosensitizer-free conditions, nitroaromatics smoothly undergo C-N coupling reactions with their boronic acid partners, delivering structurally diverse secondary amines in good yields (>50 examples, yields up to 96%). This methodology is both scalable and highly chemoselective and engages both aliphatic and aromatic boronic acid partners. The catalysis is initiated by the deoxygenation of nitroaromatics by a trans-[W(CO)₄(PPh₃)₂] (trans-W, PPh₃ = triphenylphosphine) complex, which forms in situ via ligand replacement.

Mechanistic Details of Asymmetric Bromocyclization with BINAP Monoxide: Identification of Chiral Proton-Bridged Bisphosphine Oxide Complex and Its Application to Parallel Kinetic Resolution

The mechanism of our previously reported catalytic asymmetric bromocyclization reactions using 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (BINAP) monoxide was examined in detail by the means of control experiments, NMR studies, X-ray structure analysis, and CryoSpray electrospray ionization mass spectrometry (ESI-MS) analysis. The chiral BINAP monoxide was transformed to a key catalyst precursor, proton-bridged bisphosphine oxide complex (POHOP·Br), in the presence of N-bromosuccinimide (NBS) and contaminating water. The thus-formed POHOP further reacts with NBS to afford BINAP dioxide and molecular bromine (Br₂) simultaneously in equimolar amounts. While the resulting Br₂ is activated by NBS to form a more reactive brominating reagent (Br₂─NBS), BINAP dioxide serves as a bifunctional catalyst, acting as both a Lewis base that reacts with Br₂─NBS to form a chiral brominating agent (P═O⁺─Br) and also as a Brønsted base for the activation of the substrate. By taking advantage of this novel concerted Lewis/Brønsted base catalysis by BINAP dioxide, we achieved the first regio- and chemodivergent parallel kinetic resolutions (PKRs) of racemic unsymmetrical bisallylic amides via bromocyclization.

Copper-promoted cross-coupling of nitroarenes with 4-alkyl-1,4-dihydropyridines using a peroxide-driven radical reductive strategy

Direct radical-mediated reductive coupling of nitroarenes with 4-alkyl-1,4-dihydropyridines to build the C(sp3)–N bond using 4-alkyl-1,4-dihydropyridines as internal reducing agents and alkyl sources is presented.

Manganese-mediated reductive N,N-dialkylation of nitroarenes: a dramatic NiI2 effect

A dramatic NiI2 effect has been found for Mn-mediated reductive N,N-dialkylation of nitroaromatics.

Computational mechanism investigation of bismuth (BiIII/BiV) redox-catalyzed fluorination of arylboronic esters

DFT calculations reveal details of the redox catalytic mechanism of a heavier main group element, bismuth.

Computational mechanism investigation of Bi(i)/Bi(iii) redox-catalyzed hydrodefluorination (HDF) of polyfluoroarenes

DFT calculations reveal details of the redox catalytic mechanism of non-transition-metal bismuth.

Electrochemical Reductive Arylation of Nitroarenes with Arylboronic Acids

The synthesis of diarylamine is extremely important in organic chemistry. Herein, a novel electrochemical reductive arylation of nitroarenes with arylboronic acids was developed. A variety of diarylamines were synthesized without the need for transition-metal catalysts. The reaction could be scaled up efficiently in a flow cell and several derivatization reactions were carried out smoothly. Cyclic voltammetry experiments and mechanism studies showed that acetonitrile, formic acid, and triethyl phosphite all played a role in promoting this reductive arylation transformation.

SET activation of nitroarenes by 2-azaallyl anions as a straightforward access to 2,5-dihydro-1,2,4-oxadiazoles

The use of nitroarenes as amino sources in synthesis is challenging. Herein is reported an unusual, straightforward, and transition metal-free method for the net [3 + 2]-cycloaddition reaction of 2-azaallyl anions with nitroarenes. The products of this reaction are diverse 2,5-dihydro-1,2,4-oxadiazoles (>40 examples, up to 95% yield). This method does not require an external reductant to reduce nitroarenes, nor does it employ nitrosoarenes, which are often used in N-O cycloadditions. Instead, it is proposed that the 2-azaallyl anions, which behave as super electron donors (SEDs), deliver an electron to the nitroarene to generate a nitroarene radical anion. A downstream 2-azaallyl radical coupling with a newly formed nitrosoarene is followed by ring closure to afford the observed products. This proposed reaction pathway is supported by computational studies and experimental evidence. Overall, this method uses readily available materials, is green, and exhibits a broad scope.

Visible-Light- and PPh3-Mediated Direct C-N Coupling of Nitroarenes and Boronic Acids at Ambient Temperature

We present here a metal-free, visible-light- and triphenylphosphine-mediated intermolecular, reductive amination between nitroarenes and boronic acids at ambient temperature without any photocatalyst. Mechanistically, a slow reduction of nitroarenes to a nitroso and, finally, a nitrene intermediate occurs that leads to the amination product with concomitant 1,2-aryl/-alkyl migration from a boronate complex. A wide range of nitroarenes underwent C-N coupling with aryl-/alkylboronic acids providing high yields.

Saturated Boronic Acids, Boronates, and Trifluoroborates: An Update on Their Synthetic and Medicinal Chemistry

This review discusses recent advances in the chemistry of saturated boronic acids, boronates, and trifluoroborates. Applications of the title compounds in the design of boron-containing drugs are surveyed, with special emphasis on α-amino boronic derivatives. A general overview of saturated boronic compounds as modern tools to construct C(sp³ )-C and C(sp³ )-heteroatom bonds is given, including recent developments in the Suzuki-Miyaura and Chan-Lam cross-couplings, single-electron-transfer processes including metallo- and organocatalytic photoredox reactions, and transformations of boron "ate" complexes. Finally, an attempt to summarize the current state of the art in the synthesis of saturated boronic acids, boronates, and trifluoroborates is made, with a brief mention of the "classical" methods (transmetallation of organolithium/magnesium reagents with boron species, anti-Markovnikov hydroboration of alkenes, and the modification of alkenyl boron compounds) and a special focus on recent methodologies (boronation of alkyl (pseudo)halides, derivatives of carboxylic acids, alcohols, and primary amines, boronative C-H activation, novel approaches to alkene hydroboration, and 1,2-metallate-type rearrangements).

Cu-Catalyzed Cross-Coupling of Nitroarenes with Aryl Boronic Acids to Construct Diarylamines

The development and study of a simple copper-catalyzed reaction of nitroarenes with aryl boronic acids to form diarylamines that uses phenyl silane as the stoichiometric terminal reductant is described. This cross-coupling reaction requires as little as 2 mol % of CuX and 4 mol% of diphosphine for success and tolerates a broad range of functional groups on either the nitroarene or the aryl boronic acid with to afford the amine in good yield. Mechanistic investigations established that the cross-coupling reaction proceeds via a nitrosoarene intermediate and that copper is required to catalyze both the deoxygenation of the nitroarene to afford the nitrosoarene and C-NAr bond formation of the nitrosoarene with the aryl boronic acid.

Nickel-catalyzed remote hydrosilylation of unconjugated enones with bulky triphenylsilane

Herein we describe a nickel-catalyzed remote hydrosilylation of unconjugated enones with bulky triphenylsilane. A range of Z-silyl enol ethers are obtained as major isomers due to the process of nickel triggered alkene isomerization. Notably, some specific alkyl silyl enol ethers can be prepared from this protocol, which are not easily accessed by the traditional strategy using a strong base and chlorosilane. This reaction features 100% atom economy, simple reaction conditions, and good yields.

Uniting Amide Synthesis and Activation by PIII/PV-Catalyzed Serial Condensation: Three-Component Assembly of 2-Amidopyridines

An organophosphorus (P^III/P^V redox) catalyzed method for the three-component condensation of amines, carboxylic acids, and pyridine N-oxides to generate 2-amidopyridines via serial dehydration is reported. Whereas amide synthesis and functionalization usually occur under divergent reaction conditions, here a phosphetane catalyst (together with a mild bromenium oxidant and terminal hydrosilane reductant) is shown to drive both steps chemoselectively in an auto-tandem catalytic cascade. The ability to both prepare and functionalize amides under the action of a single organocatalytic reactive intermediate enables new possibilities for the efficient and modular preparation of medicinal targets.

PIII/PV═O-Catalyzed Intermolecular N-N Bond Formation: Cross-Selective Reductive Coupling of Nitroarenes and Anilines

An organophosphorus-catalyzed method for the synthesis of unsymmetrical hydrazines by cross-selective intermolecular N-N reductive coupling is reported. This method employs a small ring phosphacycle (phosphetane) catalyst together with hydrosilane as the terminal reductant to drive reductive coupling of nitroarenes and anilines with good chemoselectivity and functional group tolerance. Mechanistic investigations support an autotandem catalytic reaction cascade in which the organophosphorus catalyst drives two sequential and mechanistically distinct reduction events via P^III/P^V═O cycling in order to furnish the target N-N bond.