Photoinduced Oxygen Transfer Using Nitroarenes for the Anaerobic Cleavage of Alkenes

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.

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.

Visible-Light Induced and Iron Peroxo-Promoted Radical Difunctionalization of Alkene for the Synthesis of β-Ketosulfone and α-Chloroketone

In this work, a switchable synthesis of β-ketosulfone and α-chloroketone through a radical difunctionalization of alkenes is reported. The transformation works well under iron peroxo species/photoredox dual catalysis and an open-flask atmosphere, and the reaction is highlighted with good yields and a broad reaction scope. Mechanism studies show that the reaction is initiated by a formal [4 + 2] cyclization of the sulfonyl radical in a regioselective manner.

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.

Accessing Azetidines through Magnesium-Mediated Nitrogen Group Transfer from Iminoiodinane to Donor-Acceptor Cyclopropanes

Herein, we report a Lewis acid-mediated ring expansion of donor-acceptor cyclopropanes (DACs) to substituted azetidines via nucleophilic nitrogen group transfer from readily accessible iminoiodinane. This protocol operates under mild, transition-metal-free conditions, and showcases excellent chemoselectivity, along with broad functional group tolerance. We report for the first time that challenging alkyl donor-acceptor cyclopropanes can undergo ring expansion leading to aliphatic azetidines without relying on external oxidants or precious transition-metal catalysts. Mechanistically, the coordination of a magnesium (Mg)-Lewis acid to the DAC promotes nucleophilic ring opening with a putative Mg-amide species generated from the iminoiodinane under the reaction conditions to furnish the azetidine products.

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.

General Alkene 1,2-syn-Cyano-Hydroxylation Procedure Via Electrochemical Activation of Isoxazoline Cycloadducts

Stereoselective alkene 1,2-difunctionalization is a privileged strategy to access three-dimensional C(sp3)-rich chiral molecules from readily available "flat" carbon feedstocks. State-of-the-art approaches exploit chiral transition metal-catalysts to enable high levels of regio- and stereocontrol. However, this is often achieved at the expense of a limited alkene scope and reduced generality. 1,3-Dipolar cycloadditions are routinely used to form heterocycles from alkenes with high levels of regioselectivity and stereospecificity. Nevertheless, methods for the ring-opening of cycloadducts to reveal synthetically useful functionalities require the use of hazardous reagents or forcing reaction conditions; thus limiting their synthetic applications. Herein, we describe the implementation of a practical, general and selective electrosynthetic strategy for olefin 1,2-syn-difunctionalization, which hinges on the design of novel reagents-consisting of a nitrile oxide 1,3-dipole precursor, equipped with a sulfonyl-handle. These can selectively difunctionalize alkenes via "click" 1,3-dipolar cycloadditions, and then facilitate the telescoped electrochemical single electron transfer activation of the ensuing isoxazoline intermediate. Cathodic reduction of the cycloadduct triggers a radical fragmentation pathway delivering sought-after stereodefined 1,2-syn-hydroxy nitrile derivatives. Our telescoped electrochemical procedure tolerates a wide range of functionalities, and─crucially─enables the difunctionalization of both electron-rich, electron-poor and unactivated olefins, with diverse degree of substitution; thus providing a robust, general and selective metal-free alternative to current alkene difunctionalization strategies. Capitalizing on these features, we employed our electrosynthetic method to enable the late-stage syn-hydroxy-cyanation of natural products and bioactive compounds, and streamline the de novo synthesis of pharmaceutical agents.

Photooxidation of Polyolefins to Produce Materials with In-Chain Ketones and Improved Materials Properties

Herein, we report a selective photooxidation of commodity postconsumer polyolefins to produce polymers with in-chain ketones. The reaction does not involve the use of catalyst, metals, or expensive oxidants, and selectively introduces ketone functional groups. Under mild and operationally simple conditions, yields up to 1.23 mol % of in-chain ketones were achieved. Installation of in-chain ketones resulted in materials with improved adhesion of the materials and miscibility of mixed plastics relative to the unfunctionalized plastics. The introduction of ketone groups into the polymer backbone allows these materials to react with diamines, forming dynamic covalent polyolefin networks. This strategy allows for the upcycling of mixed plastic waste into reprocessable materials with enhanced performance properties compared to polyolefin blends. Mechanistic studies support the involvement of photoexcited nitroaromatics in consecutive hydrogen and oxygen atom transfer reactions.

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.

Triazenolysis of alkenes as an aza version of ozonolysis

Alkenes are broadly used in synthetic applications, thanks to their abundance and versatility. Ozonolysis is one of the most canonical transformations that converts alkenes into molecules bearing carbon-oxygen motifs via C=C bond cleavage. Despite its extensive use in both industrial and laboratory settings, the aza version-cleavage of alkenes to form carbon-nitrogen bonds-remains elusive. Here we report the conversion of alkenes into valuable amines via complete C=C bond disconnection. This process, which we have termed 'triazenolysis', is initiated by a (3 + 2) cycloaddition of triazadienium cation to an alkene. The triazolinium salt formed accepts hydride from borohydride anion and spontaneously decomposes to create new C-N motifs upon further reduction. The developed reaction is applicable to a broad range of cyclic alkenes to produce diamines, while various acyclic C=C bonds may be broken to generate two separate amine units. Computational analysis provides insights into the mechanism, including identification of the key step and elucidating the significance of Lewis acid catalysis.

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.

Machine-Learning-Assisted Discovery of Mechanosynthesized Lead-Free Metal Halide Perovskites for the Oxidative Photocatalytic Cleavage of Alkenes

Lead-free metal halide perovskites can potentially be air- and water-stable photocatalysts for organic synthesis, but there are limited studies on them for this application. Separately, machine learning (ML), a critical subfield of artificial intelligence, has played a pivotal role in identifying correlations and formulating predictions based on extensive datasets. Herein, an iterative workflow by incorporating high-throughput experimental data with ML to discover new lead-free metal halide perovskite photocatalysts for the aerobic oxidation of styrene is described. Through six rounds of ML optimization guided by SHapley Additive exPlanations (SHAP) analysis, BA2CsAg0.95Na0.05BiBr7 as a photocatalyst that afforded an 80% yield of benzoic acid under the standard conditions is identified, which is a 13-fold improvement compared to the 6% with when using Cs2AgBiBr6 as the initial photocatalyst benchmark that is started. BA2CsAg0.95Na0.05BiBr7 can tolerate various functional groups with 22 styrene derivatives, highlighting the generality of the photocatalytic properties demonstrated. Radical scavenging studies and density functional theory calculations revealed that the formation of the reactive oxygen species superoxide and singlet oxygen in the presence of BA2CsAg0.95Na0.05BiBr7 are critical for photocatalysis.

Water mediated redox-neutral cleavage of arylalkenes via photoredox catalysis

Cleavage of carbon-carbon bonds remains a challenging task in organic synthesis. Traditional methods for splitting Csp2=Csp2 bonds into two halves typically involve non-redox (metathesis) or oxidative (ozonolysis) mechanisms, limiting their synthetic potential. Disproportionative deconstruction of alkenes, which yields one reduced and one oxidized fragment, remains an unexplored area. In this study, we introduce a redox-neutral approach for deleting a Csp2 carbon unit from substituted arylalkenes, resulting in the formation of an arene (reduction) and a carbonyl product (oxidation). This transformation is believed to proceed through a mechanistic sequence involving visible-light-promoted anti-Markovnikov hydration, followed by photoredox cleavage of Csp3-Csp3 bond in the alcohol intermediate. A crucial consideration in this design is addressing the compatibility between the highly reactive oxy radical species in the latter step and the required hydrogen-atom-transfer (HAT) reagent for both steps. We found that ethyl thioglycolate serves as the optimal hydrogen-atom shuttle, offering remarkable chemoselectivity among multiple potential HAT events in this transformation. By using D2O, we successfully prepared dideuteromethylated (-CD2H) arenes with good heavy atom enrichment. This work presents a redox-neutral alternative for alkene deconstruction, with considerable potential in late-stage modification of complex 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.

Recent Advances in Photoinduced Oxidative Cleavage of Alkenes

Oxidative cleavage of alkenes leading to valuable carbonyl derivatives is a fundamental transformation in synthetic chemistry. In particular, ozonolysis is the mainstream method for the oxidative cleavage of alkenes that has been widely implemented in the synthesis of natural products and pharmaceutically relevant compounds. However, due to the toxicity and explosive nature of ozone, alternative approaches employing transition metals and enzymes in the presence of oxygen and/or strong oxidants have been developed. These protocols are often conducted under harsh reaction conditions that limit the substrate scope. Photochemical approaches can provide milder and more practical alternatives for this synthetically useful transformation. In this review, we outline recent visible-light-promoted oxidative cleavage reactions that involve photocatalytic activation of oxygen via electron transfer and energy transfer. Also, an emerging field featuring visible-light-promoted oxidative cleavage under anaerobic conditions is discussed. The methods highlighted in this review represent a transformative step toward more sustainable and efficient strategies for the oxidative cleavage of alkenes.

Multiple photofluorochromic luminogens via catalyst-free alkene oxidative cleavage photoreaction for dynamic 4D codes encryption

Controllable photofluorochromic systems with high contrast and multicolor in both solutions and solid states are ideal candidates for the development of dynamic artificial intelligence. However, it is still challenging to realize multiple photochromism within one single molecule, not to mention good controllability. Herein, we report an aggregation-induced emission luminogen TPE-2MO2NT that undergoes oxidation cleavage upon light irradiation and is accompanied by tunable multicolor emission from orange to blue with time-dependence. The photocleavage mechanism revealed that the self-generation of reactive oxidants driving the catalyst-free oxidative cleavage process. A comprehensive analysis of TPE-2MO2NT and other comparative molecules demonstrates that the TPE-2MO2NT molecular scaffold can be easily modified and extended. Further, the multicolor microenvironmental controllability of TPE-2MO2NT photoreaction within polymer matrices enables the fabrication of dynamic fluorescence images and 4D information codes, providing strategies for advanced controllable information encryption.

Visible photons as ideal reagents for the activation of coloured organic compounds

In recent decades, the traceless nature of visible photons has been exploited for the development of efficient synthetic strategies for the photoconversion of colourless compounds, namely, photocatalysis, chromophore activation, and the formation of an electron donor/acceptor (EDA) complex. However, the use of photoreactive coloured organic compounds is the optimal strategy to boost visible photons as ideal reagents in synthetic protocols. In view of such premises, the present review aims to provide its readership with a collection of recent photochemical strategies facilitated via direct light absorption by coloured molecules. The protocols have been classified and presented according to the nature of the intermediate/excited state achieved during the transformation.

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.

Interrupted Dance of Five Heteroatoms: Reinventing Ozonolysis to Make Geminal Alkoxyhydroperoxides from C═N Bonds

Four heteroatoms dance in the cascade of four pericyclic reactions initiated by ozonolysis of C═N bonds. Switching from imines to semicarbazones introduces the fifth heteroatom that slows this dance, delays reaching the thermodynamically favorable escape path, and allows efficient interception of carbonyl oxides (Criegee intermediates, CIs) by an external nucleophile. The new three-component reaction of alcohols, ozone, and oximes/semicarbazones greatly facilitates synthetic access to monoperoxyacetals (alkoxyhydroperoxides).

Aziridination via Nitrogen-Atom Transfer to Olefins from Photoexcited Azoxy-Triazenes

Herein, we report that readily accessible azoxy-triazenes can serve as nitrogen atom sources under visible light excitation for the phthalimido-protected aziridination of alkenes. This approach eliminates the need for external oxidants, precious transition metals, and photocatalysts, marking a departure from conventional methods. The versatility of this transformation extends to the selective aziridination of both activated and unactivated multisubstituted alkenes of varying electronic profiles. Notably, this process avoids the formation of competing C-H insertion products. The described protocol is operationally simple, scalable, and adaptable to photoflow conditions. Mechanistic studies support the idea that the photofragmentation of azoxy-triazenes results in the generation of a free singlet nitrene. Furthermore, a mild photoredox-catalyzed N-N cleavage of the protecting group to furnish the free aziridines is reported. Our findings contribute to the advancement of sustainable and practical methodologies for the synthesis of nitrogen-containing compounds, showcasing the potential for broader applications in synthetic chemistry.

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.

Carbonyl cross-metathesis via deoxygenative gem-di-metal catalysis

Carbonyls and alkenes are versatile functional groups, whose reactivities are cornerstones of organic synthesis. The selective combination of two carbonyls to form an alkene-a carbonyl cross-metathesis-would be a valuable tool for their exchange. Yet, this important synthetic challenge remains unsolved. Although alkene/alkene and alkene/carbonyl cross-metathesis reactions are known, there is a lack of analogous methods for deoxygenative cross-coupling of two carbonyl compounds. Here we report a pair of strategies for the cross-metathesis of unbiased carbonyls, allowing an aldehyde to be chemo- and stereoselectively combined with another aldehyde or ketone. These mild, catalytic methods are promoted by earth-abundant metal salts and enable rapid access to an unprecedentedly broad range of either Z- or E-alkenes by two distinct mechanisms-entailing transiently generated (1) carbenes and ylides (via Fe catalysis) or (2) doubly nucleophilic gem-di-metallics (via Cr catalysis).

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.

Iron-incorporated metal-organic frameworks for oxidative cleavage of trans-anethole to p-anisaldehyde

An iron-incorporated Zn-MOF catalyst Zn-bpydc·Fe was fabricated for the oxidative cleavage of trans-anethole to p-anisaldehyde under facile conditions, under 1 atm of O2. The Fe coordinated bipyridine serves as the catalytically active center inside the structural skeleton of Zn-MOFs. This work affords a new avenue for the mild oxidation of olefins.

Visible-light-induced iron-catalyzed reduction of nitroarenes to anilines

An efficient visible-light-induced iron-catalyzed reduction of nitroarenes to anilines by using N-ethylmorpholine (NEM) as a reductant under mild conditions has been developed. The reaction proceeds with photosensitizer-free conditions and features good to excellent yields and broad functional group tolerance. Preliminary mechanistic investigations showed that this reaction was conducted via ligand-to-metal (NEM to Fe3+) charge transfer and nitro triplet biradical-induced hydrogen atom transfer processes.

Carbon-Carbon Bond Cleavage for Late-Stage Functionalization

Late-stage functionalization (LSF) introduces functional group or structural modification at the final stage of the synthesis of natural products, drugs, and complex compounds. It is anticipated that late-stage functionalization would improve drug discovery's effectiveness and efficiency and hasten the creation of various chemical libraries. Consequently, late-stage functionalization of natural products is a productive technique to produce natural product derivatives, which significantly impacts chemical biology and drug development. Carbon-carbon bonds make up the fundamental framework of organic molecules. Compared with the carbon-carbon bond construction, the carbon-carbon bond activation can directly enable molecular editing (deletion, insertion, or modification of atoms or groups of atoms) and provide a more efficient and accurate synthetic strategy. However, the efficient and selective activation of unstrained carbon-carbon bonds is still one of the most challenging projects in organic synthesis. This review encompasses the strategies employed in recent years for carbon-carbon bond cleavage by explicitly focusing on their applicability in late-stage functionalization. This review expands the current discourse on carbon-carbon bond cleavage in late-stage functionalization reactions by providing a comprehensive overview of the selective cleavage of various types of carbon-carbon bonds. This includes C-C(sp), C-C(sp2), and C-C(sp3) single bonds; carbon-carbon double bonds; and carbon-carbon triple bonds, with a focus on catalysis by transition metals or organocatalysts. Additionally, specific topics, such as ring-opening processes involving carbon-carbon bond cleavage in three-, four-, five-, and six-membered rings, are discussed, and exemplar applications of these techniques are showcased in the context of complex bioactive molecules or drug discovery. This review aims to shed light on recent advancements in the field and propose potential avenues for future research in the realm of late-stage carbon-carbon bond functionalization.

Escape from Hydrofunctionalization: Palladium Hydride-Enabled Difunctionalization of Conjugated Dienes and Enynes

Palladium hydrides are traditionally employed in hydrofunctionalization (i.e. monofunctionalization) of conjugated dienes and enynes, owning to its facile protic hydropalladation of electron-rich (or neutral) unsaturated bonds. Herein, we report a mild PdH-catalyzed difunctionalization of conjugated dienes and enynes. This protocol is enabled by the chemoselectivity switch of the initial hydropalladation step achieved by visible light enhancement of hydricity of PdH species. This method allows for cascade annulation of dienes and enynes with various easily available and abundant substrates, such as acrylic acids, acrylic amides, and Baylis-Hillman adducts, toward a wide range of alkenyl or alkynyl lactones, lactams, and tetrahydrofurans. This protocol also provides an easy access to complex spiro-fused tricyclic frameworks.

Carbon-to-nitrogen single-atom transmutation of azaarenes

When searching for the ideal molecule to fill a particular functional role (for example, a medicine), the difference between success and failure can often come down to a single atom1. Replacing an aromatic carbon atom with a nitrogen atom would be enabling in the discovery of potential medicines2, but only indirect means exist to make such C-to-N transmutations, typically by parallel synthesis3. Here, we report a transformation that enables the direct conversion of a heteroaromatic carbon atom into a nitrogen atom, turning quinolines into quinazolines. Oxidative restructuring of the parent azaarene gives a ring-opened intermediate bearing electrophilic sites primed for ring reclosure and expulsion of a carbon-based leaving group. Such a 'sticky end' approach subverts existing atom insertion-deletion approaches and as a result avoids skeleton-rotation and substituent-perturbation pitfalls common in stepwise skeletal editing. We show a broad scope of quinolines and related azaarenes, all of which can be converted into the corresponding quinazolines by replacement of the C3 carbon with a nitrogen atom. Mechanistic experiments support the critical role of the activated intermediate and indicate a more general strategy for the development of C-to-N transmutation reactions.

Copper-catalyzed nitration of electron-deficient BN-naphthalene

Under Cu-catalysis, a regioselective nitration of 1,8-dihalogenated BN-naphthalene (ABN) compounds (4a-4c) has been established with the use of tert-butyl nitrite as the nitrating reagent. The syntheses of dihalo-ABN nitro products (6a-6c; halo = Cl, Br and I) were case-studied in conjunction with the first synthesis and characterization of diiodo-ABN compound 4c. The molecular structures of these compounds have been spectroscopically characterized and further confirmed by X-ray single crystal diffraction experiments. This method allows direct regioselective nitration of electron-deficient ABN systems, providing a step-economical entry to novel nitro-ABN structural motifs with potential applications in agrochemicals, materials sciences, and the medicinal and pharmaceutical industries.

Donor-Acceptor Covalent Organic Frameworks as a Heterogeneous Photoredox Catalyst for Scissoring Alkenes to Carbonyl Constituents

The conversion of alkenes to carbonyl constituents via the cleavage of the C═C bond is unique due to its biological and pharmacological significance. Though a number of oxidative C═C cleavage protocols have been demonstrated for terminal and electron-rich alkene systems, none of them were optimized for electron-deficient and conjugated alkenes. In this work, a covalent organic framework containing triphenylamine and triazine units was revealed to cleave the C═C bond of alkenes under very mild conditions involving visible light irradiation due to its photoredox property. The alkenes can be conveniently broken across the double bond to their constituent carbonyl derivatives on light irradiation in the presence of air and the covalent organic framework photocatalyst. This protocol is applicable for a wide range of alkenes in an aqueous acetonitrile medium with high functional group tolerance and regioselectivity. Though the electron-deficient alkenes required tetramethylethylene diamine as a sacrificial donor, the electron-rich alkenes do not demand any additives.

Iron-Promoted Catalytic Activity of Selenium Endowing the Aerobic Oxidative Cracking Reaction of Alkenes

Oxidative cracking of alkenes is a significant process in industry. In this work, it was found that catalyzed by Se/Fe via hybrid mechanisms, the carbon-carbon double bond in alkenes can break to produce carbonyls under mild conditions. Since O2 can be used as a partial oxidant, the employed H2O2 amount can be reduced (90 mol % vs 250 mol %) to avoid the peroxide residues, making the process even safer for operation.

Photoinduced Nitroarenes as Versatile Anaerobic Oxidants for Accessing Carbonyl and Imine Derivatives

Herein, we report a protocol for the anaerobic oxidation of alcohols, amines, aldehydes, and imines promoted by photoexcited nitroarenes. Mechanistic studies support the idea that photoexcited nitroarenes undergo double hydrogen atom transfer (HAT) steps with alcohols and amines to provide the respective ketone and imine products. In the presence of aldehydes and imines, successive HAT and oxygen atom transfer (OAT) events occur to yield carboxylic acids and amides, respectively. This transformation is amenable to a continuous-photoflow setup, which led to reduced reaction times.

Oxidative transformations of olefins employing persulfate/visible light irradiation in water

We present a green and economical approach for the photooxidation of diverse olefins through the use of ammonium persulfate and blue light irradiation, resulting in the formation of vicinal diols from styrenes and aliphatic alkenes, and vinyl esters and diacids from α,β-unsaturated ketones. The involvement of sulfate radicals in the reaction medium was established as the primary species responsible for the selective generation of the products. A significant advantage of the method lies in its broad substrate scope and economic feasibility, making it a promising alternative to conventional transition metal photocatalysis.

Selective Hydrolysis of Primary and Secondary Amides Enabled by Visible Light

Amide hydrolysis is a fundamentally important transformation in organic chemistry. Developing hydrolysis procedures under mild conditions with a broad substrate scope is desirable. Herein, by leveraging a photoresponsive auxiliary o-nitroanilide, we established a mild two-step protocol for the hydrolysis of primary and secondary amides. This protocol is driven by visible light irradiation at room temperature under neutral conditions, which tolerates numerous acid- and base-sensitive functional groups. Various drugs, natural product-, and amino acid-derived amides can be selectively hydrolyzed.

Olefin Dihydroxylation Using Nitroarenes as Photoresponsive Oxidants

Vicinal diols are abundant among natural and synthetic molecules, and also represent valuable intermediates throughout organic synthesis. Olefin dihydroxylation is an effective strategy to access these derivatives owing to the broad range and availability of alkene feedstocks. OsO₄ is among the most used reagents to achieve this transformation, yet its high toxicity and cost remain concerning. Herein, we present a mechanistically distinct strategy for olefin dihydroxylation using nitroarenes as photoresponsive oxidants. Upon purple LEDs irradiation, these species undergo a [3+2]-photocycloaddition with a wide range of olefins to give stable 1,3,2-dioxazolidine intermediates. These species can be accumulated in solution and then reduced in situ to the desired diols, utilising readily accessible and easy to handle solid reagents as H₂ surrogates.

Syntheses of functionalized benzocoumarins by photoredox catalysis

Direct functionalization of inert C(sp³)-H bonds is an attractive synthetic technology for the preparation of pharmaceutically significant compounds in modern synthetic organic chemistry. In this work, we report a new method for the synthesis of functionalized benzocoumarins through the strategy of activation of multiple C-H bonds on 2-aryl toluenes under visible-light-enabled photoredox conditions. This method has the advantages of high functional group compatibility, mild reaction conditions, and effectively avoiding the use of strong oxidants and precious metal catalysts. Detailed mechanistic investigations, including spectroscopic and electrochemical studies, support the reaction's mechanistic course.

Anaerobic Hydroxylation of C(sp3)-H Bonds Enabled by the Synergistic Nature of Photoexcited Nitroarenes

A photoexcited-nitroarene-mediated anaerobic C-H hydroxylation of aliphatic systems is reported. The success of this reaction is due to the bifunctional nature of the photoexcited nitroarene, which serves as the C-H bond activator and the oxygen atom source. Compared to previous methods, this approach is cost- and atom-economical due to the commercial availability of the nitroarene, the sole mediator of the reaction. Because of the anaerobic conditions of the transformation, a noteworthy expansion in substrate scope can be obtained compared to prior reports. Mechanistic studies support that the photoexcited nitroarenes engage in successive hydrogen atom transfer and radical recombination events with hydrocarbons, leading to N-arylhydroxylamine ether intermediates. Spontaneous fragmentation of these intermediates leads to the key oxygen atom transfer products.

Photo-induced redox cascade reaction of nitroarenes and amines

A photo-induced redox cascade reaction has been developed for the chemoselective construction of isoxazolidine derivatives from stable and easily available nitroarenes and amines.

Direct Reaction of Nitroarenes and Thiols via Photodriven Oxygen Atom Transfer for Access to Sulfonamides

Sulfonamide is a common motif in medicines and agrochemicals. Typically, this class of functional groups is prepared by reacting amines with sulfonyl chlorides that are presynthesized from nitro compounds and thiols, respectively. Here, we report a novel strategy that directly couples nitro compounds and thiols to form sulfonamides atom- and redox-economically. Mechanistic studies suggest our reaction proceeds via direct photoexcitation of nitroarenes that eventually transfers the oxygen atoms from the nitro group to the thiol unit.

Palladium Hydride-Enabled Hydroalkenylation of Strained Molecules

We report the first palladium hydride enabled hydroalkenylation of strained molecules. This new mild protocol proceeds via a regio- and chemoselective hydropalladation step, followed by a photoinduced radical alkyl Heck reaction. This methodology represents a new reactivity mode for strained molecules and opens new avenues for photoinduced palladium catalysis. The reaction is compatible with a wide range of functional groups and can be applied to complex structures, delivering a diverse array of highly valuable and modifiable alkenylated cyclobutanes and cyclopropanes. A hydroalkenylation/diastereoselective rearrangement cascade toward a cyclopentene scaffold has also been demonstrated.

A switchable redox annulation of 2-nitroarylethanols affording N-heterocycles: photoexcited nitro as a multifunctional handle

The efficient transformation of nitroaromatics to functional molecules such as N-heterocycles has been an attractive and significant topic in synthesis chemistry. Herein, a photoexcited nitro-induced strategy for switchable annulations of 2-nitroarylethanols was developed to construct N-heterocycles including indoles, N-hydroxyl oxindoles and N-H oxindoles. The metal- and photocatalyst-free reaction proceeds through intramolecular redox C-N coupling of branched hydroxyalkyl and nitro units, which is initiated by a double hydrogen atom abstraction (d-HAA) process. The key to the switchable reaction outcomes is the mediation of a diboron reagent by its favorable oxy-transfer reactivity to in situ generated nitroso species. The utility of this protocol was well demonstrated by broad substrate scope, excellent yields, functional group tolerance and wide applications. Finally, detailed mechanistic studies were performed, and kinetic isotope effect (KIE) experiments indicate that the homolysis of the C-H bond is involved in the rate-determining step.