Generation and Application of Aryl Radicals Under Photoinduced Conditions

Photoinduced aryl radical generation is a powerful strategy in organic synthesis that facilitates the formation of diverse carbon-carbon and carbon-heteroatom bonds. The synthetic applications of photoinduced aryl radical formation in the synthesis of complex organic compounds, including natural products, physiologically significant molecules, and functional materials, have received immense attention. An overview of current developments in photoinduced aryl radical production methods and their uses in organic synthesis is given in this article. A generalized idea of how to choose the reagents and approach for the generation of aryl radicals is described, along with photoinduced techniques and associated mechanistic insights. Overall, this article offers a critical assessment of the mechanistic results as well as the selection of reaction parameters for specific reagents in the context of radical cascades, cross-coupling reactions, aryl radical functionalization, and selective C-H functionalization of aryl substrates.

Synthesis of β,β-Dithioketones by Merging C-C and C-S Bond Cleavage in [1 + 1 + 1 + 1 + 1 + 1] Annulation

An unconventional [1 + 1 + 1 + 1 + 1 + 1] annulation process was developed for the construction of β,β-dithioketones by merging C-C and C-S bond cleavage. In this reaction, rongalite concurrently served as triple C1 units, dual sulfur(II) synthons, and a reductant for the first time. Mechanism investigation indicated that the reaction involved the self-mediated valence state change of rongalite. By performing this step-economical method, the challenging construction of C5-substituted 1,3-dithiane can be achieved under mild and simple conditions.

Aryl sulfonyl fluoride synthesis via organophotocatalytic fluorosulfonylation of diaryliodonium salts

A mild and efficient synthesis of various aryl sulfonyl fluorides from diaryliodonium salts under organophotocatalysis via a radical sulfur dioxide insertion and fluorination strategy is presented. Diaryliodonium salts are used as aryl radical precursors, the 1,4-diazabicyclo[2.2.2]octane bis(sulfur dioxide) adduct (DABSO) as a sulfonyl source and cheap KHF2 as a desirable fluorine source, respectively. Notably, the electronic properties of substituents on the aromatic rings in diaryliodonium salts have a significant influence on the reaction yields.

Catalyst-free thiophosphorylation of in situ formed ortho-quinone methides

A metal-, chloride reagent and base-free thiophosphorylation reaction of in situ formed ortho-quinone methide (o-QM) to synthesize functionalized thiophosphates has been developed. The reaction is an atom-economical process, producing water as the sole byproduct. (EtO)₂P(O)SH functions as both a Brønsted acid and nucleophilic thiolate to produce the o-QM intermediate and the thiophosphate product, respectively. The aza o-QMs were also successfully thiophosphorylated in the presence of catalytic TsOH to form sulfonamido thiophosphates.

Mechanochemical Electrophilic Mono- or Disulfur Transfer: Construction of P(O)-S or P(O)-S-S Bonds

Under mechanochemically induced conditions, a wide range of diarylphosphine oxides or H-phosphonates react with trisulfide dioxides to afford various thiophosphate derivatives in good yields. Selective S-S bond cleavage of trisulfide dioxides determined by connecting groups is proposed as the key step in the construction of P(O)-S or P(O)-S-S bonds, which is supported by calculations.

Multicomponent reactions and photo/electrochemistry join forces: atom economy meets energy efficiency

Visible-light photoredox catalysis has been regarded as an extremely powerful tool in organic chemistry, bringing the spotlight back to radical processes. The versatility of photocatalyzed reactions has already been demonstrated to be effective in providing alternative routes for cross-coupling as well as multicomponent reactions. The photocatalyst allows the generation of high-energy intermediates through light irradiation rather than using highly reactive reagents or harsh reaction conditions. In a similar vein, organic electrochemistry has experienced a fruitful renaissance as a tool for generating reactive intermediates without the need for any catalyst. Such milder approaches pose the basis toward higher selectivity and broader applicability. In photocatalyzed and electrochemical multicomponent reactions, the generation of the radical species acts as a starter of the cascade of events. This allows for diverse reactivity and the use of reagents is usually not covered by classical methods. Owing to the availability of cheaper and more standardized photo- and electrochemical reactors, as well as easily scalable flow-setups, it is not surprising that these two fields have become areas of increased research interest. Keeping these in view, this review is aimed at providing an overview of the synthetic approaches in the design of MCRs involving photoredox catalysis and/or electrochemical activation as a crucial step with particular focus on the choice of the difunctionalized reagent.

DABSO – A Reagent to Revolutionize Organosulfur Chemistry

The introduction of easy-to-handle SO2 surrogates has transformed the field of sulfur chemistry, enabling methodologies utilizing SO2 to be carried out without specialized apparatus, and paving the way for the development of new procedures. This review highlights some of the varied and significant developments associated with one of the most prominent SO2 surrogates: DABSO.

1 Introduction

2 DABSO

3 Reactions with Nucleophilic Reagents

4 Metal-Catalyzed Reactions

4.1 Palladium-Catalyzed Reactions

4.2 Other Transition-Metal Catalysis

5 Radical Reactions

5.1 Aryldiazonium Salts

5.2 Other Aryl Radical Precursors

5.3 Alkyl Radical Precursors

6 Conclusion

Electrochemical oxygenation of sulfides with molecular oxygen or water: switchable preparation of sulfoxides and sulfones

A practical and eco-friendly method for the controllable aerobic oxygenation of sulfides by electrochemical catalysis was developed. The switchable preparation of sulfoxides and sulfones was effectively controlled by reaction time, in which both molecular oxygen and water can be used as the oxygen source under catalyst and external oxidant-free conditions. The electrochemical protocol features a broad substrate scope and excellent site selectivity and is successfully applied to the modification of some sulfide-containing pharmaceuticals and their derivatives.

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.

Photoredox-catalyzed synthesis of sulfones through deaminative insertion of sulfur dioxide

Katritzky salts are used as the alkyl radical precursors with the insertion of sulfur dioxide under photoredox catalysis. This transformation first enables direct generation of various alkylsulfonyl radicals by photoinduced single electron reduction, leading to diverse dialkyl sulfones in good to excellent yields. A radical pathway is proposed under visible-light induced conditions with the insertion of sulfur dioxide.

Photoredox-catalyzed sulfonylation of alkyl iodides, sulfur dioxide, and electron-deficient alkenes

A photoredox-catalyzed sulfonylation of alkyl iodides, sulfur dioxide, and electron-deficient alkenes under mild conditions is achieved. This reaction proceeds through alkyl radicals formed in situ from alkyl iodides under visible light irradiation in the presence of a photoredox catalyst. The alkyl radical intermediates would react with sulfur dioxide leading to alkylsulfonyl radicals, which would be trapped by electron-deficient alkenes giving rise to alkyl sulfones. Various functional groups including nitro, halo, acetyl, sufonyl, and pyridinyl are all tolerated under the photoredox conditions.

Visible-light mediated sulfonylation of thiols via insertion of sulfur dioxide

Photocatalytically generated sulfenyl radicals were coupled with arylsulfonyl radicals at room temperature for accessing unsymmetrical organic thiosulfonates.

Substituted Hantzsch esters as radical reservoirs with the insertion of sulfur dioxide under photoredox catalysis

A three-component reaction between 4-substituted Hantzsch esters, DABCO·(SO₂)₂, and vinyl azides in the presence of photoredox catalysts under visible light irradiation is developed. A range of (Z)-2-(alkylsulfonyl)-1-arylethen-1-amines is obtained in moderate to good yields with good regioselectivity and stereoselectivity.

Visible-light-induced multicomponent cascade cycloaddition involving N-propargyl aromatic amines, diaryliodonium salts and sulfur dioxide: rapid access to 3-arylsulfonylquinolines

A visible-light-induced, Eosin Y catalyzed three-component synthesis of 3-arylsulfonylquinoline derivatives through N-propargyl aromatic amines, diaryliodonium salts and sulfur dioxide has been discovered. This transformation represents an efficient and attractive method for the straightforward synthesis of 3-arylsulfonylquinoline derivatives via the formation of C-S bonds and quinolines in one step. In addition, it exhibits good substrate scope and functional group tolerance. The use of easy-to-handle diaryliodonium salts, sulfur dioxide sources and the cheap photocatalyst Eosin Y together with facile operation at room temperature makes this protocol very practical.

Insertion of sulfur dioxide via a radical process: an efficient route to sulfonyl compounds

This review is focused on the recent advances in the chemistry of sulfur dioxide fixation through a radical process. Diverse sulfonyl compounds can be obtained efficiently under mild conditions.

Photoinduced synthesis of (E)-vinyl sulfones through the insertion of sulfur dioxide

Under visible light irradiation, a three-component reaction of potassium alkyltrifluoroborates, sulfur dioxide and alkynes with the assistance of photocatalysis and in the presence of copper(ii) triflate is described. This transformation affords (E)-vinyl sulfones efficiently with excellent regioselectivity and stereoselectivity under mild conditions.

Recent advances in the sulfonylation of C–H bonds with the insertion of sulfur dioxide

Recent advances in the sulfonylation of C–H bonds with the insertion of sulfur dioxide are summarized. C–H bond sulfonylation under transition metal catalysis or through a radical process has been developed. In some cases, the sulfonylation can be performed under catalyst- and additive-free conditions, or can be facilitated by visible light irradiation. The efficiency is also studied by merging the radical process and metal catalysis.

Photocatalytic acylarylation of unactivated alkenes with diaryliodonium salts toward indanones and related compounds

A photocatalytic acylarylation of unactivated alkenes using diaryliodonium salts is described, giving 2-benzyl indanones and related compounds in promising yields with excellent diastereoselectivity.