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.

Metal-free photocatalytic cross-electrophile coupling enables C1 homologation and alkylation of carboxylic acids with aldehydes

In contemporary drug discovery, enhancing the sp3-hybridized character of molecular structures is paramount, necessitating innovative synthetic methods. Herein, we introduce a deoxygenative cross-electrophile coupling technique that pairs easily accessible carboxylic acid-derived redox-active esters with aldehyde sulfonyl hydrazones, employing Eosin Y as an organophotocatalyst under visible light irradiation. This approach serves as a versatile, metal-free C(sp3)-C(sp3) cross-coupling platform. We demonstrate its synthetic value as a safer, broadly applicable C1 homologation of carboxylic acids, offering an alternative to the traditional Arndt-Eistert reaction. Additionally, our method provides direct access to cyclic and acyclic β-arylethylamines using diverse aldehyde-derived sulfonyl hydrazones. Notably, the methodology proves to be compatible with the late-stage functionalization of peptides on solid-phase, streamlining the modification of intricate peptides without the need for exhaustive de-novo synthesis.

Visible-Light-Mediated Divergent and Regioselective Vicinal Difunctionalization of Styrenes with Arylazo Sulfones

Activated by visible light, arylazo sulfones can serve as multifaceted reactants and are employed in diazenylation, sulfonylation, and arylation reactions under (photo)catalyst-free conditions. Such versatile reactivity enabled us to develop an operationally simple, regioselective, and tunable difunctionalization of styrenes with arylazo sulfones to produce α-sulfonyl arylhydrazones and 1,2-alkoxyarylated products in moderate to excellent yields. Furthermore, such difunctionalized products have been exploited as key building blocks for the synthesis of various heterocycles.

Single-Atoms on Crystalline Carbon Nitrides for Selective C─H Photooxidation: A Bridge to Achieve Homogeneous Pathways in Heterogeneous Materials

Single-atom catalysis is a field of paramount importance in contemporary science due to its exceptional ability to combine the domains of homogeneous and heterogeneous catalysis. Iron and manganese metalloenzymes are known to be effective in C─H oxidation reactions in nature, inspiring scientists to mimic their active sites in artificial catalytic systems. Herein, a simple and versatile cation exchange method is successfully employed to stabilize low-cost iron and manganese single-atoms in poly(heptazine imides) (PHI). The resulting materials are employed as photocatalysts for toluene oxidation, demonstrating remarkable selectivity toward benzaldehyde. The protocol is then extended to the selective oxidation of different substrates, including (substituted) alkylaromatics, benzyl alcohols, and sulfides. Detailed mechanistic investigations revealed that iron- and manganese-containing photocatalysts work through a similar mechanism via the formation of high-valent M═O species. Operando X-ray absorption spectroscopy (XAS) is employed to confirm the formation of high-valent iron- and manganese-oxo species, typically found in metalloenzymes involved in highly selective C─H oxidations.

Selective Nsp2- and Csp2- photografting of Au-Surface by aryldiazonium salts and arylazo sulfonates

Arylazo sulfonates (Ar-N=N-SO3Na) were found to undergo photografting on gold surface through both Au-Nsp2- and Au-Csp2- bond formation. The functionalized materials have been fully characterized by Infrared reflection absorption spectroscopy (IRRAS), Raman, XPS, DFT calculations and UV-Vis absorption spectroscopy. These methods permit to evidence aromatic substituents (IRRAS), the Au-N=N signature (Raman and XPS spectroscopy) and the Bond Dissociation Energy values of the two linkages (DFT calculation). The grafting proceeds through two competitive paths, namely a stepwise reaction involving an aryl radical (for the formation of the Au-Ar bonds) and a concerted reaction on the surface of gold (for Au-N=N-Ar bond formation). The occurrence of an aryl radical upon irradiation has been fully evidenced via EPR spectroscopy. Finally, E/Z photoisomerisation of the N=N bonds present on prepared few layers films was observed by means of UV-Vis absorption spectroscopy.

A General Metal-Free Protocol for the Visible-Light-Driven Protection of Carbonyls

The protection of a carbonyl is a standard procedure to mask the reactivity of this group towards nucleophiles and reductants. The present work pointed out the potentialities of arylazo sulfones as non-ionic photoacid generators (PAGs) to promote the efficient conversion of aldehydes and ketones into acetals, ketals, and 1,3-oxazolidines upon visible-light irradiation. This approach employed mild conditions and exhibited an easy scalability and a broad scope (80 examples included).

Effect of Arylazo Sulfones on DNA: Binding, Cleavage, Photocleavage, Molecular Docking Studies and Interaction with A375 Melanoma and Non-Cancer Cells

A set of arylazo sulfones, known to undergo N-S bond cleavage upon light exposure, has been synthesized, and their activity in the dark and upon irradiation towards DNA has been investigated. Their interaction with calf-thymus DNA has been examined, and the significant affinity observed (most probably due to DNA intercalation) was analyzed by means of molecular docking "in silico" calculations that pointed out polar contacts, mainly via the sulfonyl moiety. Incubation with plasmid pBluescript KS II revealed DNA cleavage that has been studied over time and concentration. UV-A irradiation considerably improved DNA damage for most of the compounds, whereas under visible light the effect was slightly lower. Moving to in vitro experiments, irradiation was found to slightly enhance the death of the cells in the majority of the compounds. Naphthylazosulfone 1 showed photo-disruptive effect under UV-A irradiation (IC₅₀ ~13 μΜ) followed by derivatives 14 and 17 (IC₅₀ ~100 μΜ). Those compounds were irradiated in the presence of two non-cancer cell lines and were found equally toxic only upon irradiation and not in the dark. The temporal and spatial control of light, therefore, might provide a chance for these novel scaffolds to be useful for the development of phototoxic pharmaceuticals.

Friedel-Crafts arylation of aldehydes with indoles utilizing arylazo sulfones as the photoacid generator

A versatile, inexpensive and sustainable protocol for the preparation of valuable bis-indolyl methanes via visible light-mediated, metal-free Friedel-Crafts arylation has been developed. The procedure, that exploits the peculiar behavior of arylazo sulfones as non-ionic photoacid generators (PAGs), was applied to the conversion of a variety of aliphatic and aromatic aldehydes into diarylmethanes in good to highly satisfactory yields, employing a low-catalyst loading (0.5 mol%) and irradiation at 456 nm.

Alkyl Radical Generation via C–C Bond Cleavage in 2-Substituted Oxazolidines

There is an urgent need to develop uncharged radical precursors to be activated under mild photocatalyzed conditions. 2-Substituted-1,3-oxazolidines (E_ox < 1.3 V vs SCE, smoothly prepared from the corresponding aldehydes) have been herein employed for the successful release of tertiary, α-oxy, and α-amido radicals under photo-organo redox catalysis. The reaction relies on the unprecedented C–C cleavage occurring from the radical cation of these heterocyclic derivatives. Such a protocol is applied to the visible-light-driven conjugate radical addition onto Michael acceptors and vinyl (hetero)arenes under mild metal-free conditions.

Photocatalyzed Functionalization of Alkenoic Acids in 3D-Printed Reactors

The valorization of alkenoic acids possibly deriving from biomass (fumaric and citraconic acids) was carried out through conversion in important building blocks, such as γ-keto acids and succinic acid derivatives. The functionalization was carried out by addition onto the C=C double bond of radicals generated under photocatalyzed conditions from suitable hydrogen donors (mainly aldehydes) and by adopting a decatungstate salt as the photocatalyst. Syntheses were performed under batch (in a glass vessel) and flow (by using 3D-printed reactors) conditions. The design of the latter reactors allowed for an improved yield and productivity.

Recent Advances in Light-Induced Selenylation

Selenium-containing organic molecules have recently found a plethora of applications, ranging from organic synthesis to pharmacology and material sciences. In view of these concepts, the development of mild, efficient, and general protocols for the formation of C-Se bonds is desirable, and light induced approaches are appealing ways. The aim of this Review is to provide the reader with the most recent examples of light promoted selenylation processes.

Arylazo Sulfones as Nonionic Visible-Light Photoacid Generators

The selective visible-light-driven generation of a weak acid (sulfinic acid, in nitrogen-purged solutions) or a strong acid (sulfonic acid, in oxygen-purged solutions) by using shelf-stable arylazo sulfones was developed. These sulfones were then used for the green, smooth, and efficient photochemical catalytic protection of several (substituted) alcohols (and phenols) as tetrahydropyranyl ethers or acetals.

Diradicals Photogeneration from Chloroaryl‐Substituted Carboxylic Acids

With the aim of generating new, thermally inaccessible diradicals, potentially able to induce a double‐strand DNA cleavage, the photochemistry of a set of chloroaryl‐substituted carboxylic acids in polar media was investigated. The photoheterolytic cleavage of the Ar−Cl bond occurred in each case to form the corresponding triplet phenyl cations. Under basic conditions, the photorelease of the chloride anion was accompanied by an intramolecular electron‐transfer from the carboxylate group to the aromatic radical cationic site to give a diradical species. This latter intermediate could then undergo CO₂ loss in a structure‐dependent fashion, according to the stability of the resulting diradical, or abstract a hydrogen atom from the medium. In aqueous environment at physiological pH (pH=7.3), both a phenyl cation and a diradical chemistry was observed. The mechanistic scenario and the role of the various intermediates (aryl cations and diradicals) involved in the process was supported by computational analysis.

Visible Light-Driven, Gold(I)-Catalyzed Preparation of Symmetrical (Hetero)biaryls by Homocoupling of Arylazo Sulfones

The preparation of symmetrical (hetero)biaryls via arylazo sulfones has been successfully carried out upon visible light irradiation in the presence of PPh₃AuCl as the catalyst. The present protocol led to the efficient synthesis of a wide range of target compounds in an organic-aqueous solvent under photocatalyst-free conditions.

Photogenerated aryl mesylate and aryl diethyl phosphate radical cations: a time-resolved spectroscopy investigation

The photoinduced electron transfer reaction of selected aryl sulfonates and phosphates with K2S2O8 in a MeCN water (9 : 1) mixture has been investigated by LFP experiments.

Aryl-Cl vs heteroatom-Si bond cleavage on the route to the photochemical generation of σ,π-heterodiradicals

The photochemistry of aryl chlorides having a X-SiMe₃ group (X = O, NR, S, SiMe₂) tethered to the aromatic ring has been investigated in detail, with the aim to generate valuable ϭ,π-heterodiradicals. Two competitive pathways arising from the excited triplet state of the aromatics have been observed, namely heterolysis of the aryl-chlorine bond and homolysis of the X-silicon bond. The former path is found in chlorinated phenols and anilines, whereas the latter is exclusive in the case of silylated thiophenols and aryl silanes. A combined experimental/computational approach was pursued to explain such a photochemical behavior.Graphical abstract.

Dyedauxiliary Group Strategy for the α-Functionalization of Ketones and Esters

The synthesis of α-arylketones and α-arylazo esters has been achieved in mixed organic-aqueous media under photocatalyst- and metal-free conditions via visible light activation of arylazo sulfones in the presence of enol silyl ethers and ketene silyl acetals, respectively. The process took place efficiently and exhibits an excellent tolerance for a broad variety of functional groups.

Direct Photocatalyzed Hydrogen Atom Transfer (HAT) for Aliphatic C-H Bonds Elaboration

Direct photocatalyzed hydrogen atom transfer (d-HAT) can be considered a method of choice for the elaboration of aliphatic C–H bonds. In this manifold, a photocatalyst (PC_HAT) exploits the energy of a photon to trigger the homolytic cleavage of such bonds in organic compounds. Selective C–H bond elaboration may be achieved by a judicious choice of the hydrogen abstractor (key parameters are the electronic character and the molecular structure), as well as reaction additives. Different are the classes of PCs_HAT available, including aromatic ketones, xanthene dyes (Eosin Y), polyoxometalates, uranyl salts, a metal-oxo porphyrin and a tris(amino)cyclopropenium radical dication. The processes (mainly C–C bond formation) are in most cases carried out under mild conditions with the help of visible light. The aim of this review is to offer a comprehensive survey of the synthetic applications of photocatalyzed d-HAT.

Photohomolysis and Photoheterolysis in Aryl Sulfonates and Aryl Phosphates

The photochemical behaviour of selected aryl sulfonates and phosphates (ArOX) in polar and nonpolar media has been investigated by laser flash photolysis (LFP) experiments. Two main pathways have been identified, namely the photohomolysis of the ArO-X bond or the photoheterolysis of the Ar-OX bond depending on the nature of the leaving group (OX) and on the nature of the substituents on the aromatic ring. In nonpolar solvents the esters are quite photostable due to an efficient triplet deactivation. In polar solvents, the homolytic fragmentation of the ArO-S bond from the exited singlets was found in aryl sulfonates bearing moderately electron-donating groups as well as electron-withdrawing groups. In electron-rich aryl phosphates and sulfonates photoheterolysis of the Ar-OP/Ar-OS bond took place as the exclusive pathway.

Blue light driven free-radical polymerization using arylazo sulfones as initiators

The polymerization of a broad range of electron-poor olefins has been achieved under free-radical conditions by using arylazo sulfones as visible light photoinitiators.

Metal-Free Trifluoromethylthiolation of Arylazo Sulfones

A visible-light-driven protocol for the synthesis of aryl trifluoromethyl thioethers under photocatalyst- and metal-free conditions has been pursued. The procedure exploits the peculiar properties of arylazo sulfones (having electron-rich or electron-poor substituents on the (hetero)aromatic ring) as photochemical precursors of aryl radicals and S-trifluoromethyl arylsulfonothioates as easy-to-handle trifluoromethylthiolating agents.

Metal-free synthesis of biarenes via photoextrusion in di(tri)aryl phosphates

A metal-free route for the synthesis of biarenes has been developed. The approach is based on the photoextrusion of a phosphate moiety occurring upon irradiation of biaryl- and triaryl phosphates. The reaction involves an exciplex as the intermediate and it is especially suitable for the preparation of electron-rich biarenes.

Dyedauxiliary Groups, an Emerging Approach in Organic Chemistry. The Case of Arylazo Sulfones

The number of research papers that report photocatalyst-free protocols is currently increasing. Among the different approaches proposed, the conversion of a strong C-X bond of a stable substrate into a photolabile reactive moiety has been recently proposed. In this Synopsis, we introduce the so-dubbed dyedauxiliary group strategy by focusing on arylazo sulfones that are bench stable and visible-light responsive derivatives of anilines that have been exploited as precursors of a wide range of intermediates, including carbon-centered radicals as well as aryl cations.

Generation of Alkyl Radicals: From the Tyranny of Tin to the Photon Democracy

Alkyl radicals are key intermediates in organic synthesis. Their classic generation from alkyl halides has a severe drawback due to the employment of toxic tin hydrides to the point that "flight from the tyranny of tin" in radical processes was considered for a long time an unavoidable issue. This review summarizes the main alternative approaches for the generation of unstabilized alkyl radicals, using photons as traceless promoters. The recent development in photochemical and photocatalyzed processes enabled the discovery of a plethora of new alkyl radical precursors, opening the world of radical chemistry to a broader community, thus allowing a new era of photon democracy.

Antimony-Oxo Porphyrins as Photocatalysts for Redox-Neutral C-H to C-C Bond Conversion

The use of high-valent antimony-oxo porphyrins as visible-light photocatalysts operating via direct hydrogen atom transfer has been demonstrated. Computational analysis indicates that the triplet excited state of these complexes shows an oxyl radical behavior, while the Sb^V center remains in a high-valent oxidation state, serving uniquely to carry the oxo moiety and activate the coordinated ligands. This porphyrin-based system has been exploited upon irradiation to catalyze C-H to C-C bond conversion via the addition of hydrogen donors (ethers and aldehydes) onto Michael acceptors in a redox-neutral fashion without the need of any external oxidant. Laser flash photolysis experiments confirmed that the triplet excited state of the photocatalyst triggers the desired C-H cleavage.

C(sp3)–H functionalizations of light hydrocarbons using decatungstate photocatalysis in flow

Direct activation of gaseous hydrocarbons remains a major challenge for the chemistry community. Because of the intrinsic inertness of these compounds, harsh reaction conditions are typically required to enable C(sp3)–H bond cleavage, barring potential applications in synthetic organic chemistry. Here, we report a general and mild strategy to activate C(sp3)–H bonds in methane, ethane, propane, and isobutane through hydrogen atom transfer using inexpensive decatungstate as photocatalyst at room temperature. The corresponding carbon-centered radicals can be effectively trapped by a variety of Michael acceptors, leading to the corresponding hydroalkylated adducts in good isolated yields and high selectivity (38 examples).

Photocatalyzed syntheses of phenanthrenes and their aza-analogues. A review

Phenanthrenes and their aza-analogues have important applications in materials science and in medicine. Aim of this review is to collect recent reports describing their synthesis, which make use of radical cyclizations promoted by a visible light-triggered photocatalytic process.

Simultaneous Photografting of Two Organic Groups on a Gold Surface by using Arylazo Sulfones as Single Precursors

Arylazo sulfones have been exploited as photoactivatable substrates for the simultaneous photografting of both aryl and methanesulfonyl groups on a gold surface. The obtained samples have been characterized by different spectroscopic techniques including ellipsometry and electrochemistry, infrared reflection absorption, surface-enhanced Raman spectroscopy, XPS, and AFM. Grafting occurs through a simple N-S cleavage and not, as usually observed with aromatic precursors, by electron transfer.

Hydro/Deutero Deamination of Arylazo Sulfones under Metal- and (Photo)Catalyst-Free Conditions

Hydrodeaminated and monodeuterated aromatics were obtained via a visible-light driven reaction of arylazo sulfones. Deuteration occurs efficiently in deuterated media such as isopropanol-d8 or in THF-d8/water mixtures and exhibits a high tolerance to the nature and the position of the aromatic substituents.

Unraveling the Thermal Isomerization Mechanisms of Heteroaryl Azoswitches: Phenylazoindoles as Case Study

The research on heteroaromatic azoswitches has been blossoming in recent years due to their astonishingly broad range of properties. Minimal chemical modifications can drastically change the demeanor of these switches, regarding photophysical and (photo)chemical properties, promoting them as ideal scaffolds for a vast variety of applications based on bistable light-addressable systems. However, most of the characteristics exhibited by heteroaryl azoswitches were found empirically, and only a few works focus on their rationalization. Herein we report on a mechanistic study employing phenylazoindoles as a model reference, combining spectroscopic experiments with comprehensive computational analysis. This approach will elucidate the intrinsic correlations between the molecular structure of the switch and its thermal behavior, allowing a more rational design transferable to various heteroaryl azoswitches.

Wavelength dependence and wavelength selectivity in photochemical reactions

Our study describes how organic photochemists can modify the outcome of a reaction by tuning the wavelength.

Selective C(sp3 )-H Aerobic Oxidation Enabled by Decatungstate Photocatalysis in Flow

A mild and selective C(sp3 )-H aerobic oxidation enabled by decatungstate photocatalysis has been developed. The reaction can be significantly improved in a microflow reactor enabling the safe use of oxygen and enhanced irradiation of the reaction mixture. Our method allows for the oxidation of both activated and unactivated C-H bonds (30 examples). The ability to selectively oxidize natural scaffolds, such as (-)-ambroxide, pregnenolone acetate, (+)-sclareolide, and artemisinin, exemplifies the utility of this new method.

Versatile cross-dehydrogenative coupling of heteroaromatics and hydrogen donors via decatungstate photocatalysis

A facile sunlight-induced derivatization of heteroaromatics via photocatalyzed C-H functionalization in amides, ethers, alkanes and aldehydes is described. Tetrabutylammonium decatungstate (TBADT) was used as the photocatalyst and allowed to carry out the process under mild conditions.

Search for a photoinduced (site-selective) cleavage of the Ar-Cl bond in dichloroanisoles

The site-selective cleavage of an Ar-X bond in polyhalogenated aromatics is an important tool in synthetic planning especially when more than one identical halogen atom is present. An alternative to the usual metal-catalyzed cleavage is represented by photochemistry although only a few examples have been reported. We then investigated the feasibility of the site-selective photodechlorination of some dichloroanisoles through a combined experimental and computational study. In the case of 2,4-dichloroanisole, selective detachment of the chlorine atom at the ortho position with respect to the OMe group was observed upon photohomolysis (in cyclohexane) or photoheterolysis (in MeOH) of the Ar-Cl bond. In the latter case, 5-chloro-2-methoxy-1,1'-biphenyl was exclusively formed upon reaction of the resulting phenyl cation with benzene. The substitution of an OH group for a OMe group was detrimental since a lower photoreactivity resulted with no improvement in the selectivity.