Radical difluoromethylthiolation of aromatics enabled by visible light

Review and Theoretical Analysis of Fluorinated Radicals in Direct CAr-H Functionalization of (Hetero)arenes

We highlight key contributions in the field of direct radical CAr- H (hetero)aromatic functionalization involving fluorinated radicals. A compilation of Functional Group Transfer Reagents and their diverse activation mechanisms leading to the release of radicals are discussed. The substrate scope for each radical is analyzed and classified into three categories according to the electronic properties of the substrates. Density functional theory computational analysis provides insights into the chemical reactivity of several fluorinated radicals through their electrophilicity and nucleophilicity parameters. Theoretical analysis of their reduction potentials also highlights the remarkable correlation between electrophilicity and oxidizing ability. It is also established that highly fluorinated radicals (e.g. ⋅OCF3) are capable of engaging in single-electron transfer (SET) processes rather than radical addition, which is in good agreement with experimental literature data. A reactivity scale, based on activation barrier of addition of these radicals to benzene is also elaborated using the high accuracy DLPNO-(U)CCSD(T) method.

Synthesis of 2H-Chromeno[2,3-b]tetrahydropyridine Frameworks through Cs2CO3-Promoted Annulation of 2-Amino-4H-chromen-4-ones and 4-Benzylideneoxazol-5(4H)-ones

This innovative reaction involved the [3+3] annulation of 2-amino-4H-chromen-4-ones and 4-benzylideneoxazol-5(4H)-ones. The process provided quick and easy access to a broad range of structurally diverse and highly functionalized 1,3,4,5-tetrahydro-2H-chromeno[2,3-b]pyridines in moderate to good yields, which possess trans-form C3 and C4 substituents.

Copper-Catalyzed, Interrupted Remote Fluoromethylthiolation of Unactivated C(sp3)-H Bonds

An efficient copper-catalyzed selective fluoromethylthiolation of an inert δ-C(sp3)-H bond in sulfonamides was reported. In the presence of a copper catalyst and PhSO2SRf, the radical generated through 1,5-hydrogen atom transfer (HAT) was sufficiently trapped by PhSO2SRf, instead of copper, which was prevalent in metal-catalyzed radical-relay processes, incorporating a fluoromethylthio group into molecules. The general substrate scope and mild conditions endowed the method with wide potential applications in pharmaceuticals and agrochemicals.

Visible light-promoted difluoromethylthiolation of cycloalkanols by C-C bond cleavage

A mild and general methodology for the difluoromethylthiolation of cycloalkanols has been developed by employing N-difluoromethylthiophthalimide as the SCF2H radical source, in combination with an acridinium-derived organo-photosensitizer, under redox-neutral conditions. This reaction protocol demonstrates high efficiency, scalability, and mild reaction conditions, thus presenting a green approach for the rapid synthesis of distal difluoromethylthiolated alkyl ketones that are challenging to be synthesized through alternative means.

Recent Advances in Visible Light-Mediated Radical Fluoro-alkylation, -alkoxylation, -alkylthiolation, -alkylselenolation, and -alkylamination

In the last few years, many reagents and protocols have been developed to allow for the efficient fluorofunctionalization of a diverse set of scaffolds ranging from alkanes, alkenes, alkynes, and (hetero)arenes. The concomitant rise of organofluorine chemistry and visible light-mediated synthesis have synergistically expanded the fields and have mutually benefitted from developments in both fields. In this context, visible light driven formations of radicals containing fluorine have been a major focus for the discovery of new bioactive compounds. This review details the recent advances and progress made in visible light-mediated fluoroalkylation and heteroatom centered radical generation.

Intramolecular Heterocyclization/Fluoromethylthiolation of Alkynes Enabled by a Multicomponent Reagent System

The BnSR_f (R_f = CF₂H or CF₃)/mCPBA/Tf₂O system was found to be an effective multicomponent reagent system for the one-pot synthesis of di/trifluoromethylthiolated heterocycles from alkynes. The reaction was postulated to proceed via a cascade sequence involving the oxidation of BnSR_f by mCPBA, activation of the in situ-generated sulfoxide by Tf₂O, and intramolecular cyclization/fluoromethylthiolation of the alkyne substrates enabled by the formed electrophilic sulfonium salt to give di/trifluoromethylthiolated heterocycles.

Synthesis of 3-SCF2H-/3-SCF3-chromones via Interrupted Pummerer Reaction/Intramolecular Cyclization Mediated by Difluoromethyl or Trifluoromethyl Sulfoxide and Tf2O

The reaction of alkynyl aryl ketones bearing an o-methoxy group with difluoromethyl sulfoxide in the presence of Tf₂O was found to conveniently afford the corresponding 3-SCF₂H-substituted chromones. The combining use of difluoromethyl sulfoxide/Tf₂O could represent the first reagents system that can introduce the biologically important SCF₂H moiety under base-free conditions via an interrupted Pummerer reaction. The same protocol could also be applied to the synthesis of 3-SCF₃-substituted chromones by replacing difluoromethyl sulfoxide with trifluoromethyl sulfoxide and CH₃CN with toluene.

Atom- and step-economic 1,3-thiosulfonylation of activated allenes with thiosulfonates to access vinyl sulfones/sulfides

A new type of organocatalyzed 1,3-thiosulfonylation has been developed to straightforwardly access highly functionalized vinyl sulfones, which features mild conditions, atom- and step-economy, practicability, conciseness, and environmental friendliness. Moreover, these valuable products can be transformed to vinyl sulfides via a base-promoted isomerization. The versatile route can efficiently and rapidly introduce SCD₃ groups with excellent levels of deuterium content (>99% D) by utilizing our newly developed SCD₃ reagents. Gram-scale operations and further transformations are smoothly carried out, providing promising applications for drug discovery.

Robust, scalable construction of an electrophilic deuterated methylthiolating reagent: facile access to SCD3-containing scaffolds

We have established a practical and concise method for the straightforward access of a universal deuterated methylthiolating reagent through a one-pot gram-scale operation under mild conditions. This odourless electrophilic SCD₃ reagent was widely applied to react with numerous representative nucleophiles and approached various valuable SCD₃ analogues with excellent levels of deuterium content (>99% D). The divergent further transformations were smoothly carried out to obtain the significant derivatives with different oxidative states in high efficiency.

Deoxygenative Functionalizations of Aldehydes, Ketones and Carboxylic Acids

Conversion of carbonyl compounds, including aldehydes, ketones and carboxylic acids, into functionalized alkanes via deoxygenation would be highly desirable from a sustainability perspective and very enabling in chemical synthesis. This review covers the recent methodology development in carbonyl and carboxyl deoxygenative functionalizations, highlighting some typical and significant contributions in this field. These advances will be categorized based on types of bond formation, and in each part, selected examples will be discussed from their generalized mechanistic perspectives. Four summarized reactivity modes of aldehydes and ketones during the deoxygenation, namely, bis-electrophile, carbenoid, bis-nucleophile and alkyl radical, are presented, while the carboxylic acids are deoxygenated mainly via activated carbonyl or acetal intermediates.

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.

HCF2Se/HCF2S Installation by Tandem Substitutions from Alkyl Bromides

Herein we describe an efficient construction of HCF₂Se and HCF₂S groups by tandem substitutions between alkyl bromides and a reagent system consisting of MSeCN (or MSCN) and Ph₃P⁺CF₂H Br^-. The tandem process occurs via the first nucleophilic substitution of alkyl bromides by -SeCN (or -SCN) and the subsequent nucleophilic difluoromethylation.

Difluoromethylthiolator: A Toolbox of Reagents for Difluoromethylthiolation

Recently, the strategic installation of a fluorine atom or a fluoroalkyl group site-selectively at the specific position of the target molecule has become a routine approach and daily practice for medicinal chemists in their endeavor to fine tune the structure of the lead compound to improve its physicochemical properties such as the cell membrane permeability and metabolic stability. Among many fluoroalkyl groups, the difluoromethylthio group (-SCF₂H) has attracted recent intense attention. Largely due to the weak acidity of the proton in the difluoromethylthio group, the difluoromethylthio group is generally considered to be a lipophilic hydrogen-bonding donor and a bioisostere of the hydroxy/thio group that might interact with the heteroatom of the enzyme via a hydrogen bond to improve the binding selectivity of the drug molecule. Besides, the difluoromethylthio group is less lipophilic, less electron-withdrawing, and less stable to the acidic or basic environment than its analogue trifluoromethylthio group (-SCF₃), making it easier to regulate the metabolic stability of drug molecules. These beneficial effects render the difluoromethylthio group one of the most favorable functional groups in drug design; consequently, there is an urgent need to develop new strategies for the efficient introduction of the difluoromethylthio group into small molecules under mild conditions. Over the last few decades, several different approaches to the preparation of difluoromethylthiolated compounds have been developed, including the difluoromethylation of thiolated substrates with an electrophilic/nucleophilic difluoromethylating reagent or the insertion of a difluoromethyl carbene into the S-H bond of the thiols. In contrast, we adopt an alternative approach to the preparation of difluoromethylthiolated compounds by late-stage direct difluoromethylthiolation of the specific substrates with a difluoromethylthiolating reagent. With this aim in mind, in the last 6 years we have successfully developed a toolbox of reagents that are capable of the direct introduction of the difluoromethylthio group into the target molecules, including nucleophilic difluoromethylthiolating reagent [(SIPr)AgSCF₂H] I, electrophilic difluoromethylthiolating reagent PhthSCF₂H II, three optically pure difluoromethylthiolating reagents camphorsultam-SCF₂H III, radical difluoromethylthiolating reagent PhSO₂SCF₂H IV, and reagent PhSO₂SCFClH V that could be used for the preparation of ¹⁸F-labeled [¹⁸F]ArSCF₂H. These reagents reacted with a broad range of substrates to get access to difluoromethylthiolated compounds efficiently, thus providing medicinal chemists a powerful weapon for the direct introduction of the difluoromethylthio group into promising molecules during the search for new drugs.

Visible-Light Driven Selective C-N Bond Scission in anti-Bimane-Like Derivatives

In the present study, we report the photochemical transformation of pyrazolo[1,2-a]pyrazolone substrates that reach an excited state upon irradiation with visible light to initiate the homolytic C-N bond cleavage process that yields the corresponding N1-substituted pyrazoles. Moreover, chemoselective heterolytic C-N bond cleavage is possible in the pyrazolo[1,2-a]pyrazole core in the presence of bromomalonate.

Flow Photochemistry as a Tool in Organic Synthesis

Photochemical transformations of molecular building blocks have become an important and widely recognized research field in the past decade. Detailed and deep understanding of novel photochemical catalysts and reaction concepts with visible light as the energy source has enabled a broad application portfolio for synthetic organic chemistry. In parallel, continuous-flow chemistry and microreaction technology have become the basis for thinking and doing chemistry in a novel fashion with clear focus on improved process control for higher conversion and selectivity. As can be seen by the large number of scientific publications on flow photochemistry in the recent past, both research topics have found each other as exceptionally well-suited counterparts with high synergy by combining chemistry and technology. This review will give an overview on selected reaction classes, which represent important photochemical transformations in synthetic organic chemistry, and which benefit from mild and defined process conditions by the transfer from batch to continuous-flow mode.

Novel usage of 2-BTSO2CF2H for metal-free electrophilic difluoroalkanethiolation of indoles

2-BTSO₂CF₂H was used as an efficient electrophilic –SCF₂H synthon for the first time for the metal-free electrophilic difluoromethylthiolation of indoles.

External-photocatalyst-free visible-light-mediated aerobic oxidation and 1,4-bisfunctionalization of N-alkyl isoquinolinium salts

Visible-light-induced aerobic alternate transformations of N-alkyl isoquinolinium/quinolinium salts in the absence of any external photocatalyst have been developed.

Synthesis of 3-HCF2S-Chromones through Tandem Oxa-Michael Addition and Oxidative Difluoromethylthiolation

A simple protocol for the synthesis of difluoromethylthiolated chromen-4-ones using elemental sulfur and ClCF₂CO₂Na as the difluoromethylthiolating agent is described. Three-component reactions of 2'-hydroxychalcones, ClCF₂CO₂Na, and sulfur under basic conditions using TEMPO as the oxidant afforded HCF₂S-containing 4H-chromen-4-one and 9H-thieno[3,2-b]chromen-9-one derivatives in good yield. The protocol is practical and efficient, and the starting materials are cheap and readily available.

Difluoromethylation of Phenols and Thiophenols with the S-(Difluo-romethyl)sulfonium Salt: Reaction, Scope, and Mechanistic Study

A facile and practical approach for the difluoromethylation of phenols and thiophenols was described. Making use of the recently developed bench-stable S-(difluoromethyl)sulfonium salt as the difluorocarbene precursor, a wide variety of diversely functionalized phenols and thiophenols were readily converted to their corresponding aryl difluoromethyl ethers in good to excellent yields in the presence of lithium hydroxide. Chemoselectivity of various O,S-nucleophiles toward difluorocarbene was systematically studied, suggesting the reactivity order ArS- > RS-, ArO- > ROH > RO-, ArSH, ArOH, RSH.

Nickel-Catalyzed Defluorinative Reductive Cross-Coupling Reaction of gem-Difluoroalkenes with Thiosulfonate or Selenium Sulfonate

A nickel-catalyzed defluorinative reductive cross-coupling of gem-difluoroalkenes with thiosulfonate or selenosulfonates is described. The reaction involves the formation of thiolated or selenylated monofluoroolefins via regioselective C-F bond cleavage and C-S or C-Se bond formation and features easily available substrates, mild reaction conditions, and high E-selectivity. One of the derivatives by further cross coupling with PhMgBr exhibited an aggregation-induced emission enhancement effect.

Use of ArSO2SRf reagents: an efficient tool for the introduction of SRf moieties

In recent years, a renewal of interest was shown to the quest for new synthetic solutions to directly introduce emergent fluorinated groups (SRf) onto molecules. In this context, a new generation of reagents (ArSO2SRf) as efficient sources of SCF3, SCF2H and more generally SRf groups, was designed. Hence, potent solutions were developed for the synthesis of SRf-containing molecules, compounds of interest for drug and agrochemical research. This review highlights the recent advances made in the synthesis and the use of this new class of reagents, considerably extending the portfolio of tools for the direct introduction of SRf moieties.

Photoinduced Transition-Metal-Free Cross-Coupling of Aryl Halides with H-Phosphonates

Photoinduced transition-metal- and photosensitizer-free cross-coupling of aryl halides (including Ar-Cl, Ar-Br, and Ar-I) with H-phosphonates (including dialkyl phosphonates and diarylphosphine oxides) is reported. Various functional groups were tolerated, including ester, methoxy, dimethoxy, alkyl, phenyl, trifluoromethyl, and heterocyclic compounds. This simple and green strategy provides a practical pathway to synthesize arylphosphine oxides.

Exploration of new reaction tools for late-stage functionalization of complex chemicals

Chemistry has always had as a target the conversion of molecules into valuable materials. Nevertheless, the aim of past synthesis has primarily focused on achieving a given transformation, regardless of the environmental impact of the synthetic route. Given the current global situation, the demand for sustainable alternatives has substantially increased. Our group focuses on developing selective chemical transformations that benefit from mild conditions, improved atom economy, and that can make use of renewable feedstocks as starting materials. This account summarizes our work over the past two decades specifically regarding the selective removal, conversion, and addition of functional groups that can, later on, be applied at a late stage for the modification of complex molecules.

Visible-light-promoted direct C–H/S–H cross-coupling of quinoxalin-2(1H)-ones with thiols leading to 3-sulfenylated quinoxalin-2(1H)-ones in air

A new and efficient visible-light-mediated strategy has been developed for the synthesis of 3-sulfenylated quinoxalin-2(1H)-ones via rhodamine B catalyzed C–H/S–H cross-coupling of quinoxalin-2(1H)-ones with thiols in air at room temperature.