Visible-Light-Driven Synthesis of Arylstannanes from Arylazo Sulfones

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.

Synthesis of Diaryl Tellurides with Sodium Aryltellurites under Mild Conditions

A highly efficient new protocol has been developed for the formation of C-Te bonds, leading to both symmetrical and unsymmetrical diaryl tellurides. This protocol utilizes sodium aryltellurites (4), which can be easily prepared from low-cost aryltelluride trichlorides and NaOH. The synthesis involves the use of 4 and arylazo sulfones as starting materials in the presence of (MeO)2P(O)H. A variety of diaryl tellurides are obtained in moderate to good yields using this method. Importantly, this innovative protocol eliminates the need for traditional, highly toxic aryltellurolating reagents such as diaryl ditellurides and elemental tellurium. This study will bring new vitality to the synthesis of tellurides.

Arylazo sulfones: multifaceted photochemical reagents and beyond

The photochemical action of arylazo sulfones under visible light irradiation has recently gained considerable attention for the construction of carbon-carbon and carbon-heteroatom bonds in organic synthesis. The inherent dyedauxiliary group (-N2SO2R) embedded in the reagent is responsible for the absorption of visible light even in the absence of a photocatalyst, additive or oxidant, leading to the generation of three different radicals, viz. aryl (carbon-centred), sulfonyl (sulphur-centred) and diazenyl (nitrogen-centred) radicals, under different reaction conditions. Encountering a reagent with such a versatile behaviour is quite rare, which makes arylazo sulfones a highly interesting class of compounds. The mild reaction conditions under which these reagents can operate are an added advantage. Recently, they are also being used as non-ionic photoacid generators (PAGs), electron acceptors, and hydrogen atom transfer (HAT) and imination reagents in a number of synthetic transformations. They have displayed substantial damaging effect on the structure of DNA in the presence of light which can lead to their use as phototoxic pharmaceuticals for cancer treatment. Moreover, their photochemistry is also being exploited in polymerization reactions (as photoinitiators) and in materials chemistry (surface modification).

Csp3-H Imination Using Arylazo Sulfone as a N Source: An Approach to Access Imines

An unprecedented Csp3-H imination reaction using arylazo sulfones as the readily accessible and stable N source is reported. The synthetic virtues are demonstrated through mild conditions, simple operation, good air compatibility, and functional group tolerance, as well as suitability for gram-scale reaction. The resulting imines can be further converted to α-amino acids. The presented results shed light on an unusual usage of arylazo sulfones and will inspire novel experimental design by using arylazo sulfones as the N source.

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.

Copper-Promoted One-Pot Sandmeyer-Type Reaction for the Synthesis of N-Aryltriazoles

Herein, we report the copper-catalyzed one-pot Sandmeyer-type reaction of aromatic amines with triazoles to afford N-aryl-1,2,3-triazoles. Diazonium salts, formed from aromatic amines and tert-butyl nitrite in the presence of fluoroboric acid, reacted with triazoles in a copper-catalyzed Sandmeyer-type reaction. The reaction proceeded under mild conditions to afford N-aryltriazoles in moderate to good yields. This method is amenable to a wide range of aromatic amines and triazoles and shows diverse functional group tolerance. Inhibition of the reaction upon the addition of free radical scavengers suggested a radical pathway, in which the aryl radical, copper, and triazole formed a complex that underwent reductive elimination to give aryltriazole compounds; this is consistent with the mechanism underlying the Sandmeyer reaction. Thus, we demonstrate a new effective strategy for the construction of C-N bonds via Sandmeyer-type reactions and a valuable alternative approach for the synthesis of aryltriazole derivatives.

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.

Visible-light-induced one-pot synthesis of sulfonic esters via multicomponent reaction of arylazo sulfones and alcohols

Sulfonic ester is a chemical structure common to many organic molecules, including biologically active compounds. Herein, a visible-light-induced synthetic method to prepare aryl sulfonic ester from arylazo sulfones was developed. In the present study, a one-pot reaction was carried out using arylazo sulfones, DABSO (DABCO·(SO2)2), and alcohols in the presence of CuI as a coupling catalyst and HCl as an additive to yield sulfonic esters via multicomponent reaction. This synthetic method afforded a wide range of sulfonic esters with high yields under mild conditions.

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.

Desulfonylation via Radical Process: Recent Developments in Organic Synthesis

As the "chemical chameleon", sulfonyl-containing compounds and their variants have been merged with various types of reactions for the efficient construction of diverse molecular architectures by taking advantage of their incredible reactive flexibility. Currently, their involvement in radical transformations, in which the sulfonyl group typically acts as a leaving group via selective C-S, N-S, O-S, S-S, and Se-S bond cleavage/functionalization, has facilitated new bond formation strategies which are complementary to classical two-electron cross-couplings via organometallic or ionic intermediates. Considering the great influence and synthetic potential of these novel avenues, we summarize recent advances in this rapidly expanding area by discussing the reaction designs, substrate scopes, mechanistic studies, and their limitations, outlining the state-of-the-art processes involved in radical-mediated desulfonylation and related transformations. With a specific emphasis on their synthetic applications, we believe this review will be useful for medicinal and synthetic organic chemists who are interested in radical chemistry and radical-mediated desulfonylation in particular.

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.

A photocatalyst-free visible-light-mediated solvent-switchable route to stilbenes/vinyl sulfones from β-nitrostyrenes and arylazo sulfones

Photocatalyst-free visible-light-mediated reactions, based on the presence of a visible-light-absorbing functional group in the starting material itself in order to exclude the often costly, hazardous, degradable and difficult to remove or recover photoredox catalysts, have been gaining momentum recently. We have employed this approach to develop a denitrative photocatalyst-free visible-light-mediated protocol for the arylation/sulfonylation of β-nitrostyrenes employing arylazo sulfones (bench-stable photolabile compounds) in a switchable solvent-controlled manner. Arylazo sulfones served as the aryl and sulfonyl radical precursors under blue LED irradiation for the synthesis of trans-stilbenes and (E)-vinyl sulfones in CH3CN and dioxane/H2O 2 : 1, respectively. The absence of any metal, photocatalyst and additive; excellent selectivity (E-stereochemistry) and solvent-switchability; and the use of visible light and ambient temperature are the prime assets of the developed method. Moreover, we report the first photocatalyst-free visible light-driven route to synthesize stilbenes and vinyl sulfones from readily available β-nitrostyrenes.

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.

Generation of aryl radicals by redox processes. Recent progress in the arylation methodology

Arylation methods based on the generation and use of aryl radicals have been a rapidly growing field of research in recent years and currently represent a powerful strategy for carbon – carbon and carbon – heteroatom bond formation. The progress in this field is related to advances in the methods for generation of aryl radicals. The currently used aryl radical precursors include aryl halides, aryldiazonium and diaryliodonium salts, arylcarboxylic acids and their derivatives, arylboronic acids, arylhydrazines, organosulfur(II, VI) compounds and some other compounds. Aryl radicals are generated under mild conditions by single electron reduction or oxidation of precursors induced by conventional reagents, visible light or electric current. A crucial role in the development of the radical arylation methodology belongs to photoredox processes either catalyzed by transition metal complexes or organic dyes or proceeding without catalysts. Unlike the conventional transition metal-catalyzed arylation methods, radical arylation reactions proceed very often at room temperature and have high functional group tolerance. Without claiming to be exhaustive, this review covers the most important advances of the current decade in the generation and synthetic applications of (het)aryl radicals. Examples of reactions are given and mechanistic insights are highlighted.

The bibliography includes 341 references.

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.

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.