Cp2Fe-Mediated Electrochemical Synthesis of Phosphorylated Oxindoles and Indolo[2,1-a]isoquinolin-6(5H)-ones

An efficient and environmentally friendly electrochemical synthesis of phosphorylated oxindoles and indolo[2,1-a]isoquinolin-6(5H)-ones mediated by readily available Cp2Fe has been developed, which illustrated a broad substrate scope and diverse functional group compatibility. This protocol featured an external oxidant-free process and was at room temperature, which was proposed to be driven by the anodic oxidation of Cp2Fe.

Metal-Free One-Pot Multi-Functionalization of Unsaturated Compounds with Interelement Compounds by Radical Process

In recent years, the importance of "environmentally friendly manufacturing" has been increasing toward the establishment of a resource-recycling society. In organic synthesis, as well, it is becoming increasingly important to develop new synthetic strategies with resource conservation and the recycling of elemental resources in mind, rather than just only synthesis. Many studies on the construction of frameworks of functional molecules using ionic reactions and transition-metal-catalyzed reactions have been reported, but most of them have focused on the formation of carbon-carbon bonds. However, it is essential to introduce appropriate functional groups at appropriate positions in molecules in order for the molecules to express their functions, and furthermore, the highly selective preparation of multiple functional groups is considered important for the creation of new functional molecules. In this review, we focus on radical reactions with high functional group selectivity and overview the recent progress in practical methods for the simultaneous introduction of multiple functional groups and propose future synthetic strategies that emphasize the recycling of elemental resources and environmental friendliness.

Metal-free hydrosulfonylation of α,β-unsaturated ketones: synthesis and application of γ-keto sulfones

γ-Keto sulfones are versatile building blocks and valuable intermediates in organic synthesis and pharmaceutical chemistry. Motivated by their excellent properties, we herein report a green, convenient, metal-free hydrosulfonylation method for a variety of ynones, vinyl ketones, and sodium sulfinates in the absence of stoichiometric oxidants. This operationally simple protocol provides straightforward and practical access to a wide range of γ-keto sulfones with broad functional group tolerance from easily available starting materials. Moreover, the β,γ-unsaturated keto sulfones could further react with 2,3-butadienoate to generate cyclopentenes in phosphine-mediated [3 + 2] cycloaddition.

Visible-Light-Mediated Three-Component Cascade Sulfonylative Annulation

Visible-light-promoted cascade radical cyclization for the synthesis of sulfonylated benzimidazo/indolo[2,1-a]iso-quinolin-6(5H)-ones has been reported. The reaction provides transition-metal-free and expeditious access to sulfonylated polyaromatics. The use of sodium metabisulfite as a SO₂ surrogate and the rapid generation of molecular complexity using a three-component photochemical protocol are the salient features of this reaction manifold.

Copper(I)-Catalyzed Cross-Coupling of Arylsulfonyl Radicals with Diazo Compounds: Assembly of Arylsulfones

A novel copper-catalyzed cross-coupling of arylsulfonyl radicals with diazo compounds is described for the synthesis of various arylsulfones under mild conditions. In this reaction, the cheap, environmentally friendly, and readily available inorganic K₂S₂O₅ is employed as the sulfur dioxide source for providing arylsulfonyl radicals. In addition, a radical mechanism involving the insertion of sulfur dioxide with aryl radicals followed by the coupling of arylsulfonyl radicals with copper carbenes is proposed.

Regioselective Radical-Relay Sulfonylation/Cyclization Protocol to Sulfonylated Pyrrolidones under Transition-Metal-Free Conditions

A simple and low-cost tandem sulfonylation/cyclization of 1,5-diene, aryldiazonium salt, and DABCO·(SO₂)₂ is disclosed. This base-promoted multicomponent reaction can provide a "green" and economic synthesis of sulfonylated pyrrolidones under transition-metal-free and moisture/oxygen-insensitive reaction conditions, thus delivering a wide range of sulfonylated pyrrolidones in moderate to high yields with excellent functional group compatibility. A plausible mechanism involving a radical process is proposed, which demonstrates highly chemoselective trapping of the aryl radical with "SO₂" species, and a regioselective sulfonylation/cyclization protocol in this reaction.

Unconventional approaches for the introduction of sulfur-based functional groups

Organosulfur compounds have a pivotal role in the functionalities of many natural products, pharmaceuticals and organic materials. For these reasons, the search for new methodologies for the formation of carbon-sulfur bonds has been the object of intensive work for organic chemists. However, the proposed strategies suffer from various drawbacks, such as volatility, toxicity, and instability of the sulfur sources or the use of VOC solvents. In this review, we summarise the recent protocols which have the goal of obtaining sulfones, thioethers, thiazines, thiazepines and sulfonamides in an unconventional and/or sustainable way. The use of starting materials less invasive and toxic with respect to the traditional reagents, alternative solvents such as water, ionic liquids or deep eutectic solvents, the exploitation of ultrasound and electrochemistry, increasing the efficiency of the process, are reported. Moreover, representative reaction mechanisms are also discussed.

Transition-Metal-Free Decarboxylative Cyclization of N-Arylacrylamides with 2,2-Difluoro-2-(phenylthio)acetic Acid: Synthesis of Thiodifluorooxindole Derivatives

An efficient transition-metal-free decarboxylative cyclization of N-arylacrylamides with 2,2-difluoro-2-(phenylthio)acetic acid for the construction of thiodifluoroindoleone derivatives is described. This strategy features stable and readily available substrates, mild reaction conditions, and transition-metal-free catalysts. Notably, this protocol has successfully applied to synthesis of gem-difluoroalkenes, which exist in numerous biologically active compounds.

Hydroxysulfonylation of alkenes: an update

The direct difunctionalization of inexpensive and widely available alkenes has been recognized as a strong and straightforward tool for the rapid fabrication of complex molecules and pharmaceutical targets by introducing two different functional groups on adjacent carbon atoms of common alkene moieties in a single operation. This synthetic strategy avoids the purification and isolation of the intermediates and thus makes synthetic schemes shorter, simpler and cleaner. In this family of reactions, the hydroxysulfonylation of alkenes has emerged as an increasingly promising strategy for the synthesis of β-hydroxysulfones, which are found in many biologically important molecules and widespread applications in organic synthesis. The objective of this review is to illustrate the advancements in the field of hydroxysulfonylation of alkenes with special emphasis on the mechanistic details of the reaction pathways.

The direct difunctionalization of alkenes recognized as a straightforward tool for the rapid fabrication of complex molecules and pharmaceutical targets by introducing two different functional groups on adjacent carbon atoms of common alkene moieties.

Metal-Free Radical Annulation of Oxygen-Containing 1,7-Enynes: Configuration-Selective Synthesis of (E)-3-((Arylsulfonyl)methyl)-4-Substituted Arylidenechromene Derivatives

A novel strategy for the synthesis of (E)-3-((arylsulfonyl)methyl)-4-substituted benzylidenechromene derivatives via a metal-free radical annulation reaction of oxygen-containing 1,7-enynes with thiosulfonates has been developed. The reaction shows broad substrate scope, wide functional group tolerance, and moderate to excellent yields. Moreover, thiosulfonates were well driven to achieve the bifunctionalization reaction of oxo-1,7-enynes which derived from aliphatic alkynes. In addition, the (E)-configuration of the products was highly controlled by the structure of 1,7-enyne.

Metal-free alkynylsulfonylation of vinylarenes

A metal-free two-component alkynylsulfonylation of vinylarenes with aryl alkynylsulfones to afford various β-sulfonyl alkynes in moderate to excellent yields under mild conditions is developed.

An efficient iodine pentoxide-triggered iodocarbocyclization for the synthesis of iodooxindoles in water

An efficient iodocarbocyclization of alkenes for the synthesis of iodooxindoles has been developed. This reaction proceeds in a chemoselective manner and shows excellent tolerance of various functional groups, including a chemosensitive hydroxymethyl group. Nonmetal inorganic iodine pentoxide was used as both the oxidant and iodine source, making this protocol very practical. On the basis of experimental observations, a plausible electrophilic reaction mechanism was proposed.

Synthesis of difluoromethylated 2-oxindoles and quinoline-2,4-diones via visible light-induced tandem radical cyclization of N-arylacrylamides

Visible light-induced difluoromethylation of N-arylacrylamides to afford difluoromethylated 2-oxindoles and quinoline-2,4-diones with difluoromethyl 2-pyridyl sulfones as radical precursors has been disclosed.

Ag/Pyridine Co-Mediated Oxidative Arylthiocyanation of Activated Alkenes

An efficient Ag/pyridine co-mediated oxidative arylthiocyanation of activated alkenes via radical addition/cyclization cascade process was developed. This reaction could be carried out under mild conditions to provide biologically interesting 3-alkylthiocyanato-2-oxindoles in good to excellent yields. Mechanistic studies suggested a unique NCS• radical addition path and clarified the dual roles of catalytic pyridine as base and crucial ligand to accelerate the oxidation of Ag(I) to Ag(II), which is likely oxidant responsible for the formation of NCS• radical. These mechanistic results may impact the design and refinement of other radical based reactions proceeding through catalytic oxidations mediated by Ag(I)-pyridine/persulfate. The chemical versatility of thiocyanate moiety was also highlighted via SCN-tailoring chemistry in post-synthetic transformation for new S-C(sp³/sp²/sp), S-P, and S-S bonds constructions. The protocol provides an easy access to many important bioisosteres in medicinal chemistry and an array of sulfur-containing 2-oxindoles that are difficult to prepare by other approaches.

Synthesis of 3-((arylsulfonyl)methyl)indolin-2-ones via insertion of sulfur dioxide using anilines as the aryl source

Facile assembly of 3-((arylsulfonyl)methyl)indolin-2-ones via insertion of sulfur dioxide starting from anilines, N-arylacrylamides and DABCO·(SO₂)₂ is realized.

A practical synthesis of benzothiophenes via visible-light-promoted cyclization of disulfides and alkynes

Visible-light-promoted radical cyclization of disulfides and alkynes provided benzothiophenes in good yields.