Metal‐Free Sulfonylation of α,β‐Conjugated Systems by Using Sulfonyl Hydrazides

Photoinduced Single Electron Transfer via EDA Complexation Enables Decarbonylative C(sp2)-S Bond Formation

We have developed a photoinduced transition-metal-free decarbonylative strategy at ambient temperature through non-covalent interactions to achieve vinyl sulfones. Traditionally, decarbonylative functionalization is accomplished using transition metal catalysts at elevated temperatures. The π-π interaction facilitates the elimination of CO to generate vinyl radical, thereby promoting the creation of C-S bonds with the sulfonyl radical. These interactions and the overall process were illuminated by spectroscopic investigations and mechanistic studies.

Copper-Catalyzed Radical Sulfonylation: Divergent Construction of C(sp3)-Sulfonyl Bonds with Sulfonylhydrazones

Sulfonylhydrazones have been proven to be versatile synthetic intermediates in a panel of transformations. However, radical sulfonylation with sulfonylhydrazone as sulfonyl radical source is relatively rare. Here, we found that sulfonylhydrazone can serve as a new sulfonyl radical precursor to couple various partners such as arylacetic acids, ene-yne-ketones, and para-quinone methides under copper catalysis and microwave irradiation. The reactions of sulfonyl radicals have been successively developed to enable the divergent synthesis of C(sp3)-sulfonyl bonds. In addition, when alkynes and alkenes are used as radical receptors, this method can also promote the formation of C(sp2)-sulfonyl bonds. This finding suggests that sulfonylhydrazone could be regarded as a potentially useful sulfonyl radical in sulfone synthesis.

Ni-Catalyzed Hydrosulfonylation of Alkenes with Aromatic Iodides and K2S2O5

Hydrosulfonylation of alkenes with readily available aromatic iodides via a SO2-insetion strategy is presented. The combination of non-noble Ni catalysis with (iPr)3SiH as the final reductant enables the efficient formation of aryl and heteroaryl sulfinate intermediates, which undergo Michael-type additions to electron-deficient alkenes for initiating the hydrosulfonylation process. Moreover, the superiority of this protocol is demonstrated by broad substrate scope and good functional group compatibility.

The green chemistry of chalcones: Valuable sources of privileged core structures for drug discovery

The sustainable use of resources is essential in all production areas, including pharmaceuticals. However, the aspect of sustainability needs to be taken into consideration not only in the production phase, but during the whole medicinal chemistry drug discovery trajectory. The continuous progress in the fields of green chemistry and the use of artificial intelligence are contributing to the speed and effectiveness of a more sustainable drug discovery pipeline. In this light, here we review the most recent sustainable and green synthetic approaches used for the preparation and derivatization of chalcones, an important class of privileged structures and building blocks used for the preparation of new biologically active compounds with a broad spectrum of potential therapeutic applications. The literature here reported has been retrieved from the SciFinder database using the term "chalcone" as a keyword and filtering the results applying the concept: "green chemistry", and from the Reaxys database using the keywords "chalcone" and "green". For both databases the time-frame was 2017-2022. References were manually selected based on relevance.

A Bi(OTf)3-Promoted Hydrosulfonylation of Alkenes with Sulfonyl Hydrazides: An Approach to Branched Sulfones

The Bi(OTf)₃-promoted hydrosulfonylation of alkenes with sulfonyl hydrazides to produce branched sulfones is reported, in which various branched sulfones (>40 examples) have been prepared in moderate to good yields. The gram-scale reaction and synthesis of the experimental inhibitor precursor showed the potential application. A preliminary mechanistic study revealed that double-bond migration to form the α,β-conjugated alkene is crucial for this transformation.

A radical-radical cross-coupling reaction of xanthene with sulfonyl hydrazides: facile access to xanthen-9-sulfone derivatives

A straightforward strategy for direct incorporation of sulfonyl units into a xanthene moiety for accessing xanthen-9-sulfone derivatives in good to excellent yields has been established via metal-free radical-radical cross-coupling reaction of xanthenes and sulfonyl hydrazides. Using easily accessible starting materials, this methodology proceeds efficiently with a high degree of functional group compatibility and with a wide scope of both xanthenes and sulfonyl hydrazides under operationally simple reaction conditions. Mechanistic investigations revealed that sulfonyl radicals could be generated from sulfonyl hydrazides in the presence of TBHP under an oxygen atmosphere.

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.

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.

Denitrative imino-diaza-Nazarov cyclization: synthesis of pyrazoles

An iodine-catalyzed denitrative imino-diaza-Nazarov cyclization (DIDAN) methodology has been developed for the synthesis of pyrazoles with high to excellent yields by using α-nitroacetophenone derivatives and in situ generated hydrazones. The key transformation of this oxidative 4π-electrocyclization proceeds through an enamine-iminium ion intermediate. This rapid one-pot DIDAN protocol results in the selective generation of C-C and C-N bonds and cleavage of a C-N bond.

Denitrative Cross-Couplings of Nitrostyrenes

Interestingly, β-nitrostyrenes, typically bench stable compounds, are highly promising cross-coupling partners, due to their excellent availability and well understood reactivity. In this review, we report on the discovery and advancements, in the field of stereoselective, denitrative cross-couplings of β-nitrostyrenes with miscellaneous organic reagents. The rapidly expanding field offers alternative access to a broad range of functionalized alkenes, including β-alkylated styrenes, chalcones, stilbenes, cinnamic acids, and conjugated sulfones and phosphonates. The most important mechanistic pathways are briefly discussed, to familiarize readers with the elementary reactions occurring during the coupling.

Silver-promoted synthesis of vinyl sulfones from vinyl bromides and sulfonyl hydrazides in water

The synthesis of vinyl sulfones via silver-promoted cross-coupling of vinyl bromides with sulfonyl hydrazides was realized. Water was used as the sole solvent. Multisubstituted vinyl sulfones were easily prepared with excellent alkyl group tolerance. A mechanism involving nucleophilic attack of a sulfinate anion was proposed.

Electrochemical sulfonylation of alkenes with sulfonyl hydrazides: a metal- and oxidant-free protocol for the synthesis of (E)-vinyl sulfones in water

An electrochemical sulfonylation of alkenes with sulfonyl hydrazides for the synthesis of (E)-vinyl sulfones in water is reported.