Desulfonative photoredox alkylation of N-heteroaryl sulfones - an acid-free approach for substituted heteroarene synthesis

Desulfonylative Functionalization of Organosulfones via Inert (Hetero)Aryl C(sp2)-SO2 Bond Cleavage

As "chemical chameleons," organosulfones have been widely applied in various desulfonylative functionalization reactions. However, the desulfonylative functionalization of (hetero)arylsulfones through the cleavage of inert C(sp2)-SO2 bonds remains a challenging and underexplored task. Over the past twenty years, the use of (hetero)arylsulfones as arylation reagents has gradually gained attention in diverse cross-coupling reactions under specific catalytic conditions, especially in transition metal-catalysis and photocatalysis chemistry. In this review, we discuss the representative accomplishments and mechanistic insights achieved in desulfonylative reactions of inactive C(sp2)-SO2 bonds in (hetero)arylsulfones, including: (i) transition-metal-catalyzed desulfonylative cross-coupling reactions and (ii) photo-/electrocatalytic radical desulfonylative coupling reactions. We anticipate that this review will provide an overall perspective in this area to a general audience of researchers and stimulate further innovative strategies for desulfonylative functionalization of inert arylsulfones.

Visible Light-Induced, Nickel-Catalyzed Late-Stage 4-Alkylation of Hantzsch Esters with Alkyl Bromide

Herein, visible light-induced, nickel-catalyzed direct functionalization of the Hantzsch esters (HEs) with readily accessible alkyl bromides has been successfully achieved by taking advantage of HE as the reductant and substrate through an aromatization-dearomatization process. In this strategy, the single electron reduction of alkyl bromides by reactive Ni(I) species is essential for the success of this late-stage transformation. A wide range of 4-alkyl-1,4-dihydropyridines were rapidly assembled in moderate to good yields under mild conditions, rendering this photoinduced approach attractive for synthetic and medicinal chemistry.

Triply Selective & Sequential Diversification at Csp 3: Expansion of Alkyl Germane Reactivity for C-C & C-Heteroatom Bond Formation

We report the triply selective and sequential diversification of a single Csp 3 carbon carrying Cl, Bpin and GeEt3 for the modular and programmable construction of sp3-rich molecules. Various functionalizations of Csp 3-Cl and Csp 3-BPin (e.g. alkylation, arylation, homologation, amination, hydroxylation) were tolerated by the Csp 3-GeEt3 group. Moreover, the methodological repertoire of alkyl germane functionalization was significantly expanded beyond the hitherto known Giese addition and arylation to alkynylation, alkenylation, cyanation, halogenation, azidation, C-S bond formation as well as the first demonstration of stereo-selective functionalization of a Csp 3-[Ge] bond.

A visible-light-catalyzed sulfonylation reaction of an aryl selenonium salt via an electron donor-acceptor complex

An efficient synthesis of sulfone structures through selenonium salts and sodium sulfinates was developed. Under the irradiation of a blue LED lamp, the two substrates generate aryl and sulfonyl radicals through the activation of the intermediate electron donor acceptor (EDA) complex, thereby synthesizing aromatic, heteroaromatic and aliphatic sulfones in medium to good yields. The advantages of this strategy are metal-free, mild conditions and the leaving group is recycled to construct new selenonium salts.

Visible-Light-Driven Deoxygenative Heteroarylation of Alcohols with Heteroaryl Sulfones

The visible-light-promoted deoxygenative radical heteroarylation of alcohols was achieved in the absence of any external photosensitizers. The processes occur through the generation of xanthate salts from alcohols, followed by SET and fragmentation, delivering alkyl radicals to react with heteroaryl sulfones. This method is amenable for a wide range of alcohols with good functional group tolerance, providing a practical strategy for the alkylation of benzo-heteroaromatics. Mechanism studies indicate that direct visible-light excitation of xanthate anions and subsequent SET initiate the reactions.

A General and Practical Route to Functionalized Bicyclo[1.1.1]Pentane-Heteroaryls Enabled by Photocatalytic Multicomponent Heteroarylation of [1.1.1]Propellane

1-Aryl-substituted bicyclo[1.1.1]pentanes (BCPs) are an important class of BCP derivatives with widespread application in drug development. Most syntheses of these materials require multiple chemical steps via BCP electrophiles or nucleophiles derived from [1.1.1]propellane. Although one-step, multicomponent radical cross-coupling reactions could provide a more sustainable and rapid route to access diverse heteroarylated BCPs, current approaches are limited to tertiary alkyl radicals, leading to a decrease in their practical value. In this study, a conceptually different approach enabled by a radical multicomponent heteroarylation of [1.1.1]propellane to access functionalized heteroarylated BCPs is described. Importantly, this protocol is compatible with primary-, secondary-, and tertiary aliphatic radicals, as well as various fluoroalkyl radical sources, thus enabling rapid library generation of sought-after BCP derivatives for drug development.

Tuning the Photocatalytic Performance of Ruthenium(II) Polypyridine Complexes Via Ligand Modification for Visible-Light-Induced Phosphorylation of Tertiary Aliphatic Amines

Tuning the redox potential of commonly available photocatalyst to improve the catalytic performance or expand its scope for challenging synthetic conversions is an ongoing demand in synthetic chemistry. Herein, the excited state properties and redox potential of commercially available [Ru(bpy)₃ ]²⁺ photocatalyst were tuned by modifying the structure of the bipyridine ligands with electron-donating/withdrawing units. The visible-light-mediated photoredox phosphorylation of tertiary aliphatic amines was demonstrated under mild conditions. A series of cross-dehydrogenative coupling reactions were performed employing the Ru^II complexes as photocatalyst giving the corresponding α-aminophosphinoxides and α-aminophosphonates via carbon-phosphorus (C-P) bond formation.

Exploiting photoredox catalysis for carbohydrate modification through C–H and C–C bond activation

Photoredox catalysis has recently emerged as a powerful synthetic platform for accessing complex chemical structures through non-traditional bond disconnection strategies that proceed through free-radical intermediates. Such synthetic strategies have been used for a range of organic transformations; however, in carbohydrate chemistry they have primarily been applied to the generation of oxocarbenium ion intermediates in the ubiquitous glycosylation reaction. In this Review, we present more intricate light-induced synthetic strategies to modify native carbohydrates through homolytic C-H and C-C bond cleavage. These strategies allow access to glycans and glycoconjugates with profoundly altered carbohydrate skeletons, which are challenging to obtain through conventional synthetic means. Carbohydrate derivatives with such structural motifs represent a broad class of natural products integral to numerous biochemical processes and can be found in active pharmaceutical substances. Here we present progress made in C-H and C-C bond activation of carbohydrates through photoredox catalysis, focusing on the operational mechanisms and the scope of the described methodologies.

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.

Iron-Promoted Oxidative Alkylation/Cyclization of Ynones with 4-Alkyl-1,4-dihydropyridines: Access to 2-Alkylated Indenones

An iron-promoted oxidative tandem alkylation/cyclization of ynones with 4-alkyl-substituted 1,4-dihydropyridines for the efficient synthesis of 2-alkylated indenones is described. The process occurs via oxidative homolysis of a C-C σ-bond in 1,4-dihydropyridines to generate an alkyl radical followed by the addition of C-C triple bonds in ynones and intramolecular cyclization. A wide range of alkyl radicals could be efficiently transferred to generate a series of synthetically useful 2-alkylated indenones with excellent selectivity under mild conditions.

Thianthrenium-Enabled Sulfonylation via Electron Donor-Acceptor Complex Photoactivation

Sulfone-containing compounds are prevalent building blocks in pharmaceuticals and other biomolecules, and they serve as key intermediates in the synthesis of complex scaffolds. During the past decade, several methods have been developed to access sulfones. These strategies, however, require the use of strong reaction conditions, limiting their substrate scope. Recently, visible light-mediated transformations have emerged as novel platforms to access unprecedented structural motifs. This report demonstrates a thianthrenium-enabled sulfonylation via intra-complex charge transfer to generate transient aryl- and persistent sulfonyl radicals that undergo selective coupling to generate alkyl- and (hetero)aryl sulfones under ambient conditions. Importantly, this strategy allows retention of halide handles, presenting a complementary approach to transition metal-mediated photoredox couplings. Furthermore, this sulfonylation allows high functional group tolerance and is amenable to late-stage functionalization of complex biomolecules. Mechanistic investigations support the intermediacy of electron donor-acceptor (EDA) complexes.

Transition-Metal-Free Alkylation Strategy: A Facile Access of Alkylated Oxindoles via Alkyl Transfer

An efficient transition-metal-free alkylation/cyclization of activated alkenes using Hantzsch ester derivatives as effective alkyl reagents was described. A wide variety of valuable oxindoles were constructed in a single step with...

Synthesis of Reverse Glycosyl Fluorides via Organophotocatalytic Decarboxylative Fluorination of Uronic Acids

An efficient protocol for organophotocatalytic synthesis of reverse glycosyl fluorides (RGFs) is established relying on 9-mesityl-10-methyl-acridinium (Mes-Acr+)-mediated oxidative decarboxylative fluorination of uronic acids. Both pentofuranoid and hexopyranoid uronic acids are...

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.

Radical Anion Promoted Chemoselective Cleavage of Csp2-S Bond Enables Formal Cross-Coupling of Aryl Methyl Sulfones with Alcohols

A novel formal cross-coupling of aryl methyl sulfones and alcohols affording alkyl aryl ethers via an S_RN1 pathway is developed. Two marketed antitubercular drugs were efficiently prepared employing this approach as the key step. A dimsyl-anion initiated radical chain process was revealed as the major pathway. DFT calculations indicate that the formation of a radical anion via nucleophilic addition of alkoxide to the aryl radical is the key step in determining the observed chemoselectivity.

Recent advances in visible light-activated radical coupling reactions triggered by (i) ruthenium, (ii) iridium and (iii) organic photoredox agents

Photoredox chemistry with organic or transition metal agents has been reviewed in earlier years, but such is the pace of progress that we will overlap very little with earlier comprehensive reviews. This review first presents an overview of the area of research and then examines recent examples of C-C, C-N, C-O and C-S bond formations via radical intermediates with transition metal and organic radical promoters. Recent successes with Birch reductions are also included. The transition metal chemistry will be restricted to photocatalysts based on the most widely used metals, Ru and Ir, but includes coupling chemistries that take advantage of low-valent nickel, or occasionally copper, complexes to process the radicals that are formed. Our focus is on developments in the past 10 years (2011-2021). This period has also seen great advances in the chemistry of organic photoredox reagents and the review covers this area. The review is intended to present highlights and is not comprehensive.

Visible-light-promoted photocatalyst-free alkylation and acylation of benzothiazoles

Herein we report a protocol for the visible-light-mediated alkylation/acylation reaction of benzothiazoles. Alkyl/acyl substituted Hantzsch esters are easily prepared and rationally used as radical precursors. In the presence of BF3·Et2O and Na2S2O8, various benzothiazole derivatives were readily obtained in good yields. Our user-friendly protocol can proceed by simple irradiation with blue LEDs (λ = 465 nm) and without the assistance of external photocatalysts. The reaction is also characterized by mild conditions and scalability, thus offering an alternative and efficient tool for the synthesis of 2-functionalized benzothiazoles.

A Parisian Vision of the Chemistry of Hypercoordinated Silicon Derivatives

Less than ten years of acquaintance with hypercoordinated silicon derivatives in our lab is described in this account. Martin's spirosilane derivatives open new opportunities as ligands and as agents for the activation of small molecules and bis-catecholato silicates have proven to be exquisite radical precursors in photoredox conditions for broad synthetic applications.

Organic thermally activated delayed fluorescence (TADF) compounds used in photocatalysis

Organic compounds that show Thermally Activated Delayed Fluorescence (TADF) have become wildly popular as next-generation emitters in organic light emitting diodes (OLEDs). Since 2016, a subset of these have found increasing use as photocatalysts. This review comprehensively highlights their potential by documenting the diversity of the reactions where an organic TADF photocatalyst can be used in lieu of a noble metal complex photocatalyst. Beyond the small number of TADF photocatalysts that have been used to date, the analysis conducted within this review reveals the wider potential of organic donor-acceptor TADF compounds as photocatalysts. A discussion of the benefits of compounds showing TADF for photocatalysis is presented, which paints a picture of a very promising future for organic photocatalyst development.

Theoretical investigation on the nature of 4-substituted Hantzsch esters as alkylation agents

Recently, a variety of 4-substituted Hantzsch esters (XRH) with different structures have been widely researched as alkylation reagents in chemical reactions, and the key step of the chemical process is the elementary step of XRH˙⁺ releasing R˙. The purpose of this work is to investigate the essential factors which determine whether or not an XRH is a great alkylation reagent using density functional theory (DFT). This study shows that the ability of an XRH acting as an alkylation reagent can be reasonably estimated by its ΔG^≠_RD(XRH˙⁺) value, which can be conveniently obtained through DFT computations. Moreover, the data also show that ΔG^≠_RD(XRH˙⁺) has no simple correlation with the structural features of XRH, including the electronegativity of the R substituent group and the magnitude of steric resistance; therefore, it is difficult to judge whether an XRH can provide R˙ solely by experience. Thus, these results are helpful for chemists to design 4-substituted Hantzsch esters (XRH) with novel structures and to guide the application of XRH as a free radical precursor in organic synthesis.

This work presents a convenient computation method to estimate whether a 4-substituted Hantzsch ester can be a good alkyl radical donor.

Site-Selective Thiolation of (Multi)halogenated Heteroarenes

A general and simple strategy for the site-selective thiolation of various pharmaceutically relevant electron-rich heteroarenes with thiols is reported. This mild and reliable photocatalytic protocol enables C-S coupling at the most electron-rich position of the (multi)halogenated substrates, complementing established methodologies. Experimental and computational studies suggest a radical chain mechanism with the key step being a homolytic aromatic substitution of the heteroaryl halide by an electrophilic thiyl radical, highlighting an underdeveloped reactivity mode.

Radical Dehydroxymethylative Fluorination of Carbohydrates and Divergent Transformations of the Resulting Reverse Glycosyl Fluorides

A mild and convenient method for the synthesis of reverse glycosyl fluorides (RGFs) has been developed that is based on the silver-promoted radical dehydroxymethylative fluorination of carbohydrates. A salient feature of the reaction is that furanoid and pyranoid carbohydrates furnish structurally diverse RGFs bearing a wide variety of functional groups in good to excellent yields. Intramolecular hydrogen atom transfer experiments revealed that the reaction involves an underexploited radical fluorination that proceeds via β-fragmentation of sugar-derived primary alkoxyl radicals. Structurally divergent RGFs were obtained by catalytic C-F bond activation, and our method thus offers a concise and efficient strategy for the synthesis of reverse glycosides by late-stage diversification of RGFs. The potential of this method is showcased by the preparation and diversification of sotagliflozin, leading to the discovery of a promising SGLT2 inhibitor candidate.

Direct radical alkylation and acylation of 2H-indazoles using substituted Hantzsch esters as radical reservoirs

A platform approach for the direct synthesis of 3-substituted 2H-indazole derivatives has been developed using a Ag(i)/Na₂S₂O₈ system.

Terminal-oxidant-free photocatalytic C–H alkylations of heteroarenes with alkylsilicates as alkyl radical precursors

Alkylsilicates bearing C,O-bidentate ligands could achieve photocatalytic C–H alkylations of heteroarenes under acidic conditions without adding any terminal oxidant.

Regioselective Single-Electron Tsuji-Trost Reaction of Allylic Alcohols: A Photoredox/Nickel Dual Catalytic Approach

A radical-mediated functionalization of allyl alcohol derived partners with a variety of alkyl 1,4-dihydropyridines via photoredox/nickel dual catalysis is described. This transformation transpires with high linear and E-selectivity, avoiding the requirement of harsh conditions (e.g., strong base, elevated temperature). Additionally, using aryl sulfinate salts as radical precursors, allyl sulfones can also be obtained. Kinetic isotope effect experiments implicated oxidative addition of the nickel catalyst to the allylic electrophile as the turnover-limiting step, supporting previous computational studies.

Hantzsch esters: an emerging versatile class of reagents in photoredox catalyzed organic synthesis

This minireview highlights the recent advances in the chemistry of Hantzsch esters in photoredox catalyzed organic synthesis, with particular emphasis placed on reaction mechanisms.

Metal-, photocatalyst-, and light-free late-stage C–H alkylation of N-heteroarenes with organotrimethylsilanes using persulfate as a stoichiometric oxidant

Minisci C–H alkylation of N-heteroarenes with readily available benzylsilanes and heteroatom substituted silanes was developed.