Olefin Dihydroxylation Using Nitroarenes as Photoresponsive Oxidants

Photoexcited nitroarene-enabled carbon chain-elongated oxidation of alkenes via tandem oxidative cleavage and dipolar cycloaddition

Oxidation of alkenes with O3 and photoexcited nitroarenes represents one of the most attractive organic chemical transformations for the synthesis of oxygen-enriched molecules. However, known achievements are mainly limited to carbon chain-shortened oxidation and carbon chain-retained oxidation of alkenes. Given that constructing higher molecular complexity is the core goal of modern synthesis, the development of chain-elongated oxidation of alkenes would be in high demand but still remains an elusive challenge so far. Herein, we report a photoexcited nitroarene-enabled highly regioselective chain-elongated oxidation of alkenes via tandem oxidative cleavage and dipolar cycloaddition, providing a broad range of synthetically-useful isoxazolidines in up to 92% yield from readily available enol ethers or styrene and derivatives under simple and mild conditions.

Light-promoted aromatic denitrative chlorination

Nitroarenes are readily accessible bulk chemicals and can serve as versatile starting materials for a series of synthetic reactions. However, due to the inertness of the CAr-NO2 bond, the direct denitrative substitution reaction with unactivated nitroarenes remains challenging. Chemists rely on sequential reduction and diazotization followed by the Sandmeyer reaction or the nucleophilic aromatic substitution of activated nitroarenes to realize nitro group transformations. Here we develop a general denitrative chlorination reaction under visible-light irradiation, in which the chlorine radical replaces the nitro moiety through the cleavage of the CAr-NO2 bond. This practical method works with a wide range of unactivated nitro(hetero)arenes and nitroalkenes, is not sensitive to air or moisture and can proceed smoothly on a decagram scale. This transformation differs fundamentally from previous nucleophilic aromatic substitution reactions under thermal conditions in both synthesis and mechanism. Density functional theory calculations reveal the possible pathway for the substitution reaction.

Merging Photoexcited Nitroarenes with Lewis Acid Catalysis for the Anti-Markovnikov Oxidation of Alkenes

Herein we describe the oxidation of alkenes to carbonyls and acetonides via the interplay of photoexcited nitroarenes and Lewis acid catalysis. A wide range of alkenes were oxidized to aldehyde and ketone products with anti-Markovnikov selectivity and to acetonides when acetone was employed as a co-solvent. Mechanistic studies support that Lewis acid coordination to the 1,3,2-dioxazolidine intermediate results in a 1,2-shift to generate carbonyl derivatives and a nucleophilic substitution pathway for the formation of acetonides.

Transformative reactions in nitroarene chemistry: C-N bond cleavage, skeletal editing, and N-O bond utilization

Nitroarenes are highly versatile building blocks in organic synthesis, playing a pivotal role in various reactions. Common transformations involving nitroarenes include nucleophilic aromatic substitution (SNAr) reactions, where the nitro group functions both as a potent electron-withdrawing group that activates the aromatic ring and as a leaving group facilitating the substitution. Additionally, the direct transformation of nitro groups, such as reduction-driven syntheses of amines and carboxylic acids, as well as ipso-substitution SNAr reactions, have been extensively explored. Interactions between ortho-nitro groups and neighboring substituents also provide unique opportunities for selective transformations. However, beyond these well-established processes, direct transformations of nitro groups have been relatively limited. In recent years, significant advancements have been made in alternative methodologies for nitro group transformations. This review focuses on the latest progress in novel transformations of nitroarenes, with emphasis on three major categories: (i) functional group transformations involving C-N bond cleavage in nitroarenes, (ii) skeletal editing via nitrene intermediates generated by N-O bond cleavage, and (iii) the utilization of nitroarenes as an oxygen source through N-O bond cleavage. These developments under-score the expanding utility of nitroarenes in modern organic synthesis.

Visible-light-induced aerobic oxidative cyclization of nitroarenes with triethylamine using an organophotocatalyst

Isoxazolidines are structurally important scaffolds in many natural products and bioactive compounds. Herein, we report a novel synthetic method for isoxazolidine derivatives through visible-light-induced photoredox cascade cyclization of nitroarenes with triethylamine under aerobic conditions. The resultant 5-hydroxyl isoxazolidine compounds were generally obtained in moderate yields with a broad range of compatible functionalities.

Photoinduced Bartoli Indole Synthesis by the Oxidative Cleavage of Alkenes with Nitro(hetero)arenes

Given the unique charm of dipole chemistry, intercepting N-O=C dipoles precisely generated by designed processes to develop novel reactivity has become a seminal challenge. The polar fragmentation of 1,3,2-dioxazolidine species generated through the radical addition of excited nitro(hetero)arenes to alkenes represents a significantly underappreciated mechanism for generating N-O=C dipoles. Herein, we present a photoinduced Bartoli indole synthesis by the oxidative cleavage of alkenes with nitro(hetero)arenes. Various indoles and azaindoles are constructed through the multi-step spontaneous rearrangement of carbonyl imine intermediates generated by the polar fragmentation of 1,3,2-dioxazolidine species. Mechanism studies and DFT calculations support that the reaction involves radical cycloaddition, ozonolysis-type cycloreversion, intramolecular H-shift of carbonyl imines, and 3,3-sigmatropic shift of O-Alkenyl hydroxylamines, etc. The implementation of continuous- flow photochemistry, in particular, significantly enhances efficiency, thereby overcoming obstacles to the commercialization process.

Anaerobic 1,2-/1,3-Hydroxytrifluoromethylation of Unactivated Alkenes Enabled by Photoexcited Nitroarenes

An anaerobic 1,2-/1,3-hydroxytrifluoromethylation of unactivated alkenes is described. This reaction proceeds in mild and environmentally friendly conditions without photocatalyst and metal catalyst, allowing access to a wide range of β- and γ-trifluoromethyl alcohols. Preliminary mechanistic investigations indicate that the accomplishment of this protocol relies on the dual functionality of the photoexcited triplet nitroarenes, which serve as the oxygen atom source and enable the single-electron transfer (SET) process with CF3SO2Na.

Oxydiazomethylation of Alkenes via Photoredox Catalysis

α-Diazoesters belong to significantly important carbenoid precursors in synthetic chemistry. Diazomethylation-based difunctionalization of alkenes is highly valuable but remain nontrivial. Herein, we reported a general and modular approach for the direct 1,2-oxydiazomethylation of alkenes through visible-light photoredox catalysis. This process exploits photocatalyzed strategy to convert alkenes to γ-formyloxyl-α-diazoesters using α-diazo iodonium salts as carbyne precursors, featuring wide substrate tolerance and broad late-stage diversifications. Mechanistic studies suggest that the formation of γ-carbocation-tethered α-diazoesters plays a crucial role in trapping DMF and H2O to allow for this transformation.

Synthesis of N-Sulfonyl Formamidines by Direct Condensation between Sulfonamide and Formamide Enabled by a Photogenerated Vilsmeier-Type Reagent

Herein, we report the synthesis of N-sulfonyl formamidines from carbon tetrabromide and formamide under UVA irradiation without any additional catalysts. This approach represents a straightforward methodology for accessing this class of structural units and has been applied to a wide range of readily available sulfonamides and formamides, providing the corresponding products in moderate to excellent yields (30 examples, 16-99% yields). Mechanistic investigations associated with previous reports suggest the implication of an activated iminium intermediate (Vilsmeier-Haack reagent derivatives), obtained by the photoinduced reaction between carbon tetrabromide and formamides.

KBr-Mediated Electrochemical Dihydroxylation of Alkenes Using H2O as the Hydroxyl Source

Dihydroxylation of alkenes provides direct access to vicinal diols. Herein, a new electrochemical strategy for dihydroxylation of alkenes in only the presence of KBr is disclosed. Water serves as a green and sustainable hydroxyl source. Cheap KBr acts as both an electrolyte and a catalyst. Both styrenes and unactivated alkenes proceed in the dihydroxylation reactions smoothly to furnish vicinal diols in good yields. The successful synthesis of Cyclandelate, DTD derivative precursors, and a key intermediate for the synthesis of herbicide Metamitron highlights its synthetic utility.

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.

Metal and Photocatalyst-Free Amide Synthesis via Decarbonylative Condensation of Alkynes and Photoexcited Nitroarenes

Depending on the intrinsic photoactivity of nitroarenes, we herein developed a practical Brønsted acid-catalyzed decarbonylative amide synthesis from alkynes and photoexcited nitroarenes without any metal or photocatalyst. This method exhibited compatibility with water and air, broad substrate applicability, marvelous functional group tolerance, and wide applications in scale-up synthesis, late-stage functionalization, and total synthesis. Mechanism studies and DFT calculations supported that a 1,3,2-dioxazole intermediate was involved, and gaseous carbon monoxide was the only byproduct during amide construction.

Photoexcited nitroarenes for alkylation of quinoxalin-2(1H)-ones

A straightforward method for the dehydrogenative alkylation of quinoxalin-2(1H)-ones with alkylbenzenes has been developed, facilitated by a photoexcited nitroarene. The reaction's success hinges on the dual role of the photoexcited nitroarene molecule, acting as both a hydrogen atom transfer (HAT) reagent and an oxidant. This technique is both atom-economical and cost-effective, due to the readily available nitroarene, which serves as the sole intermediary in the reaction 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.

Discovery of Photoexcited 2-Chloro-3,5-Dinitrobenzoic Acid as a Chemical Deprenylase of i6A RNA

RNA modifications play crucial roles in regulating gene expression and cellular homeostasis. Modulating RNA modifications, particularly by targeting the enzymes responsible for their catalysis, has emerged as a promising therapeutic strategy. However, limitations, such as the lack of identified modifying enzymes and compensatory mechanisms, hinder targeted interventions. Chemical approaches independent of enzymatic activity offer an alternative strategy for RNA modification modulation. Here, we present the identification of 2-chloro-3,5-dinitrobenzoic acid as a highly effective photochemical deprenylase of i6A RNA. This method demonstrates exceptional selectivity towards i6A, converting its substituent into a "N-doped" ozonide, which upon hydrolysis releases natural adenine. We believe that this chemical approach will pave the way for a better understanding of RNA modification biology and the development of novel therapeutic modalities.

Photocatalytic, α-Aminoalkyl Radical-Mediated, Methylene-Extrusive Ring-Closing Transformation of o-Alkynyl and o-Cyano Acrylamides

Herein we report a visible-light-induced, α-aminoalkyl radical-mediated cascade reaction of 1,7-enynes that establishes a unique ring-closing enyne transformation pathway which occurs with concomitant loss of a methylene moiety. The α-aminoalkyl radical derived from N,N-dimethylaniline was demonstrated to be a traceless promoter of enyne reorganization leading to 4-alkylquinolinones. The reaction can also be extended to nitrile-substituted acrylamide systems, leading to carbostyrils. Experiments with deuterated N,N-dimethylaniline-d6 (PhN(CD3)2) established the involvement of 1,5-H atom transfer in the mechanism.

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.

A Photochemical Strategy for the Conversion of Nitroarenes into Rigidified Pyrrolidine Analogues

Bicyclic amines are important motifs for the preparation of bioactive materials. These species have well-defined exit vectors that enable accurate disposition of substituents toward specific areas of chemical space. Of all possible skeletons, the 2-azabicyclo[3.2.0]heptane framework is virtually absent from MedChem libraries due to a paucity of synthetic methods for its preparation. Here, we report a modular synthetic strategy that utilizes nitroarenes as flat and easy-to-functionalize feedstocks for the assembly of these sp3-rich materials. Mechanistically, this approach exploits two concomitant photochemical processes that sequentially ring-expand the nitroarene into an azepine and then fold it into a rigid bicycle pyrroline by means of singlet nitrene-mediated nitrogen insertion and excited-state-4π electrocyclization. A following hydrogenolysis provides, with full diastereocontrol, the desired bicyclic amine derivatives whereby the aromatic substitution pattern has been translated into the one of the three-dimensional heterocycle. These molecules can be considered rigid pyrrolidine analogues with a well-defined orientation of their substituents. Furthermore, unsupervised clustering of an expansive virtual database of saturated N-heterocycles revealed these derivatives as effective isosteres of rigidified piperidines. Overall, this platform enables the conversion of nitroarene feedstocks into complex sp3-rich heterocycles of potential interest to drug development.

Oxidative cleavage of ketoximes to ketones using photoexcited nitroarenes

The methoxime group has emerged as a versatile directing group for a variety of C-H functionalizations. Despite its importance as a powerful functional handle, conversion of methoximes to the parent ketone, which is often desired, usually requires harsh and functional group intolerant reaction conditions. Therefore, the application of methoximes and their subsequent conversion to the corresponding ketone in a late-stage context can be problematic. Here, we present an alternative set of conditions to achieve mild and functional group tolerant conversion of methoximes to the parent ketones using photoexcited nitroarenes. The utility of this methodology is showcased in its application in the total synthesis of cephanolide D. Furthermore, mechanistic insight into this transformation obtained using isotope labeling studies as well as the analysis of reaction byproducts is provided.

Metal-free electrochemical dihydroxylation of unactivated alkenes

Herein, a metal-free electrochemical dihydroxylation of unactivated alkenes is described. The transformation proceeds smoothly under mild conditions with a broad range of unactivated alkenes, providing valuable and versatile dihydroxylated products in moderate to good yields without the addition of costly transition metals and stoichiometric amounts of chemical oxidants. Moreover, this method can be applied to a range of natural products and pharmaceutical derivatives, further demonstrating its synthetic utility. Mechanistic studies have revealed that iodohydrin and epoxide intermediate are formed during the reaction process.

Recent Advances in C-H Functionalisation through Indirect Hydrogen Atom Transfer

The functionalisation of C-H bonds has been an enormous achievement in synthetic methodology, enabling new retrosynthetic disconnections and affording simple synthetic equivalents for synthons. Hydrogen atom transfer (HAT) is a key method for forming alkyl radicals from C-H substrates. Classic reactions, including the Barton nitrite ester reaction and Hofmann-Löffler-Freytag reaction, among others, provided early examples of HAT. However, recent developments in photoredox catalysis and electrochemistry have made HAT a powerful synthetic tool capable of introducing a wide range of functional groups into C-H bonds. Moreover, greater mechanistic insights into HAT have stimulated the development of increasingly site-selective protocols. Site-selectivity can be achieved through the tuning of electron density at certain C-H bonds using additives, a judicious choice of HAT reagent, and a solvent system. Herein, we describe the latest methods for functionalizing C-H/Si-H/Ge-H bonds using indirect HAT between 2018-2023, as well as a critical discussion of new HAT reagents, mechanistic aspects, substrate scopes, and background contexts of the protocols.