Intramolecular Reductive Amidation of Unactivated Esters with Nitroarenes: A Telescoped Synthesis of Tetrahydropyrrolo/Pyrido[1,2-a]quinoxalinones

A reductive amidation of unactivated esters with nitroarenes, a key step in the telescopic synthesis of tetrahydropyrrolo[1,2-a]quinoxalinones and tetrahydropyrido[1,2-a]quinoxalinones, is reported. The process involves an intermolecular base-mediated SNAr reaction, followed by intramolecular reductive amidation employing sodium dithionite. The substrate scope coupled with the demonstration of the synthesis of pharmaceuticals is reported. The key features include nitro reduction at room temperature, easy purification without chromatography, amidation of unactivated esters without any externally added activating agent, and a telescopic process.

Synthetic and Mechanistic Studies into the Reductive Functionalization of Nitro Compounds Catalyzed by an Iron(salen) Complex

We report on the use of a simple, bench-stable [Fe(salen)2]-μ-oxo precatalyst in the reduction of nitro compounds. The reaction proceeds at room temperature across a range of substrates, including nitro aromatics and aliphatics. By changing the reducing agent from pinacol borane (HBpin) to phenyl silane (H3SiPh), we can chemoselectively reduce nitro compounds while retaining carbonyl functionality. Our mechanistic studies, which include kinetics, electron paramagnetic resonance (EPR), mass spectrometry, and quantum chemistry, indicate the presence of a nitroso intermediate and the generation of an on-cycle iron hydride as a key catalytic intermediate. Based on this mechanistic insight, we were able to extend the chemistry to hydroamination and identified a simple substrate feature (alkene lowest unoccupied molecular orbital (LUMO) energy) that could be used to predict which alkenes would result in productive catalysis.

Modular synthesis of pyrrole-fused heterocycles via glucose-mediated nitro-reductive cyclization

A novel biomass-derived glucose-mediated one-pot multicomponent nitro-reductive cyclization method is presented for the direct synthesis of diverse pyrrole-fused heterocycles. The process involves two-component reactions of alkyl (NH)-pyrrole-2-carboxylates and 2-fluoronitroarenes, yielding pyrrolo[1,2-a]quinoxalin-4(5H)-ones, as well as three-component reactions utilizing (NH)-pyrroles, nitroarenes, and DMSO as carbon sources, resulting in various pyrrolo[1,2-a]quinoxaline derivatives. High yields were achieved with broad substrate scope and gram-scale synthesis capability, including pharmaceuticals featuring pyrroloquinoxaline scaffolds. The method's key innovation lies in enabling three or four reactions in a single-pot setup, previously unexplored in pyrrole chemistry. The simplicity of nitro group reduction by biomass-derived glucose ensures practical safety during scale-up, while mechanistic insights from control experiments reveal a new paradigm in pyrrole chemistry. The tandem process demonstrates low PMI values and high step and atom economies, aligning well with green chemistry principles.

Visible light-induced reductive aza-6π electrocyclization access to phenanthridines

Visible light-induced aza-6π electrocyclization was developed for the synthesis of aza-arenes from nitroarenes with diverse aldehydes. This protocol allows the reduction of nitroarenes by B2nep2 and subsequent 6π-electrocyclization of the in situ formed imine under visible light. An array of 6- and multi-substituted phenanthridines were constructed in moderate to good yields under purple LEDs at room temperature. A wide scope of substrates with diverse functional groups were well tolerated. In addition, the synthetic utility of this methodology was further demonstrated in the late-stage functionalization of celecoxib.

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.

Ru-Mg promoted reductive cross-coupling of allyl bromides and alkenes to synthesize 1,7-octadienes with an all-carbon quaternary center

A Ru-promoted reductive cross-coupling of allyl bromides and electron-deficient alkenes to provide terminal 1,7-octadienes with magnesium as a reductant is reported herein. This approach enables the facile construction of a series of complex terminal 1,7-octadienes with an all-carbon quaternary center under mild reaction conditions, and the synthetic utility of the current method has been demonstrated by a gram scale synthesis. Preliminary mechanism investigations suggested that a radical pathway might not be involved in this transformation.

Design of a gold nanoparticles site in an engineered lipase: an artificial metalloenzyme with enantioselective reductase-like activity

The conjugation of gold complexes with proteins has proved to be interesting and effective in obtaining artificial metalloenzymes as catalysts with improved properties such as higher stability, activity and selectivity. However, the design and precise regulation of their structure as protein nanostructured forms level remains a challenge. Here, we have designed and constructed a gold nanoparticles-enzyme bioconjugate, by tailoring the in situ formation of gold nanoparticles (AuNPs) at two specific sites on the structure of an alkalophilic lipase from Geobacillus thermocatenulatus (GTL). For this purpose, two genetically modified variants of GTL were created by inserting a unique cysteine residue into the catalytic active site by replacing the active serine (GTL-114) and into the lid site (GTL-193). The enzyme, after a first protein-gold coordination, induced the in situ formation of AuNPs, generating a homogeneous artificial enzyme. The size and morphology of the nanoparticles in the AuNPs-enzyme conjugate have been controlled by specific pH conditions in synthesis and the specific protein region where they are formed. Reductase activity of all of them was confirmed in the hydrogenation of nitroarenes in aqueous media. The protein area seemed to be key for the AuNPs, with the best TOF values obtained for the bioconjugates with AuNPs in the lid site. Finally, the protein environment and the asymmetric properties of the AuNPs were tested in the reduction of acetophenone to 1-phenylethanol in aqueous medium at room temperature. A high reductive conversion and an enantiomeric excess of up to 39% towards (R)-1-phenylethanol was found using Au-Mt@GTL-114 pH 10 as a catalyst. Moderate enantioselectivity towards the opposite isomer was also observed using the Au-Mt@GTL-193 pH 10 conjugate.

Iron-Mediated Reductive Amidation of Triazine Esters with Nitroarenes

A reductive amidation of triazine esters with nitroarenes by using cheap iron as a reducing metal in the presence of TMSCl in DMF was developed. The reactions proceeded efficiently under transition metal-free conditions to give the corresponding amides in moderate to good yields with good functional group compatibility. Preliminary mechanistic investigations indicated that nitrosobenzene, N-phenyl hydroxylamine, azoxybenzene, azobenzene, aniline, and N-arylformamide possibly served as the intermediates of the reaction.

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.

Molybdenum-Mediated Reductive Hydroamination of Vinylcyclopropanes with Nitroarenes: Synthesis of Homoallylamines

A molybdenum-mediated reductive hydroamination of vinylcyclopropanes with nitroarenes has been developed. A broad range of substituted homoallylamines were prepared in good to excellent yields from readily available starting materials. No noble metal catalysts were used in this reaction, and Mo(CO)₆ acted as both catalyst and reductant. This protocol provides an effective method for the selective synthesis of substituted homoallylamines from easily available nitroarenes.

Magnetic boron nitride adorned with Pd nanoparticles: an efficient catalyst for the reduction of nitroarenes in aqueous media

Hexagonal boron nitride (h-BN) is an excellent support material for nanocatalysts due to its two-dimensional (2D) architectural morphology and physicochemical stability. In this study, a chemically stable, recoverable, eco-friendly, and magnetic h-BN/Pd/Fe₂O₃ catalyst was prepared by a one-step calcination process, in which Pd and Fe₂O₃ nanoparticles (NPs) were uniformly decorated on the surface of h-BN via a typical adsorption-reduction procedure. In detail, nanosized magnetic (Pd/Fe₂O₃) NPs were derived from a Prussian blue analogue prototype, a well-known porous metal-organic framework, and then further surface-engineered to produce magnetic BN nanoplate-supported Pd nanocatalysts. The structural and morphological features of h-BN/Pd/Fe₂O₃ were investigated by spectroscopic and microscopic characterization techniques. Moreover, the h-BN nanosheets endow it with stability and appropriate chemical anchoring sites which solve the problems of inefficient reaction rate and high consumption caused by the inevitable agglomeration of precious metal NPs. Under mild reaction conditions, the developed nanostructured h-BN/Pd/Fe₂O₃ as the catalyst shows high yield and efficient reusability in reducing nitroarenes into the corresponding anilines using sodium borohydride (NaBH₄) as a reductant.

Nitroarenes and nitroalkenes as potential amino sources for the synthesis of N-heterocycles

Nitro-compounds are one of the cheapest and most readily available materials in the chemical industry and are commonly utilized as versatile building blocks. Previously, the synthesis of N-heterocycles was largely based on anilines. The utilization of nitroarenes and nitroalkenes for the synthesis of N-heterocyclic compounds can save at least one step, however, as compared to anilines. Thus, considerable attention has been paid to nitroarenes and nitroalkenes as new potential amino sources. Significant progress has been made in the reductive cyclization of nitroarenes or nitroalkenes to access various N-heterocycles in recent years. Herein, we comprehensively summarize the recent progress in the construction of N-heterocycles using nitroarenes and nitroalkenes as potential amino sources. The compatibility of the reaction substrate, its mechanism, applications, advantages, and limitations in this field are also discussed in detail.

Recent advances on catalysts for photocatalytic selective hydrogenation of nitrobenzene to aniline

Selective hydrogenation of nitrobenzene (SHN) is an important approach to synthesize aniline, an essential intermediate with extremely high research significance and value in the fields of textiles, pharmaceuticals and dyes. SHN reaction requires high temperature and high hydrogen pressure via the conventional thermal-driven catalytic process. On the contrary, photocatalysis provides an avenue to achieve high nitrobenzene conversion and high selectivity towards aniline at room temperature and low hydrogen pressure, which is in line with the sustainable development strategies. Designing efficient photocatalysts is a crucial step in SHN. Up to now, several photocatalysts have been explored for photocatalytic SHN, such as TiO2, CdS, Cu/graphene and Eosin Y. In this review, we divide the photocatalysts into three categories based on the characteristics of the light harvesting units, including semiconductors, plasmonic metal-based catalysts and dyes. The recent progress of the three categories of photocatalysts is summarized, the challenges and opportunities are pointed out and the future development prospects are described. It aims to give a clear picture to the catalysis community and stimulate more efforts in this research area.

Palladium-Catalyzed Reductive Double Carbonylation of Nitroarenes with Aryl Halides Using Mo(CO)6 as a Reductant and Carbonyl Source

A new palladium-catalyzed reductive double carbonylation of nitroarenes with aryl halides for the synthesis of benzoxazin-4-ones has been reported. The key to success was the use of Mo(CO)₆ as a reductant and bench-stable solid carbonyl sources. Various aryl iodides, bromides, and trifluoromethanesulfonates are suitable reaction partners and produce corresponding benzoxazin-4-one derivatives in moderate to good yields. Preliminary mechanistic studies indicate that nitrosoarene was first generated as the key intermediate through nitro reduction. Remarkably, this method avoids the use of toxic CO gas and is further applied to the late-stage modification of estrone.

Dearomative oxyphosphorylation of indoles enables facile access to 2,2-disubstituted indolin-3-ones

A highly efficient oxidative dearomatization of indoles with H-phosphorus oxides in the presence of TEMPO oxoammonium salt has been demonstrated. Through the intramolecular oxidative dearomatization of indoles and subsequent intermolecular nucleophilic addition with phosphorus nucleophile, a variety of structurally diverse arylphosphoryl and alkylphosphoryl indolin-3-ones were obtained in good yields with a broad substrate scope and high functional-group compatibility.

Palladium-Catalyzed Intramolecular Heck/Aminocarbonylation of Alkene-Tethered Iodobenzenes with Nitro Compounds: Synthesis of Carbamoyl-Substituted Benzoheterocycles

A palladium-catalyzed intramolecular Heck/aminocarbonylation of alkene-tethered iodobenzenes with nitro compounds has been developed for the synthesis of carbamoyl-substituted benzoheterocycles. Using Mo(CO)₆ as a solid CO source, no external reductant or additives were needed in this procedure. Both nitroarenes and nitroalkanes were well tolerated. A range of carbamoyl-substituted dihydrobenzofurans and indolines were prepared in moderate to high yields.

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.

Ni-Catalysed intramolecular reductive aminocarbonylation of 2-haloaryl-tethered nitroarenes for the synthesis of dibenzazepine-based heterocycles

A nickel-catalysed intramolecular reductive aminocarbonylation of 2-haloaryl-tethered nitroarenes is developed for rapid access to a variety of dibenzoazepinones and their derivatives.

Fe-catalyzed denitrative cyanoalkylation of nitroalkenes with cycloketone oxime esters via reductive C–C bond formation

An iron catalyzed reductive denitrative cyanoalkylation of nitroalkenes with cycloketone oxime esters using Zn as the reductant has been successfully established in which the NO2 of nitroalkenes eventually acts as a leaving group.

Nickel-Catalyzed Arylcarbamoylation of Alkenes of N-(o-Iodoaryl)acrylamides with Nitroarenes via Reductive Aminocarbonylation: Facile Synthesis of Carbamoyl-Substituted Oxindoles

Nickel-catalyzed arylcarbamoylation reactions of alkenes of N-(o-haloaryl)acrylamides with CO and nitroarenes via reductive aminocarbonylation to produce carbamoyl-substituted oxindoles with an all-carbon quaternary stereogenic center are presented. Starting with N-(o-haloaryl)acrylamides, simple CO, and inexpensive nitroarenes and using a Ni catalyst, a dinitrogen-based ligand, a Zn reductant, a TMSCl additive, and a base system, this protocol enables the synthesis of various carbamoyl-substituted oxindoles and allows the efficient late-stage derivatization of valuable molecules.

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.

3-Nitro-coumarin synthesis via nitrative cyclization of aryl alkynoates using tert-butyl nitrite

We report a one-pot metal-free synthesis of 3-nitro-coumarins from aryl alkynoates using TBN (tert-butyl nitrite) as the sole reagent. The radical triggered cyclization and nitration of the aryl alkynoates proceeded in a cascaded manner via nitro radical addition to alkynoates, then 5-exo-trig spirocyclization and ester migration.

Site-specific Umpolung amidation of carboxylic acids via triplet synergistic catalysis

Development of catalytic amide bond-forming methods is important because they could potentially address the existing limitations of classical methods using superstoichiometric activating reagents. In this paper, we disclose an Umpolung amidation reaction of carboxylic acids with nitroarenes and nitroalkanes enabled by the triplet synergistic catalysis of FeI₂, P(V)/P(III) and photoredox catalysis, which avoids the production of byproducts from stoichiometric coupling reagents. A wide range of carboxylic acids, including aliphatic, aromatic and alkenyl acids participate smoothly in such reactions, generating structurally diverse amides in good yields (86 examples, up to 97% yield). This Umpolung amidation strategy opens a method to address challenging regioselectivity issues between nucleophilic functional groups, and complements the functional group compatibility of the classical amidation protocols. The synthetic robustness of the reaction is demonstrated by late-stage modification of complex molecules and gram-scale applications.

Catalytic amide bond-forming methods is important because they could potentially address the existing limitations of classical methods using superstoichiometric activating reagents. Here the authors show an Umpolung amidation reaction of carboxylic acids with nitroarenes and nitroalkanes enabled by FeI₂, P(V)/P(III) and photoredox catalysis that avoids the production of byproducts.

NaI/PPh3-Mediated Photochemical Reduction and Amination of Nitroarenes

A mild transition-metal- and photosensitizer-free photoredox system based on the combination of NaI and PPh₃ was found to enable highly selective reduction of nitroarenes. This protocol tolerates a broad range of reducible functional groups such as halogen (Cl, Br, and even I), aldehyde, ketone, carboxyl, and cyano. Moreover, the photoredox catalysis with NaI and stoichiometric PPh₃ provides also an alternative entry to Cadogan-type reductive amination when o-nitrobiarenes were used.

Rhodium-terpyridine Catalyzed Transfer Hydrogenation of Aromatic Nitro Compounds in Water

A rhodium terpyridine complex catalyzed transfer hydrogenation of nitroarenes to anilines with i-PrOH as hydrogen source and water as solvent has been developed. The catalytic system can work at a substrate/catalyst (S/C) ratio of 2000, with a turnover frequency (TOF) up to 3360 h^-1 , which represents one of the most active catalytic transfer hydrogenation systems for nitroarene reduction. The catalytic system is operationally simple and the protocol could be scaled up to 20 gram scale. The water-soluble catalyst bearing a carboxyl group could be recycled 15 times without significant loss of activity.