Straightforward Sulfonamidation viaMetabisulfite‐Mediated Cross Coupling of Nitroarenes and Boronic Acids under Transition‐Metal‐Free Conditions †

Modular access to diarylmethyl sulfonamides via visible light-promoted cross-coupling reactions

We report a novel and efficient method for the preparation of diarylmethyl sulfonamide derivatives through visible-light-induced sulfamoylation of para-quinone methides with sulfamoyl chlorides under mild, metal-free conditions. This protocol demonstrates excellent tolerance toward a wide range of functional groups, affording the corresponding products in moderate to high yields. Preliminary mechanism studies revealed that the excited photocatalyst rhodamine 6G* was mainly quenched by para-quinone methides and the generated diarylmethyl radical intermediates then underwent radical-radical cross-coupling with sulfamoyl radicals to yield the diarylmethyl sulfonamides.

Metal-free introduction of primary sulfonamide into electron-rich aromatics

We report herein a direct and practical synthesis of arylsulfonamides from electron-rich aromatic compounds by using in situ generated N-sulfonylamine as the active electrophile. Substrates include derivatives of aniline, indole, pyrrole, furan, styrene and so on. The reaction proceeds under mild conditions and tolerates many sensitive functional groups such as alkyne, acetate, the trifluoromethoxy group or acetoxymethyl ester. Applications of this method for the construction of metal ion sensors and fluorogenic dye have been demonstrated, thus highlighting the potential of this method for probe development.

Sodium sulfide-promoted regiodefined redox condensation of o-nitroanilines with aryl ketones to benzo[a]phenazines and quinoxalines

Inexpensive sodium sulfide trihydrate was found to promote unprecedented 6e-regio-predefined redox condensation of o-nitroanilines with α-tetralones to benzo[a]phenazines. The method was also successfully extended to acetophenones and higher homologs as reducing partners to provide 2-phenylquinoxalines. Compared to traditional approaches toward benzo[a]phenazine and quinoxaline cores starting with o-phenylenediamines, the present strategy could afford these heterocycles with well-defined regiochemistry based on the structure of starting o-nitroanilines.

Nitro-sulfinate Reductive Coupling to Access (Hetero)aryl Sulfonamides

A method to assemble (hetero)aryl sulfonamides via the reductive coupling of aryl sulfinates and nitroarenes is reported. Various reducing conditions with sodium bisulfite and with or without tin(II) chloride in DMSO were developed using an ultrasound bath to improve reaction homogeneity and mixing. A range of (hetero)aryl sulfonamides bearing a selection of functional groups were prepared, and the mechanism of the transformation was investigated. These investigations have led us to propose the formation of nitrosoarene intermediates, which were established via an independent molecular coupling strategy.

Fe/S-Catalyzed Redox Condensation of o-Nitrophenols with Isothiocyanates to 2-Aminobenzoxazoles

As frequently encountered byproducts of isocyanate chemistry, hydrogen sulfide and related sulfur containing compounds should be treated in a safe way to lower their adverse health and environmental effects, especially in large scale syntheses. As a proof of concept, we report herein an example of in situ recycling of sulfur byproduct to reductant in the synthesis of bioactive 2-aminobenzoxazoles 3. Using an Fe/S catalytic system, this heterocyclic scaffold could be obtained from o-nitrophenols 1 with isothiocyates 2 via direct redox condensation consisting of reduction of the nitro group of 1 by the sulfur moiety of 2.

Mild Amide Synthesis Using Nitrobenzene under Neutral Conditions

Amide synthesis is one of the most important transformations in organic chemistry due to the broad application in pharmaceutical drugs and organic materials. In this report, we describe a mild protocol for amide formation using the readily available nitroarenes as nitrogen sources and an inexpensive iron complex as a catalyst. Because of the use of the pH-neutral conditions and the avoidance of the strong oxidant or reductant, a wide range of aromatic and aliphatic aldehydes as well as nitroarenes with various functional groups could be tolerated well. A plausible mechanism is proposed based on the detailed studies, in which iron catalyst initiates the radical process and the solvent plays a key role as O-atom acceptor.

Recent Progress in the Synthesis of Drugs and Bioactive Molecules Incorporating Nitro(het)arene Core

Aromatic nitro compounds play a unique role in the synthesis of drugs and pharmaceutically oriented molecules. This field of organic chemistry continues to be in demand and relevant. A significant number of papers are published annually on new general methods for the synthesis of nitrodrugs and related biomolecules. This review is an analysis of the literature on methods for the synthesis of both new and already-known aromatic and heteroaromatic nitrodrugs covering the period from 2010 to the present.

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.

SET activation of nitroarenes by 2-azaallyl anions as a straightforward access to 2,5-dihydro-1,2,4-oxadiazoles

The use of nitroarenes as amino sources in synthesis is challenging. Herein is reported an unusual, straightforward, and transition metal-free method for the net [3 + 2]-cycloaddition reaction of 2-azaallyl anions with nitroarenes. The products of this reaction are diverse 2,5-dihydro-1,2,4-oxadiazoles (>40 examples, up to 95% yield). This method does not require an external reductant to reduce nitroarenes, nor does it employ nitrosoarenes, which are often used in N-O cycloadditions. Instead, it is proposed that the 2-azaallyl anions, which behave as super electron donors (SEDs), deliver an electron to the nitroarene to generate a nitroarene radical anion. A downstream 2-azaallyl radical coupling with a newly formed nitrosoarene is followed by ring closure to afford the observed products. This proposed reaction pathway is supported by computational studies and experimental evidence. Overall, this method uses readily available materials, is green, and exhibits a broad scope.

The Renaissance of Alkali Metabisulfites as SO2 Surrogates

The upsurge of interest in the development of methodologies for the construction of sulfur-containing compounds via the use of expedient reagents has established sustainable tools in organic chemistry. This review focuses on sulfonylation reactions using inorganic sulfites (Na2S2O5 or K2S2O5) as the sulfur dioxide surrogates. Compared to the bis-adduct with DABCO, which is an excellent surrogate of gaseous SO2, the use of sodium or potassium metabisulfites as SO2 surrogates are equally efficient. The objective of the current review is to exemplify recent sulfonylation reactions using inorganic sulfites. For better understanding, the review is categorized according to the mode of reactions: transition-metal-catalyzed SO2 insertion, metal-free SO2 insertion, and visible-light-mediated SO2 insertion. All the reactions in each of the sections are illustrated with selected examples with a pertinent explanation of the proposed mechanism.

1 Introduction

2 Outlines of the Reactions Involving SO2 Insertion

2.1 Transition-Metal-Catalyzed SO2 Insertion

2.2 Transition-Metal-Free SO2 Insertion

2.3 Visible-Light-Mediated SO2 Insertion

3 Conclusion and Outlook

Advances in photochemical and electrochemical incorporation of sulfur dioxide for the synthesis of value-added compounds

Organic photochemistry and electrochemistry currently receive tremendous attention in organic synthesis as both techniques enable the reagent-less activation of organic molecules without using expensive and hazardous redox reagents. The incorporation of SO₂ into organic molecules is a relatively modern research topic, which likewise gains immense popularity since the discovery of the SO₂ surrogate DABSO. Sulfur-containing organic molecules are omnipresent in pharmaceuticals and agrochemicals. This review covers the recent progress in electrochemical and photochemical methodologies for the incorporation and uses of SO₂ in the synthesis of value-added compounds. Additionally, different work techniques are demonstrated for the synthetic application of SO₂.

Photocatalytic three-component annulation enabling stereoselective generation of (Z)-thiochromene 1,1-dioxides

A new photocatalytic three-component annulation of 2-alkynylaryldiazonium tetrafluoroborates with γ-hydroxyl terminal alkynes and sodium metabisulfite is reported and used to produce a series of (Z)-thiochromene 1,1-dioxides.

The same oxygenation-state introduction of hypervalent sulfur under transition-metal-free conditions

This review discusses the application of the same oxygenation-state introduction of hypervalent sulfur strategy under transition-metal-free conditions.