Diversity‐Oriented Desulfonylative Functionalization of Alkyl Allyl Sulfones

Neutral nickel-catalyzed dehydrosulfonylation of unactivated allylic alcohols under mild conditions

Allyl sulfones are important sulfur-containing compounds that have widespread applications in organic synthesis, medicinal chemistry and materials science. Herein, nickel-catalysed dehydrosulfonylation of unactivated allyl alcohols with aryl sulfonyl hydrazides without additional active agents under mild conditions was developed. A variety of functional allyl sulfones could be efficiently synthesized in the presence of air-stable Ni(acac)2 as the catalyst and 1,1'-bis(diphenylphosphino)ferrocene (DPPF) as the ligand.

Deuterium- and Electron-Shuttling Catalysis for Deoxygenative Deuteration of Alcohols

A practical and precise method for visible-light-promoted deoxygenative deuteration of common aliphatic alcohols using D2O as the deuterium source is reported. Upon intermediacy of xanthate anions, a variety of primary, secondary, and tertiary alcohols can be facilely transformed into deuterioalkanes with excellent D-incorporation at predicted sites. The deoxygenation and deuteration sequence is catalyzed by in situ formed deuterated 2-mercaptopyridine, which plays dual roles as a deuterium atom transfer catalyst and an electron shuttle as well.

Regiodivergent synthesis of sulfone-tethered lactam-lactones bearing four contiguous stereocenters

Sulfone-tethered lactones/amides/amines display a diverse spectrum of biological activities, including anti-psychotic and anti-hypertensive. Sulfones are also widely present in functional materials and fragrances. We therefore reasoned that a regiodivergent and stereocontrolled strategy that merges the sulfone, lactone, and lactam motifs would likely lead to the discovery of new pharmacophores and functional materials. Here, we report mild conditions for the sulfonyllactonization of γ-lactam-tethered 5-aryl-4(E)-pentenoic acids. The annulation is highly modular, chemoselective, and diastereoselective. With respect to regioselectivity, trisubstituted alkenoic acids display a preference for 5-exo-trig cyclization whereas disubstituted alkenoic acids undergo exclusive 6-endo-trig cyclization. The lactam-fused sulfonyllactones bear angular quaternary as well as four contiguous stereocenters. The products are post-modifiable, especially through a newly developed Co-catalyzed reductive cross-coupling protocol.

KI-Catalyzed Allylic Sulfonation of α-Methylstyrene Derivatives with Sulfonylhydrazides via Electrochemistry

A direct allylic C-H bond activation of α-methylstyrene derivatives with sulfonylhydrazines for the synthesis of allylic sulfones has been developed under exogenous oxidant- and metal-catalyst-free electrochemical conditions. Using the transfer of electrons in the current instead of a stoichiometric chemical oxidant, a series of valuable allylic sulfones were accessed with a wide substrate scope and excellent regioselectivity via radical coupling.

Catalytic asymmetric deuterosilylation of exocyclic olefins with mannose-derived thiols and deuterium oxide

Metal-free, photoinduced asymmetric deuterosilylation of exocyclic olefins has been achieved using a mannose-derived thiol catalyst.

Dealkenylative Alkynylation Using Catalytic FeII and Vitamin C

In this paper, we report the synthesis of alkyl-tethered alkynes through ozone-mediated and FeII-catalyzed dealkenylative alkynylation of unactivated alkenes in the presence of alkynyl sulfones. This one-pot reaction, which employs a combination of a catalytic FeII salt and l-ascorbic acid, proceeds under mild conditions with good efficiency, high stereoselectivity, and broad functional group compatibility. In contrast to our previous FeII-mediated reductive fragmentation of α-methoxyhydroperoxides, the FeII-catalyzed process was devised through a thorough kinetic analysis of the multiple competing radical (redox) pathways. We highlight the potential of this dealkenylative alkynylation through multiple post-synthetic transformations and late-stage diversifications of complex molecules, including natural products and pharmaceuticals.

Radical deuteration

Deuterated chemicals are becoming irreplaceable in pharmaceutical engineering, material science and synthetic chemistry. Many excellent reviews have discussed acid/base-dependent or metal-catalyzed deuteration reactions, but radical deuterations have been discussed less. With the development of radical chemistry, there has been a rapid growth in radical deuterium-labelling technology. Diverse mild, cheap and efficient strategies for deuterium atom installation have been reported, and this review summarizes the recent achievements of radical deuteration classified by the reaction types.

Beyond classical sulfone chemistry: metal- and photocatalytic approaches for C-S bond functionalization of sulfones

The exceptional versatility of sulfones has been extensively exploited in organic synthesis across several decades. Since the first demonstration in 2005 that sulfones can participate in Pd-catalysed Suzuki-Miyaura type reactions, tremendous advances in catalytic desulfitative functionalizations have opened a new area of research with burgeoning activity in recent years. This emerging field is displaying sulfone derivatives as a new class of substrates enabling catalytic C-C and C-X bond construction. In this review, we will discuss new facets of sulfone reactivity toward further expanding the flexibility of C-S bonds, with an emphasis on key mechanistic features. The inherent challenges confronting the development of these strategies will be presented, along with the potential application of this chemistry for the synthesis of natural products. Taken together, this knowledge should stimulate impactful improvements on the use of sulfones in catalytic desulfitative C-C and C-X bond formation. A main goal of this article is to bring this technology to the mainstream catalysis practice and to serve as inspiration for new perspectives in catalytic transformations.

Radical 1,2,3-tricarbofunctionalization of α-vinyl-β-ketoesters enabled by a carbon shift from an all-carbon quaternary center

Herein, we report an intermolecular, radical 1,2,3-tricarbofunctionalization of α-vinyl-β-ketoesters to achieve the goal of building molecular complexity via the one-pot multifunctionalization of alkenes. This reaction allows the expansion of the carbon ring by a carbon shift from an all-carbon quaternary center, and enables further C-C bond formation on the tertiary carbon intermediate with the aim of reconstructing a new all-carbon quaternary center. The good functional group compatibility ensures diverse synthetic transformations of this method. Experimental and theoretical studies reveal that the excellent diastereoselectivity should be attributed to the hydrogen bonding between the substrates and solvent.

Radical 1-Fluorosulfonyl-2-alkynylation of Unactivated Alkenes

Sulfonyl fluorides have found widespread use in chemical biology and drug discovery. The development of synthetic methods for the introduction of the sulfonyl fluoride moiety is therefore of importance. Herein, a transition-metal-free radical 1,2-difunctionalization of unactivated alkenes via FSO2 -radical addition with subsequent vicinal alkynylation to access β-alkynyl-fluorosulfonylalkanes is presented. Alkynyl sulfonyl fluorides are introduced as highly valuable bifunctional radical trapping reagents that also serve as FSO2 -radical precursors. The β-alkynyl-fluorosulfonylalkanes obtained in these transformations can be readily diversified by using SuFEx click chemistry to obtain sulfonates and sulfonamides.

Convergent Synthesis of 1,4-Dicarbonyl Z-Alkenes through Three-Component Coupling of Alkynes, α-Diazo Sulfonium Triflate, and Water

We report a general protocol for the convergent synthesis of 1,4-dicarbonyl Z-alkenes form alkynes using α-diazo sulfonium triflate and water. The C═O, C═C, and C-H bonds are formed under mild conditions with a wide range of functional groups tolerated. The reaction exhibits excellent Z-selectivity and complete regioselectivity. The resulting 1,4-dicarbonyl Z-alkenes can smoothly undergo follow-up conversion to a variety of heteroaromatic scaffolds. Moreover, the reaction also provides a facile access to the corresponding deuterated Z-alkenes and deuterated heteroarenes with a high level of deuterium incorporation (90-97% D-inc.) by directly using D₂O, thus rendering the method highly valuable. The comprehensive mechanistic studies indicate that a free carbyne radical intermediate is formed via the photocatalytic single electron transfer process, and KH₂PO₄ plays a crucial role in significant improvements on yield and selectivity based on density-functional theory calculations, providing a new direction for radical coupling reactions of diazo compounds.

Recent Developments for the Deuterium and Tritium Labeling of Organic Molecules

Organic compounds labeled with hydrogen isotopes play a crucial role in numerous areas, from materials science to medicinal chemistry. Indeed, while the replacement of hydrogen by deuterium gives rise to improved absorption, distribution, metabolism, and excretion (ADME) properties in drugs and enables the preparation of internal standards for analytical mass spectrometry, the use of tritium-labeled compounds is a key technique all along drug discovery and development in the pharmaceutical industry. For these reasons, the interest in new methodologies for the isotopic enrichment of organic molecules and the extent of their applications are equally rising. In this regard, this Review intends to comprehensively discuss the new developments in this area over the last years (2017-2021). Notably, besides the fundamental hydrogen isotope exchange (HIE) reactions and the use of isotopically labeled analogues of common organic reagents, a plethora of reductive and dehalogenative deuteration techniques and other transformations with isotope incorporation are emerging and are now part of the labeling toolkit.

Base-Mediated Radical Borylation of Alkyl Sulfones

A practical and direct method was developed for the production of versatile alkyl boronate esters via transition metal-free borylation of primary and secondary alkyl sulfones. The key to the success of the strategy is the use of bis(neopentyl glycolato) diboron (B2 neop2 ), with a stoichiometric amount of base as a promoter. The practicality and industrial potential of this protocol are highlighted by its wide functional group tolerance, the late-stage modification of complex compounds, no need for further transesterification, and operational simplicity. Radical clock, radical trap experiments, and EPR studies were conducted which show that the borylation process involves radical intermediates.

Radical boron migration of allylboronic esters

A photocatalyzed 1,3-boron shift of allylboronic esters is reported. The atom-switch acrobatics proceeds via cascade 1,2-boron migrations and Smiles type rearrangement to furnish a variety of terminally functionalized alkyl boronates.

SOMOphilic Alkynylation of Unreactive Alkenes Enabled by Iron-Catalyzed Hydrogen Atom Transfer

We report an efficient and practical iron-catalyzed hydrogen atom transfer protocol for assembling acetylenic motifs into functional alkenes. Diversities of internal alkynes could be obtained from readily available alkenes and acetylenic sulfones with excellent Markovnikov selectivity. An iron hydride hydrogen atom transfer catalytic cycle was described to clarify the mechanism of this reaction.

Copper-Catalyzed Intermolecular Alkynylation and Allylation of Unactivated C(sp3)-H Bonds via Hydrogen Atom Transfer

We describe Cu-catalyzed intermolecular alkynylation and allylation of unactivated C(sp³)-H bonds with singly occupied molecular orbital-philes (SOMO-philes) via hydrogen atom transfer (HAT). Employing N-fluoro-sulfonamide as a HAT reagent, a set of substituted alkene and alkyne compounds were synthesized in high yields with good regioselectivity and functional-group compatibility. Late-stage functionalization of natural products and drug molecules is also demonstrated.

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.

Direct Photoexcitation of Ethynylbenziodoxolones: An Alternative to Photocatalysis for Alkynylation Reactions*

Ethynylbenziodoxolones (EBXs) are commonly used as radical traps in photocatalytic alkynylations. Herein, we report that aryl-substituted EBX reagents can be directly activated by visible light irradiation. They act as both oxidants and radical traps, alleviating the need for a photocatalyst in several reported EBX-mediated processes, including decarboxylative and deboronative alkynylations, the oxyalkynylation of enamides and the C-H alkynylation of THF. Furthermore, the method could be applied to the synthesis of alkynylated quaternary centers from tertiary alcohols via stable oxalate salts and from tertiary amines via aryl imines. A photocatalytic process using 4CzIPN as an organic dye was also developed for the deoxyalkynylation of oxalates.

Radical-mediated vicinal addition of alkoxysulfonyl/fluorosulfonyl and trifluoromethyl groups to aryl alkyl alkynes

The addition of sulfonyl radicals to alkenes and alkynes is a valuable method for constructing useful highly functionalized sulfonyl compounds. The underexplored alkoxy- and fluorosulfonyl radicals are easily accessed by CF₃ radical addition to readily available allylsulfonic acid derivatives and then β-fragmentation. These substituted sulfonyl radicals add to aryl alkyl alkynes to give vinyl radicals that are trapped by trifluoromethyl transfer to provide tetra-substituted alkenes bearing the privileged alkoxy- or fluorosulfonyl group on one carbon and a trifluoromethyl group on the other. This process exhibits broad functional group compatibility and allows for the late-stage functionalization of drug molecules, demonstrating its potential in drug discovery and chemical biology.

An unprecedented method for vicinal addition of alkoxysulfonyl/fluorosulfonyl and trifluoromethyl groups to aryl alkyl alkynes has been developed to afford useful alkenylsulfonate esters and alkenylsulfonyl fluorides.

Remote Site-Selective Radical C(sp3 )-H Monodeuteration of Amides using D2 O

Site-selective incorporation of deuterium into biologically active compounds is of high interest in pharmaceutical industry. We present a mild and environmentally benign metal-free method for the remote selective radical C-H monodeuteration of aliphatic C-H bonds in various amides with inexpensive heavy water (D2 O) as the deuterium source. The method uses the easily installed N-allylsulfonyl moiety as an N-radical precursor that generates the remote C-radical via site-selective 1,5- or 1,6-hydrogen atom transfer (HAT). Methyl thioglycolate, that readily exchanges its proton with D2 O, serves as the radical deuteration reagent and as a chain-carrier. The highly site-selective monodeuteration has been applied to different types of unactivated sp3 -C-H bonds and also to the deuteration of C-H bonds next to heteroatoms. The potential utility of this method is further demonstrated by the site-selective incorporation of deuterium into natural product derivatives and drugs.

Nonenzymatic Stereoselective S-Glycosylation of Polypeptides and Proteins

Here we report a nonenzymatic glycosylation reaction that builds axial S-glycosidic bonds under biorelevant conditions. This strategy is enabled by the design and use of allyl glycosyl sulfones as precursors to glycosyl radicals and exploits the exceptional functional group tolerance of radical processes. Our method introduces a variety of unprotected glycosyl units to the cysteine residues of peptides in a highly selective fashion. Through developing the second-generation protocol, we applied our method in the direct glycosylation of complex polypeptides and proteins. Computational studies were performed to elucidate the reaction mechanism.

A Titanium-Catalyzed Reductive α-Desulfonylation

A titanium(III)-catalyzed desulfonylation gives access to functionalized alkyl nitrile building blocks from α-sulfonyl nitriles, circumventing traditional base-mediated α-alkylation conditions and strong single electron donors. The reaction tolerates numerous functional groups including free alcohols, esters, amides, and it can be applied also to the α-desulfonylation of ketones. In addition, a one-pot desulfonylative alkylation is demonstrated. Preliminary mechanistic studies indicate a catalyst-dependent mechanism involving a homolytic C-S cleavage.

Synthesis of quaternary centres by single electron reduction and alkylation of alkylsulfones

A new method for the generation of tertiary radicals through single electron reduction of alkylsulfones promoted by Zn and 1,10-phenanthroline has been developed. These radicals could be employed in the Giese reaction, affording structurally diverse quaternary products in good yields. With the high modularity and functional group compatibility of sulfones, the utility of this method was demonstrated by intramolecular and iterative reactions to give complex structures. The radical generation process was investigated by control experiments and theoretical calculations.

A new method for the generation of tertiary radicals through single electron reduction of alkylsulfones promoted by Zn and 1,10-phenanthroline has been developed.

Forging C-SeCF3 Bonds with Trifluoromethyl Tolueneselenosulfonate under Visible-Light

This account highlights some of our recent work on photoinduced trifluoromethylselenolation reactions. This research program relies primarily on the design of a new key shelf-stable selenating reagent that can be involved in various radical processes In particular, we demonstrated that trifluoromethylselenolation of arenes, alkenes, alkynes as well as aliphatic organic building blocks can be readily achieved under visible-light irradiation. Mechanistic investigations based on ¹⁹ F NMR studies, EPR spectroscopy, cyclic voltammetry and luminescence studies allowed us to shed the light on the different proposed mechanisms in the designed methodologies. The applicative potential of these strategies was further demonstrated through the synthesis of bioactive analogue containing SeCF₃ motif.

Visible-light photoredox-catalyzed aryl radical in situ SO2-capture reactions

An aryl radical in situ SO₂-capture reaction is developed for the synthesis of various β-keto, allyl and alkynyl arylsulfone derivatives.

SOMOphilic alkynylation using acetylenic sulfones as functional reagents

Recent advancements in SOMOphilic alkynylation reactions by using acetylenic sulfones as functional reagents are summarized.

Water Compatible Hypophosphites-d2 Reagents: Deuteration Reaction via Deutero-deiodination in Aqueous Solution

Contrary to conventional deuteration approaches which typically entail deuterated solvents and/or moisture exclusion, an unprecedented deutero-deiodination reaction attainable in aqueous (H₂O) solution is presented herein. By utilizing the stability of inorganic deuterated calcium/sodium hypophosphites against wayward H/D isotopic exchange within pH 2.5-11.7, these shelf-stable, nontoxic, cost-effective, and environmentally benign deuteration reagents mediate deuteration of a broad range alkyl and aryl iodides with ample isotopic incorporation in aqueous (H₂O) solution.

Acid-Controlled Access to β-Sulfenyl Ketones and α,β-Disulfonyl Ketones by Pummerer Reaction of β-Keto Sulfones and Sulfoxides

A convenient acid-mediated reaction of β-keto sulfones with sulfoxides under metal-free conditions has been developed, thereby delivering the acid-controlled synthesis of β-sulfenyl ketones and α,β-disulfonyl ketones in good to excellent yields. The mechanism of the transformations has been studied carefully, which suggested the involvement of a radical process in the formation of α,β-disulfonyl ketones.

Alkynylation of radicals: spotlight on the "Third Way" to transfer triple bonds

The alkynylation of radical intermediates has been known since a long time, but had not been broadly applied in synthetic chemistry, in contrast to the alkynylation of either electrophiles or nucleophiles. In the last decade however, it has been intensively investigated leading to new disconnections to introduce versatile triple bonds into organic compounds. Nowadays, such processes are important alternatives to classical nucleophilic and electrophilic alkynylations. Efficient alkyne transfer reagents, in particular arylsulfones and hypervalent iodine reagents were introduced. Direct alkynylation, as well as cascade reactions, were subsequently developed. If relatively harsh conditions were required in the past, a new era began with progress in photoredox and transition metal catalysis. Starting from various radical precursors, alkynylations under very mild reaction conditions were rapidly discovered. This review covers the evolution of radical alkynylation, from its emergence to its current intensive stage of development. It will focus in particular on improvements for the generation of radicals and on the extension of the scope of radical precursors and alkyne sources.