Organic radical reactions associated with nitrogen monoxide

Catalytic NO Reduction by NO Pre-Adsorbed RhCeO2 NO- Clusters

A fundamental understanding on the dynamically structural evolution of catalysts induced by reactant gases under working conditions is challenging but pivotal in catalyst design. Herein, in combination with state-of-the-art mass spectrometry for cluster reactions, cryogenic photoelectron imaging spectroscopy, and quantum-chemical calculations, we identified that NO adsorption on rhodium-cerium bimetallic oxide cluster RhCeO₂ ^- can create a Ce³⁺ ion in product RhCeO₂ NO^- that serves as the starting point to trigger the catalysis of NO reduction by CO. Theoretical calculations substantiated that the reduction of another two NO molecules into N₂ O takes place exclusively on the Ce³⁺ ion while Rh behaves like a promoter to buffer electrons and cooperates with Ce³⁺ to drive NO reduction. Our finding demonstrates the importance of NO in regulating the catalytic behavior of Rh under reaction conditions and provides much-needed insights into the essence of NO reduction over Rh/CeO₂ , one of the most efficient components in three-way catalysts for NO_x removal.

Blatter Radical-Decorated Silica as a Prospective Adsorbent for Selective NO Capture from Air

Nitrogen oxides are adverse poisonous gases present in the atmosphere and having detrimental effects on the human health and environment. In this work, we propose a new type of mesoporous materials capable of capturing nitrogen monoxide (NO) from air. The designed material combines the robust Santa Barbara Amorphous-15 silica scaffold and ultrastable Blatter-type radicals acting as NO traps. Using in situ electron paramagnetic resonance spectroscopy, we demonstrate that NO capture from air is selective and reversible at practical conditions, thus making Blatter radical-decorated silica highly promising for environmental applications.

Carbodicarbenes and Striking Redox Transitions of their Conjugate Acids: Influence of NHC versus CAAC as Donor Substituents

Herein, a new type of carbodicarbene (CDC) comprising two different classes of carbenes is reported; NHC and CAAC as donor substituents and compare the molecular structure and coordination to Au(I)Cl to those of NHC-only and CAAC-only analogues. The conjugate acids of these three CDCs exhibit notable redox properties. Their reactions with [NO][SbF₆ ] were investigated. The reduction of the conjugate acid of CAAC-only based CDC with KC₈ results in the formation of hydrogen abstracted/eliminated products, which proceed through a neutral radical intermediate, detected by EPR spectroscopy. In contrast, the reduction of conjugate acids of NHC-only and NHC/CAAC based CDCs led to intermolecular reductive (reversible) carbon-carbon sigma bond formation. The resulting relatively elongated carbon-carbon sigma bonds were found to be readily oxidized. They were, thus, demonstrated to be potent reducing agents, underlining their potential utility as organic electron donors and n-dopants in organic semiconductor molecules.

Modeling and insights into the structural characteristics of drug-induced autoimmune diseases

The incidence and complexity of drug-induced autoimmune diseases (DIAD) have been on the rise in recent years, which may lead to serious or fatal consequences. Besides, many environmental and industrial chemicals can also cause DIAD. However, there are few effective approaches to estimate the DIAD potential of drugs and other chemicals currently, and the structural characteristics and mechanism of action of DIAD compounds have not been clarified. In this study, we developed the in silico models for chemical DIAD prediction and investigated the structural characteristics of DIAD chemicals based on the reliable drug data on human autoimmune diseases. We collected 148 medications which were reported can cause DIAD clinically and 450 medications that clearly do not cause DIAD. Several different machine learning algorithms and molecular fingerprints were combined to develop the in silico models. The best performed model provided the good overall accuracy on validation set with 76.26%. The model was made freely available on the website http://diad.sapredictor.cn/. To further investigate the differences in structural characteristics between DIAD chemicals and non-DIAD chemicals, several key physicochemical properties were analyzed. The results showed that AlogP, molecular polar surface area (MPSA), and the number of hydrogen bond donors (nHDon) were significantly different between the DIAD and non-DIAD structures. They may be related to the DIAD toxicity of chemicals. In addition, 14 structural alerts (SA) for DIAD toxicity were detected from predefined substructures. The SAs may be helpful to explain the mechanism of action of drug induced autoimmune disease, and can used to identify the chemicals with potential DIAD toxicity. The structural alerts have been integrated in a structural alert-based web server SApredictor (http://www.sapredictor.cn). We hope the results could provide useful information for the recognition of DIAD chemicals and the insights of structural characteristics for chemical DIAD toxicity.

On the reactions of Cu(II/I)ATP complexes with methyl radicals

The Cu^I/IIATP react with methyl radicals to form methane and methanol, where Cu^IATP reacts with ^•CH₃ in a process that is surprisingly slow. The low-rate constant of this process is attributed to the significant rearrangement of the chelating ligand required for the transient's formation. These results were corroborated by DFT calculations of the relevant compounds.

Water Significantly Changes the Ring-Cleavage Process During Aqueous Photooxidation of Toluene

As a major constituent of aromatic compounds, toluene exists widely in environmental aqueous phases. This research investigated the aqueous-phase OH oxidation of toluene to determine how liquid water changes the radical chemistry of ring-cleavage pathways. Results show that ring-cleavage pathways through the C₇ bicyclic peroxy radical (BPR) account for about 70% of total aqueous-phase oxidation pathways, which is similar to that in the gas-phase oxidation. However, detailed ring-cleavage pathways in the aqueous phase change significantly compared with those in the gas phase as shown by the decreased production of glyoxal and methylglyoxal and the enhanced production of formic acid and acetic acid as primary products, which can be attributed to the presence of liquid water. Water facilitates the formation of the BPR whose structure is different from that in the gas-phase oxidation and results in different ring-cleavage pathways through hydrogen-shift reactions. Furthermore, water helps the hydration of acyl radicals formed by the BPR to produce organic acids. With the suggested ring-cleavage mechanisms, a box model can simulate aqueous-phase product distributions better than that with the classical ring-cleavage mechanisms. Given the influence of water on reaction mechanisms, aqueous-phase oxidation of hydrophobic organic compounds may be more important than previously assumed.

Rhenium-Mediated Conversion of Dinitrogen and Nitric Oxide to Nitrous Oxide

Reductive splitting of N₂ is an attractive strategy towards nitrogen fixation beyond ammonia at ambient conditions. However, the resulting nitride complexes often suffer from thermodynamic overstabilization hampering functionalization. Furthermore, oxidative nitrogen atom transfer of N₂ derived nitrides remains unknown. We here report a Re^IV pincer platform that mediates N₂ splitting upon chemical reduction or electrolysis with unprecedented yield. The N₂ derived Re^V nitrides undergo facile nitrogen atom transfer to nitric oxide, giving nitrous oxide nearly quantitatively. Experimental and computational results indicate that outer‐sphere ReN/NO radical coupling is facilitated by the activation of the nitride via initial coordination of NO.

Cleavage of Organosolv Lignin to Phenols Using Nitrogen Monoxide and Hydrazine

From the variety of methods known for the depolymerization of organosolv lignin, a broad range of diversely substituted aromatic compounds are available today. In the present work, a novel two-step reaction sequence is reported, which is focused on the formation of phenols. While the first step of the depolymerization strategy comprises the 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)-catalyzed oxidation of organosolv lignin with nitrogen monoxide so that two waste materials are combined, cleavage to the phenolic target compounds is achieved in the second step employing hydrazine and potassium hydroxide under Wolff-Kishner-type conditions. Besides the fact that the novel strategy proceeds via an untypical form of oxidized organosolv lignin, the two-step sequence is further able to provide phenols as cleavage products, which bear no substituent at the 4-position.

Photo-Mediated Intermolecular Coupling of Alkenes with Ketones via Acyloxy Nitroso Compounds

An atom-economic intermolecular radical addition reaction of acyloxy nitroso compounds to electron-deficient alkenes mediated by visible light is reported. The starting nitroso derivatives are readily prepared by oxidation of the corresponding oximes prepared from ketones and the overall transformation represents an oxidative coupling of a ketone with a Michael acceptor. The cascade proceeds smoothly under mild conditions, providing a series of valuable functionalized oximes in moderate to good yields. Mechanistic studies suggest that these cascades proceed via addition/coupling processes that are controlled by the persistent radical effect (PRE) with NO acting as the persistent species.

Controllable Z/E-selective synthesis of α-amino-ketoximes from N-nitrososulfonamides and aryl alkenes under neutral conditions

An amidoximation of alkenes with N-nitrososulfonamides enabled by triplet energy transfer under neutral conditions is presented. Both (Z)- and (E)-α-amino-ketoximes are selectively accessible depending on the triplet energy of the photosensitizer.

Iron-catalyzed hydrogen atom transfer induced cyclization of 1,6-enynes for the synthesis of ketoximes: a combined experimental and computational study

An iron-catalyzed reductive radical cyclization/hydro-oximation of 1,6-enynes with tBuONO was developed, leading to functionalized benzofuran, benzothiophene, and cyclopentenyl-based ketoximes.

Transforming electron-rich hetero-buckybowls into electron-deficient polycycles

Oxidation of trichalcogenasumanenes (TCSs) with NO species results in the simultaneous formation of ortho-quinone and diester groups. This reaction enables the transformation of electron-rich TCSs into electron-deficient polycycles.

Fabrication of ZnO/Carbon Quantum Dots Composite Sensor for Detecting NO Gas

ZnO and carbon quantum dots (CQDs) were synthesized by a hydrothermal method, and CQDs were doped into ZnO by a grinding method to fabricate a ZnO/CQDs composite. The X-ray diffraction and the scanning electron microscope revealed that the as-prepared ZnO has a structure of wurtzite hexagonal ZnO and a morphology of a flower-like microsphere which can provide more surface areas to adsorbed gases. The ZnO/CQDs composite has a higher gas sensitivity response to NO gas than ZnO microspheres. A gas sensitivity test of the ZnO/CQDs composite showed that the sensor had a high NO response (238 for 100 ppm NO) and NO selectivity. The detection limit of the ZnO/CQDs composite to NO was 100 ppb and the response and recovery times were 34 and 36 s, respectively. The active functional group provided by CQDs has a significant effect on NO gas sensitivity, and the gas sensitivity mechanism of the ZnO/CQDs composite is discussed.

Catalytic Mechanism of Aromatic Nitration by Cytochrome P450 TxtE: Involvement of a Ferric-Peroxynitrite Intermediate

The cytochromes P450 are heme-dependent enzymes that catalyze many vital reaction processes in the human body related to biodegradation and biosynthesis. They typically act as mono-oxygenases; however, the recently discovered P450 subfamily TxtE utilizes O₂ and NO to nitrate aromatic substrates such as L-tryptophan. A direct and selective aromatic nitration reaction may be useful in biotechnology for the synthesis of drugs or small molecules. Details of the catalytic mechanism are unknown, and it has been suggested that the reaction should proceed through either an iron(III)-superoxo or an iron(II)-nitrosyl intermediate. To resolve this controversy, we used stopped-flow kinetics to provide evidence for a catalytic cycle where dioxygen binds prior to NO to generate an active iron(III)-peroxynitrite species that is able to nitrate l-Trp efficiently. We show that the rate of binding of O₂ is faster than that of NO and also leads to l-Trp nitration, while little evidence of product formation is observed from the iron(II)-nitrosyl complex. To support the experimental studies, we performed density functional theory studies on large active site cluster models. The studies suggest a mechanism involving an iron(III)-peroxynitrite that splits homolytically to form an iron(IV)-oxo heme (Compound II) and a free NO₂ radical via a small free energy of activation. The latter activates the substrate on the aromatic ring, while compound II picks up the ipso-hydrogen to form the product. The calculations give small reaction barriers for most steps in the catalytic cycle and, therefore, predict fast product formation from the iron(III)-peroxynitrite complex. These findings provide the first detailed insight into the mechanism of nitration by a member of the TxtE subfamily and highlight how the enzyme facilitates this novel reaction chemistry.

Rapid and sensitive detection of nitric oxide by a BODIPY-based fluorescent probe in live cells: glutathione effects

Glutathione effects on the sensing reaction toward nitric oxide in live cells.

The Persistent Radical Effect in Organic Synthesis

Radical-radical couplings are mostly nearly diffusion-controlled processes. Therefore, the selective cross-coupling of two different radicals is challenging and not a synthetically valuable transformation. However, if the radicals have different lifetimes and if they are generated at equal rates, cross-coupling will become the dominant process. This high cross-selectivity is based on a kinetic phenomenon called the persistent radical effect (PRE). In this Review, an explanation of the PRE supported by simulations of simple model systems is provided. Radical stabilities are discussed within the context of their lifetimes, and various examples of PRE-mediated radical-radical couplings in synthesis are summarized. It is shown that the PRE is not restricted to the coupling of a persistent with a transient radical. If one coupling partner is longer-lived than the other transient radical, the PRE operates and high cross-selectivity is achieved. This important point expands the scope of PRE-mediated radical chemistry. The Review is divided into two parts, namely 1) the coupling of persistent or longer-lived organic radicals and 2) "radical-metal crossover reactions"; here, metal-centered radical species and more generally longer-lived transition-metal complexes that are able to react with radicals are discussed-a field that has flourished recently.

BODIPY-based selenides as fluorescent probes for rapid, sensitive and mitochondria-specific detection of hypochlorous acid

Hypochlorous acid (HClO) is a powerful microbicidal agent in the innate immue system; however, abnormal HClO levels can cause tissue damage and many diseases. Thus, it is vitally important to develop facile, rapid and accurate analytical methods for the detection of HClO/ClO^-in vitro and in vivo. In this work, we have constructed three meso-substituted BODIPY selenides with different hydrocarbyl groups (ethyl for BSe-Et, benzyl for BSe-Bz and phenyl for BSe-Ph) as fluorescent probes for the detection of HClO/ClO^-. All three non-fluorescent probes can sense HClO/ClO^- to form fluorescent selenoxides by blocking the photo-induced electron transfer process. Their sensing properties display a clear relationship with the structure of the hydrocarbyl. The sensing reactivity is heavily dependent on the electron-donating ability of hydrocarbyls, with the order of the response time as BSe-Et (2 s) < BSe-Bz (5 s) ≪ BSe-Ph (>100 s). Both BSe-Et and BSe-Bz afford a large fluorescence response and very low detection limits (0.3 nM and 0.8 nM), and BSe-Bz displays a higher selectivity over BSe-Et. Finally, as a representative, BSe-Bz was successfully applied to the detection of exo- and endogenous HClO in living cells, and demonstrated to be a mitochondria-localized fluorescent probe.

Nitrogen Oxides and Nitric Acid Enable the Sustainable Hydroxylation and Nitrohydroxylation of Benzenes under Visible Light Irradiation

A new type of waste recycling strategy is described in which nitrogen oxides or nitric acid are directly employed in photocatalyzed hydroxylations and nitrohydroxylations of benzenes. Through these transformations, otherwise costly denitrification can be combined with the synthesis of valuable compounds for various applications.

Metal-Involving Synthesis and Reactions of Oximes

This review classifies and summarizes the past 10-15 years of advancements in the field of metal-involving (i.e., metal-mediated and metal-catalyzed) reactions of oximes. These reactions are diverse in nature and have been employed for syntheses of oxime-based metal complexes and cage-compounds, oxime functionalizations, and the preparation of new classes of organic species, in particular, a wide variety of heterocyclic systems spanning small 3-membered ring systems to macroheterocycles. This consideration gives a general outlook of reaction routes, mechanisms, and driving forces and underlines the potential of metal-involving conversions of oxime species for application in various fields of chemistry and draws attention to the emerging putative targets.

Mn-, Fe-, and Co-Catalyzed Radical Hydrofunctionalizations of Olefins

Cofactor-mimetic aerobic oxidation has conceptually merged with catalysis of syngas reactions to form a wide range of Markovnikov-selective olefin radical hydrofunctionalizations. We cover the development of the field and review contributions to reaction invention, mechanism, and application to complex molecule synthesis. We also provide a mechanistic framework for understanding this compendium of radical reactions.

Aryl Nitriles from Alkynes Using tert-Butyl Nitrite: Metal-Free Approach to C≡C Bond Cleavage

Alkyne C≡C bond breaking, outside of alkyne metathesis, remains an underdeveloped area in reaction discovery. Recently, nitrogenation has been reported to allow nitrile formation from alkynes. A new protocol for the metal-free C≡C bond cleavage of terminal alkynes to produce nitriles is reported. This method provides an opportunity to synthesize a vast range of nitriles containing aryl, heteroaryl, and natural product derivatives (38 examples). In addition, the potential of (t)BuONO to act as a powerful nitrogenating agent for terminal aryl alkynes is demonstrated.

A simple and convenient method for the synthesis of 1,3,5-triazine-nitrolic acids. The first X-ray investigation of Z-isomeric nitrolic acid

A simple and convenient method for the preparation of 2,4-disubstituted 1,3,5-triazine-nitrolic acids has been developed. The first example of nitrolic acid with Z-configuration was revealed by X-ray crystallographic analysis.

Oxidative Nitration of Styrenes for the Recycling of Low-Concentrated Nitrogen Dioxide in Air

The oxidative nitration of styrenes in ethyl acetate represents a metal-free, environmentally friendly, and sustainable technique to recover even low concentrations of NO2 in air. Favorable features are that the product mixture comprising nitroalcohols, nitroketones, and nitro nitrates simplifies at lower concentrations of NO2 . Experiments in a miniplant-type 10 L wet scrubber demonstrated that the recycling technique is well applicable on larger scales at which initial NO2 concentrations of >10 000 ppm were reliably reduced to less than 40 ppm.

The rational design of a highly sensitive and selective fluorogenic probe for detecting nitric oxide

A small-molecule fluorogenic probe for nitric oxide (NO) detection based on a new switching mechanism is developedviaa rational design.

Chemoselective nitration of aromatic sulfonamides with tert-butyl nitrite

A methodology for the efficient conversion of aromatic sulfonamides into their mono-nitro derivatives using tert-butyl nitrite is reported. The reaction exhibits a high degree of chemoselectivity for sulfonamide functionalized aryl systems, even in the presence of other sensitive or potentially reactive functionalities.

Methods for direct alkene diamination, new & old

The 1,2-diamine moiety is a ubiquitous structural motif present in a wealth of natural products, including non-proteinogenic amino acids and numerous alkaloids, as well as in pharmaceutical agents, chiral ligands and organic reagents. The biological activity associated with many of these systems and their chemical utility in general has ensured that the development of methods for their preparation is of critical importance. While a wide range of strategies for the preparation of 1,2-diamines have been established, the diamination of alkenes offers a particularly direct and efficient means of accessing these systems. The purpose of this review is to provide an overview of all methods of direct alkene diamination, metal-mediated or otherwise.