Dual photoredox/copper-catalyzed selective difluoromethylthiolation of remote unactivated C(sp3)-H bonds at room temperature

A photocatalysis/copper dual-catalyzed difluoromethylthiolation of remote unactivated C(sp3)-H bonds using N-fluorosulfonamides was reported. The combination of photoredox and copper catalysis led to mild reaction conditions with a broad substrate scope. Luminescence quenching experiments revealed that both N-fluorosulfonamides and the copper catalyst could quench the excited state of the Ru complex. Primary, secondary and tertiary C(sp3)-H bonds could be transformed, offering an efficient way to construct difluoromethylthio-bearing compounds.

Ferric Nitrate as a Bifunctional Catalyst for Dehydration and Oxidative Cleavage-Esterification of Tertiary Alcohols

The selective oxidative cleavage and functionalization of C(OH)-C bonds in tertiary alcohols harbor immense feasibility in organic synthesis and enable the production of high value-added chemicals from renewable biomass. However, it remains a challenge, owing to the inherent kinetic inertness and thermodynamic stability of C(OH)-C bonds and the lack of Cα-H. Taking the huge potential and challenge of C(OH)-C bond activation and functionalization into consideration, herein, we show the first example of an inexpensive bifunctional ferric nitrate catalyst for catalytic direct oxidation of structurally distinct tertiary alcohols to esters with environmentally benign molecular oxygen as an oxidant and MeOH as a solvent, without the assistance of any additives. Detailed mechanistic studies reveal that this tandem catalytic oxidative process is initiated by the synergistic effects of an iron ion and nitrate ion, which serve as Lewis acids for dehydrating and a nitrogen dioxide radical precursor for inducing an oxidative cleavage, respectively.

Synthesis of ω-functionalized ketones from strained cyclic alcohols by ring-opening and cross-recombination between alkyl and N-oxyl radicals

Radical ring-opening oxyimidation of cyclobutanols and cyclopropanols with the formation of ω-functionalized ketones was discovered. The oxidative C-O coupling proceeds via the interception of a primary alkyl radical generated from a cyclic alcohol with a reactive radical generated in situ, which is an electron-deficient N-oxyl radical. The developed conditions allow for the balanced generation rates of carbon- and N-oxyl radicals, which are necessary for their selective cross-recombination. Thus, typical competitive dimerization processes of carbon-centered radicals, their intermolecular cyclization, and N-oxyl radical self-decay are suppressed. The method is applicable to a wide range of cyclobutanols and results in oxyimidated ketones in yields of up to 82%.

Photo-induced decarboxylative C-S bond formation to access sterically hindered unsymmetric S-alkyl thiosulfonates and SS-alkyl thiosulfonates

Due to the high reactivity and versatility of benzenesulfonothioates, significant advancements have been made in constructing C-S bonds. However, there are certain limitations in the synthesis of S-thiosulfonates and SS-thiosulfonates, especially when dealing with substantial steric hindrance, which poses a significant challenge. Herein, we present an innovative approach for assembling unsymmetric S-thiosulfonates and unsymmetric SS-thiosulfonates through the integration of dual copper/photoredox catalysis. Moreover, we also realized the one-pot strategy by directly using carboxylic acids as raw materials by in-situ activation of them to access S-thiosulfonates and SS-thiosulfonates without further purification and presynthesis of NHPI esters. The envisaged synthesis and utilization of these reagents are poised to pioneer an innovative pathway for fabricating a versatile spectrum of mono-, di-, and polysulfide compounds. Furthermore, they introduce a class of potent sulfenylating reagents, empowering the synthesis of intricate unsymmetrical disulfides that were previously challenging to access.

Organophotocatalytic Remote Thiocyanation Reaction via Ring-Opening Functionalization of Cycloalkanols

The remote C(sp3)-SCN bond formation via ring-opening functionalization of cycloalkanols with N-thiocyanatosaccharin as the precursor of SCN radicals and pyrylium salt as the organic photocatalyst under visible light has been developed. Thus, various terminal keto thiocyanates were prepared without transition metals and oxidants in moderate to good yields. The simplicity, wide substrate scope and mild conditions feature its synthetic application capability.

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.

Construction of C-S and C-Se Bonds from Unstrained Ketone Precursors under Photoredox Catalysis

A mild photoredox catalyzed construction of sulfides, disulfides, selenides, sulfoxides and sulfones from unstrained ketone precursors is introduced. Combination of this deacylative process with SN 2 or coupling reactions provides novel and convenient modular strategies toward unsymmetrical or symmetric disulfides. Reactivity studies favor a bromine radical that initiates a HAT (Hydrogen Atom Transfer) from the aminal intermediate resulting in expulsion of a C-centered radical that is intercepted to make C-S and C-Se bonds. Gram scale reactions, broad substrate scope and tolerance towards various functional groups render this method appealing for future applications in the synthesis of organosulfur and selenium complexes.

[(SIPr)Ag(CF2 H)]: A Shelf-Stable, Versatile Difluoromethylation Reagent

Due to its unique physical and electrophilic properties, the difluoromethyl group (-CF2 H) has been playing an irreplaceable role in the field of pharmaceutical and agrochemical industry. Methods that could efficiently incorporate the difluoromethyl group into the target molecules are increasing in the recent years. Developing a stable and efficient difluoromethylating reagent is thus highly attractive. In this review, we describe the development of a nucleophilic difluoromethylation reagent [(SIPr)Ag(CF2 H)], including its elemental reaction, difluoromethylation reaction with different types of electrophiles, and its application in the synthesis of a nucleophilic and an electrophilic difluoromethylthiolating reagent.

Copper-catalyzed ring-opening trifluoromethylthiolation/trifluoromethylselenolation of cyclopropanols with TsSCF3 or Se-(trifluoromethyl) 4-methoxybenzenesulfonoselenoate

We report a ring-opening trifluoromethylthiolation of cyclopropanols with TsSCF₃ by using Cu(OAc)₂ as the catalyst. Moreover, by using this strategy, the trifluoromethylselenolation of cyclopropanols with Se-(trifluoromethyl) 4-methoxybenzenesulfonoselenoate to access β-SeCF₃-substituted carbonyl compounds is achieved for the first time. The broad substrate scope, readily accessible reagents and cheap catalyst make this protocol an alternative and efficient method for the synthesis of β-SCF₃-substituted or β-SeCF₃-substituted carbonyl compounds.

Deacylative Thiolation by Redox-Neutral Aromatization-Driven C-C Fragmentation of Ketones

Herein we report the development of deacylative thiolation of diverse methyl ketones. The reaction is redox-neutral, and heavy-metal-free, which provides a new way to introduce thioether groups site-specifically to unactivated aliphatic positions. It also features excellent functional group tolerance and broad substrate scope, thus allowing late-stage derivatization. The process benefits from efficient condensation between the activation reagent and ketone substrates, which triggers aromatization-driven C-C fragmentation and rapid radical coupling with thiosulfonates. Experimental and computational mechanistic studies suggest the involvement of a radical chain pathway.

Free Radical Promoted Trifluoromethylthiolation of Alkynes to Access SCF3-Containing Dibenzazepines or Dioxodibenzothiazepines

Persulfate-promoted radical cascade trifluoromethylthiolation of aryl acetylenes with AgSCF₃ provides a simple reaction system for the synthesis of SCF₃-substituted dibenzazepines or dioxodibenzothiazepines with good Z/E selectivity. The single-crystal X-ray diffraction data confirms the structures of the final products. A series of scaled-up experiments, further transformations, and radical inhibition experiments were operated in the reaction system.

Lewis-Acid-Catalyzed Tandem Cyclization by Ring Expansion of Tertiary Cycloalkanols with Propargyl Alcohols

A new method for the efficient synthesis of hexahydro-1H-fluorene and octahydrobenzo[a]azulene derivatives through a ring-expansion strategy is reported. With an appropriate combination of thulium(III) trifluoromethanesulfonate and 13X molecular sieves, a range of unsaturated polycyclic compounds were obtained in good yields. Mechanism studies reveal that the reaction is more likely to undergo Meyer-Schuster rearrangement, ring expansion, and Friedel-Crafts-type pathways, which provide a conceptually different strategy for the ring opening of tertiary cycloalkanols.

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.

Catalyst-free gem-chlorosulfurization of difluoromethyl-substituted diazo compounds with disulfide and PhICl2

A series of gem-chlorosulfurization products bearing difluoromethyl substituents were synthesized in high to excellent yields directly from p-toluenesulfonyl difluorodiazoethane (TsCF₂CHN₂), disulfides and PhICl₂ without any catalysts or additives. The mild reaction conditions and high functional group compatibility indicated the utility and sustainability of the method. In addition, the gem-chlorosulfurization products could be efficiently converted to sulfur-containing and aryl substituted difluoromethyl derivatives by a feasible multi-component operation.

Electrochemical Trifluoromethylthiolation and Spirocyclization of Alkynes with AgSCF3: Access to SCF3-Containing Spiro[5,5]trienones

A novel and efficient strategy for trifluoromethylthiolation and dearomatization of activated alkynes with stable and readily available AgSCF₃ has been developed. Reported herein is the unprecedented electrochemical generation of the SCF₃ radical in the absence of persulfate for the synthesis of SCF₃-containing spiro[5,5]trienones in good yields via a 6-exo-trig radical cyclization.

Electrophotochemical Ring-Opening Bromination of tert-Cycloalkanols

An electrophotochemical ring-opening bromination of unstrained tert-cycloalkanols has been developed. This electrophotochemical method enables the oxidative transformation of cycloalkanols with 5- to 7-membered rings into synthetically useful ω-bromoketones without the use of chemical oxidants or transition-metal catalysts. Alkoxy radical species would be key intermediates in the present transformation, which generate through homolysis of the O-Br bond in hypobromite intermediates under visible light irradiation.

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.

Radical-Promoted Distal C-H Functionalization of C(sp3 ) Centers with Fluorinated Moieties

Due to their unique properties, fluorinated scaffolds are pivotal compounds in pharmaceuticals, agrochemicals, and materials science. Over the last years, the development of versatile strategies for the selective synthesis of fluorinated molecules by direct C-H bond functionalization has attracted a lot of attention. In particular, the design of novel transformations based on a radical process was a bottleneck for distal C-H functionalization reactions, offering synthetic solutions for the selective introduction of fluorinated groups. This Minireview highlights the major contributions in this blossoming field. The development of new methodologies for the remote functionalization of aliphatic derivatives with various fluorinated groups based on a 1,5-hydrogen atom transfer process and a β-fragmentation reaction will be showcased and discussed.

Alkylation-Terminated Catellani Reactions by Cyclobutanol C-C Cleavage

This report describes the first application of a cyclobutanol ring-opening procedure in the Catellani termination reaction, which includes two β-carbon elimination processes. This tandem reaction features mild conditions, high yields, good functional group tolerance, and a broad substrate scope. Meanwhile, four types of electrophiles (N-benzoyloxyamines, alkyl iodides, aryl bromides, and benzyl chlorides) are quite compatible with this termination reaction for the construction of various types of polysubstituted aromatic hydrocarbons.

Nickel-Catalyzed Arylation/Alkenylation of tert-Cyclobutanols with Aryl/Alkenyl Triflates via a CC Bond Cleavage

Herein, we first present a nickel-catalyzed arylation and alkenylation of tert-cyclobutanols with aryl/alkenyl triflates via a C-C bond cleavage. An array of γ-substituted ketones was obtained in moderate-to-good yields, thus featuring earth-abundant nickel catalysis, broad substrate scope, and simple reaction conditions. Preliminary mechanistic experiments indicated that β-carbon elimination pathways might be involved in the catalytic cycle.

δ-Regioselective heteroarylation of free alcohols through 1,5-hydrogen-atom transfer

Silver-catalyzed δ-regioselective heteroarylation of C(sp³)–H bonds (1°, 2°, and 3°) in alcohols with divergent N-heteroarenes.

TFA-Catalyzed [3+2] Spiroannulation of Cyclobutanols: A Route to Spiro[cyclobuta[a]indene-7,1'-cyclobutane] Skeletons

A straightforward method for the synthesis of spiro[cyclobuta[a]indene-7,1'-cyclobutane] derivatives from cyclobutanols has been developed via one-pot [3+2] spiroannulation. A series of new spiro[cyclobuta[a]indene-7,1'-cyclobutane] derivatives are facilely synthesized in good yields under mild reaction conditions.

Silver-promoted oxidative sulfonylation and ring-expansion of vinylcyclopropanes with sodium sulfinates leading to dihydronaphthalene derivatives

Silver-promoted sulfonylation and ring-expansion of vinylcyclopropanes with sodium sulfinates is established for the construction of 1-sulfonylmethylated 3,4-dihydronaphthalenes. This sulfonylation process involves a radical pathway, including sulfonyl radical formation, radical addition, ring-opening and cyclization. The 1-sulfonylmethylated 3,4-dihydronaphthalenes can be converted into other useful products.

Cleavage of Carbon-Carbon σ-Bonds of Four-Membered Rings

This article reviews synthetic transformations involving cleavage of a carbon-carbon bond of a four-membered ring, with a particular focus on the examples reported during the period from 2011 to the end of 2019. Most significant is the progress of catalytic reactions involving oxidative addition of carbon-carbon bonds onto transition metals or β-carbon elimination of transition metal alkoxides. When they are looked at from synthetic perspectives, they offer unique and efficient methods to build complex natural products and structures that are difficult to construct by conventional methods. On the other hand, β-scission of radical intermediates has also attracted increasing attention as an alternative elementary step to cleave carbon-carbon bonds. Its site-selectivity is often complementary to that of transition metal-catalyzed reactions. In addition, Lewis acid-mediated and thermally induced ring-opening of cyclobutanone derivatives has garnered renewed attention. On the whole, these examples demonstrate unique synthetic potentials of structurally strained four-membered ring compounds for the construction of organic skeletons.

A silver-catalyzed radical ring-opening reaction of cyclopropanols with sulfonyl oxime ethers

A silver-catalyzed ring-opening reaction of cyclopropanols with sulfonyl oxime ethers has been developed. The protocol was conducted under mild reaction conditions to provide a series of γ-keto oxime ethers with moderate to good yields. The reaction proceeded in a stereoselective manner for CF3-containing oxime ethers to provide a single stereoisomer, while an inseparable E and Z mixture was obtained for CN-containing oxime ethers. Mechanistic studies indicate that the reaction proceeded via a radical mechanism.

Remote Trifluoromethylthiolation Enabled by Organophotocatalytic C-C Bond Cleavage

The first metal-free ring opening/trifluoromethylthiolation of cycloalkanols for the formation of remote C(sp³)-SCF₃ bonds has been developed. A variety of trifluoromethylthiolated carbonyl compounds that are otherwise difficult to achieve were prepared in good yields under mild reaction conditions. The reaction is assumed to proceed via C-C bond cleavage of the alkoxyl radical species generated via a photoredox-enabled intramolecular proton-coupled electron transfer process, followed by trifluoromethylthiolation of the resulting C-centered radical with the N-(trifluoromethylthio)phthalimide reagent.

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.

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.

Oxidative radical ring-opening/cyclization of cyclopropane derivatives

The ring-opening/cyclization of cyclopropane derivatives has drawn great attention in the past several decades. In this review, recent efforts in the development of oxidative radical ring-opening/cyclization of cyclopropane derivatives, including methylenecyclopropanes, cyclopropyl olefins and cyclopropanols, are described. We hope this review will be of sufficient interest for the scientific community to further advance the application of oxidative radical strategies in the ring-opening/cyclization of cyclopropane derivatives.

Silver-catalyzed cascade radical cyclization of 2-(allyloxy)arylaldehydes with cyclopropanols: access to chroman-4-one derivatives

A convenient silver-catalyzed cascade radical cyclization of 2-(allyloxy)arylaldehydes with cyclopropanols was developed to synthesize carbonyl-containing alkyl-substituted chroman-4-one derivatives.

Visible-light promoted fluoroalkylselenolation: toward the reactivity of unsaturated compounds

The reactivity of fluoroalkylselenotoluenesulfonates with unsaturated substrates is explored herein. The direct activation of these shelf-stable reagents under visible light allows the double functionalisation of alkenes or alkynes efficiently, leading to a wide range of β-fluoroalkylselenolated sulfones. Mechanistic investigations have been undertaken supporting the formation of radical intermediates.