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.

Oxidative coupling of N-nitrosoanilines with substituted allyl alcohols under rhodium (III) catalysis

Rhodium(III) catalysis has been used for C-H activation of N-nitrosoanilines with substituted allyl alcohols. This method provides an efficient synthesis of the functional N-nitroso ortho β-aryl aldehydes and ketones with low catalyst loading, high functional group tolerance, and superior reactivity of allyl alcohols toward N-nitrosoanilines. We demonstrated that reaction also proceeds through the one-pot synthesis of N-nitrosoaniline, followed by subsequent, C-H activation. The protocol was also feasible with acyrlaldehyde and methyl vinyl ketone which furnished the same oxidative N-nitroso coupling product.

Photocatalytic regioselective C-H bond functionalizations in arenes

The direct functionalization of C-H bonds has revolutionized the field of synthetic organic chemistry by enabling efficient and atom-economical modification of arenes by avoiding prefunctionalization. However, the inherent challenges of inertness and regioselectivity in different C-H bonds, particularly for distal positions, necessitate innovative approaches. In this aspect, photoredox catalysis by utilizing both transition metal and organic photocatalysts has emerged as a powerful tool for addressing these challenges under mild reaction conditions. This review provides a comprehensive overview of recent progress in regioselective C-H functionalization in arenes via photocatalysis. Emphasizing the strategies for achieving ortho-, meta-, and para-selectivity, we explore the mechanistic insights, catalyst designs, and the novel methodologies that have expanded the scope of C-H bond functionalization. This discussion aims to offer valuable perspectives for advancing the field and developing more efficient and sustainable synthetic methodologies.

Nitroso-azomethine(ene) reaction enabled annulations of nitrosoarenes, azomethines and alkenes

An unprecedented example of a nitroso-azomethine(ene) reaction is reported. Nitroso-azomethine(ene) reaction-mediated unprecedented annulation of nitrosoarenes, azomethines, and alkenes to furnish arylquinolines via arene functionalization of nitrosoarene has been developed. DFT studies provided mechanistic insights into the newly developed nitroso-azomethine(ene) reaction.

Direct Aromatic Nitrosation Using 2-Methoxyethyl Nitrite as a NO Cation Source

This work presents an acid-free method for aromatic nitrosation using 2-methoxyethyl nitrite (MOE-ONO). While originally developed as a NOx radical source in our group, we demonstrate the utility of MOE-ONO as a NO cation source for aromatic electrophilic nitrosation. This method successfully nitrosates phenols, naphthols, and other pronucleophiles, completely suppressing undesired nitration by NOx radicals. Notably, it enables nitrosation of acid-sensitive substrates, which has been difficult to achieve with existing protocols.

Continuous Flow Synthesis of Nitrosoarenes via Photochemical Rearrangement of Aryl Imines

Nitrosoarenes are versatile organic building blocks; however, their intrinsic instability and limited synthetic accessibility have so far restricted their widespread use. Herein, we present a new continuous flow route toward these entities that is based on a direct photochemical rearrangement process using o-nitrophenylimines as starting materials. Due to the underlying redox mechanism, a new amide group accompanies the formation of the nitroso group. Crucial to the success of this approach is the use of trifluoroethanol as a solvent and high-power light-emitting diodes (365 nm) as light sources that provide uniform irradiation and high efficiency of the resulting continuous flow method. The process is fast and robust, with high functional group tolerance and high throughput. The formation of the nitroso moiety is supported by full spectroscopic analysis, including X-ray crystallography. The scalability of this flow approach allows access to gram quantities of nitroso species for which we highlight a small set of derivatization reactions underlining their synthetic utility.

Regioselective synthesis of novel nitroso-pyrazolylquinoxalines via HOAc-mediated cyclocondensation of 2-hydroxyimino-1,3-diketones with hydrazinylquinoxalines

This article describes the regioselective synthesis of novel nitroso-pyrazolylquinoxalines via cyclocondensation of 2-hydroxyimino-1,3-diketones with hydrazinylquinoxalines. Of note, this is the first time that 2-hydroxyimino-1,3-diketones are used as electrophilic reagents to cyclocondensation with hetarylhydrazines. The cyclocondensation proceeded through a hydrazone intermediates formation. A series of novel nitroso-substituted pyrazolylquinoxalines were synthesized by both "one-pot" and "two-step" procedures in up to 86% yields. Importantly, a gram-scale synthesis of nitroso-pyrazolylquinoxalines, their oxidation and reduction were successfully accomplished.

Organocatalytic asymmetric nitroso aldol reaction of α-substituted malonamates

A practical enantioselective N-selective nitroso aldol reaction of α-methylmalonamates with a nitrosoarene is reported. The reaction employs the Takemoto thiourea catalyst for the induction of enantioselectivity, and the corresponding optically active oxyaminated malonamates were obtained in reasonably good yields.

Diastereoselective synthesis of 1,1,3,3-tetrasubstituted cyclobutanes enabled by cycloaddition of bicyclo[1.1.0]butanes

A diastereoselective cycloaddition of bicyclo[1.1.0]butanes (BCBs) with triazolinedione or nitrosoarenes was developed to access multi-substituted cyclobutanes.

Photoinduced Diverse Reactivity of Diazo Compounds with Nitrosoarenes

A diverse reactivity of diazo compounds with nitrosoarene in an oxygen-transfer process and a formal [2 + 2] cycloaddition is reported. Nitosoarene has been exploited as a mild oxygen source to oxidize an in situ generated carbene intermediate under visible-light irradiation. UV-light-mediated in situ generated ketenes react with nitosoarenes to deliver oxazetidine derivatives. These operationally simple processes exemplify a transition-metal-free and catalyst-free protocol to give structurally diverse α-ketoesters or oxazetidines.

Lewis acid catalyzed reactivity switch: pseudo three-component annulation of nitrosoarenes and (epoxy)styrenes

A Lewis acid catalyzed annulation reaction via arene functionalization of nitrosoarenes and C-C cleavage of (epoxy)styrene to provide arylquinolines is reported. The Lewis acid catalyst altered the annulation pattern providing arylquinolines instead of oxazolidines. The reaction with styrene resulted in a mixture of 2,4-diarylquinoline and 4-arylquinoline, while only 3-arylquinoline was formed from the reaction of epoxystyrene.

Recent developments in photoredox-catalyzed remote ortho and para C-H bond functionalizations

In recent years, the research area of direct C-H bond functionalizations was growing exponentially not only due to the ubiquity of inert C-H bonds in diverse organic compounds, including bioactive natural and nonnatural products, but also due to its impact on the discovery of pharmaceutical candidates and the total synthesis of intricate natural products. On the other hand, more recently, the field of photoredox catalysis has become an indispensable and unparalleled research topic in modern synthetic organic chemistry for the constructions of challenging bonds, having the foremost scope in academia, pharmacy, and industry. Therefore, the development of green, simpler, and effective methodologies to accomplish direct C-H bond functionalization is well overdue and highly desirable to the scientific community. In this review, we mainly highlight the impact on, and the utility of, photoredox catalysts in inert ortho and para C-H bond functionalizations. Although a surge of research papers, including reviews, demonstrating C-H functionalizations have been published in this vital area of research, to our best knowledge, this is the first review that focuses on ortho and para C-H functionalizations by photoredox catalysis to provide atom- and step-economic organic transformations. We are certain that this review will act as a promoter to highlight the application of photoredox catalysts for the functionalization of inert bonds in the domain of synthetic organic chemistry.

Radical chemistry of nitrosoarenes: concepts, synthetic applications and directions

The radical chemistry of nitrosoarenes has recently emerged as an attractive and important topic in organic chemistry.

ortho-Substituent effect on the crystal packing and solid state speciation of aromaticC-nitroso compounds

The solid state behaviour of a short series aromaticC-nitroso compounds has been studied as a function of the electronic and steric nature of theortho-substituent on the ring.

Construction of α-methoxyimidoyl ketonitrones via phosphite-mediated addition of α-keto N-tert-butanesulfinyl imidates to nitrosoarenes

A dimethyl phosphite-mediated addition of α-keto N-tert-butanesulfinyl imidates to nitrosoarenes was developed. Nitrosoarenes were successfully used as electrophiles to trap aza-enolate intermediates that were generated from nucleophilic addition of deprotonated phosphite to α-keto N-tert-butanesulfinyl imidates and following phospha-Brook rearrangement, allowing efficient construction of ketonitrones with excellent (Z)-geometries.

N-Trifluoromethylation of Nitrosoarenes with Sodium Triflinate

A highly efficient N-trifluoromethylation of nitrosoarenes is reported. The inexpensive and convenient Langlois reagent (sodium triflinate) is employed as a CF₃-radical source in combination with a copper catalyst and an oxidant. N-Trifluoromethylated hydroxylamines are obtained in high yields within 1 h at room temperature. The addition of hydroquinone was found to be instrumental to prevent the formation of side products. The method is high-yielding, is scalable, and displays a high functional group tolerance.