Recent advances in the synthetic applications of nitrosoarene chemistry

Nitroso groups are widely present in biologically active compounds in medicinal chemistry, and nitroso compounds serve as important building blocks in organic chemistry and materials science. Nitrosoarenes, in particular, showcase remarkable versatility, functioning as both electrophilic and nucleophilic reagents in a broad spectrum of organic reactions, thereby holding significant relevance in organic chemistry. This review aims to provide a comprehensive overview of the latest advancements in nitrosoarene reactions spanning a decade. Special attention is given to the synthesis of products derived from nitrosoarenes and the conditions that promote these reactions, as well as the type of catalysts. The exploration covers various facets of nitrosoarene chemistry, including cyclization, reactions involving attacks at the oxygen or nitrogen terminus, dimerization, rearrangement, coordination, and other significant reactions. By delving into these diverse reaction pathways and mechanisms, this review aspires to serve as a valuable resource for researchers seeking to deepen their understanding of nitrosoarene chemistry and its applications in both fundamental and applied scientific research.

Photocatalyzed C3-H Nitrosylation of Imidazo[1,2-a]pyridine under Continuous Flow and External Photocatalyst-, Oxidant-, and Additive-Free Conditions

This study reports a protocol for the highly regioselective photocatalyzed C-H nitrosylation of imidazo[1,2-a]pyridine scaffolds at the C3 position under a combination of visible-light irradiation and continuous flow without any external photocatalyst. This protocol involves mild and safe conditions and shows good tolerance to air and water along with excellent functional group compatibility and site selectivity, generating various 3-nitrosoimidazo[1,2-a]pyridines in excellent yields under photocatalyst-, oxidant-, and additive-free conditions.Notably, the proposed nitrosylation reaction, which introduces the chromophore NO into imidazo[1,2-a]pyridine scaffolds, occurs efficiently under visible-light irradiation without any additional photocatalyst owing to the intense light-absorption characteristics of the nitrosylation products. This study could guide future studies on the development of green organic-synthesis strategies with a wide variety of potential applications.

Synthesis of Azoxy Compounds: from Copper Compounds to Mesoporous Silica-Encaged Ultrasmall Copper Catalysts

Azoxy compounds have aroused extensive attention due to their unique biological activities, but the chemical synthesis of these compounds often suffers from limitations due to their requirement for stoichiometric oxidants, high costs, and restricted substrate range. Herein, a series of azoxy compounds were constructed via facile coupling reactions by using cost-effective N-methoxyformamide and nitroso compounds over Cu-based catalysts, affording high product yields with excellent tolerance of functional groups. Significantly, the mesoporous silica nanosphere-encapsulated ultrasmall Cu (Cu@MSN) catalyst was developed via a one-pot synthetic method and first used for the synthesis of azoxy compounds. As compared with copper salt catalysts, the Cu@MSN catalyst exhibited remarkably enhanced catalytic activity and superior recycling stability. Such a Cu@MSN catalyst overcame the inherent drawbacks of low activity, fast deactivation, and difficult recycling of traditional metal salt catalysts in organic reactions. This work provides a green and efficient method for the construction of azoxy compounds and also creates new prospects for the application of nanoporous materials confined metal catalysts in organic synthesis.

Synthetic and biosynthetic routes to nitrogen-nitrogen bonds

The nitrogen-nitrogen bond is a core feature of diverse functional groups like hydrazines, nitrosamines, diazos, and pyrazoles. Such functional groups are found in >300 known natural products. Such N-N bond-containing functional groups are also found in significant percentage of clinical drugs. Therefore, there is wide interest in synthetic and enzymatic methods to form nitrogen-nitrogen bonds. In this review, we summarize synthetic and biosynthetic approaches to diverse nitrogen-nitrogen-bond-containing functional groups, with a focus on biosynthetic pathways and enzymes.

Convergent Paired Electrochemical Synthesis of Azoxy and Azo Compounds: An Insight into the Reaction Mechanism

A convergent paired electrochemical method was developed for the synthesis of azoxy and azo compounds starting from the corresponding nitroarenes. We propose a unique mechanism for electrosynthesis of azoxy and azo compounds. We find that both anodic and cathodic reactions are responsible for the synthesis of these compounds. The synthesis of azoxy and azo derivatives have been successfully performed in an undivided cell, using carbon rod electrodes, by constant current electrolysis at room temperature.

Copper-Catalyzed Aerobic Oxidative Cyclization of 2-Alkynylanilines with Nitrosoarenes: Synthesis of Organic Solid Mechanoluminescence Compounds of 4-Oxo-4H-cinnolin-2-ium-1-ide

An efficient Cu(I)/DMAP/air system for the one-pot synthesis of 4-oxo-4H-cinnolin-2-ium-1-ides, which are often difficult to prepare by traditional routes from substituted 2-alkynylanilines and nitrosoarenes, was developed. These 4-oxo-4H-cinnolin-2-ium-1-ides have practical applications as mechanoluminescent materials. Preliminary mechanistic experiments were performed, and a plausible mechanism for this tandem process is proposed. The use of an inexpensive copper catalyst and molecular oxygen as the oxygen source and the oxidant make this an attractive green protocol with potential synthetic applications.