Photocatalytic Defluorinative Three-Component Reaction of α-Trifluoromethyl Alkenes, Alkenes, and Sodium Sulfinates: Synthesis of Monofluorocyclopentenes

Visible-Light-Mediated Addition Reactions of Sulfur-Containing Reagents with α-Trifluoromethyl Alkenes

Herein, we described the addition reactions of sulfur-containing reagents (sodium sulfinates, dithiosulfonates) with α-trifluoromethyl alkenes under visible light. A series of trifluoromethyl sulfonates were synthesized via the visible-light-induced radical addition reaction of sodium sulfinates and α-trifluoromethyl alkenes to obtain protons from the solvent. A series of dithiosulfonated derivatives were synthesized via visible-light-induced bifunctionalization reaction of α-trifluoromethyl alkenes with dithiosulfonates.This strategy has the advantages of mild reaction conditions, good substrate universality and high yield up to 99% yield.

Photocatalyzed/Base-Mediated Defluorinative Three-Component Cascade Cyclization: Access to Monofluorocyclohexenes and 6-Fluoro-1,2,3,4-tetrahydropyridines

A three-component cascade cyclization of α-CF3 alkenes, electron-rich alkenes, and dimethyl esters or sulfonamides via a dual C-F bond cleavage process is described. This methodology provides a general and efficient strategy to access monofluorocyclohexenes and 6-fluoro-1,2,3,4-tetrahydropyridines by switching the bifunctional reagents (dimethyl esters or sulfonamides), which are valuable building blocks in synthetic chemistry and the pharmaceutical industries. The reaction mechanism and the synthetic applications of the products have been demonstrated.

Recent progress in C-S bond formation via electron donor-acceptor photoactivation

Recent advancements in C-S bond formation via electron donor-acceptor (EDA) complex photoactivation have been remarkable. EDA complexes, which are composed of electron donors and acceptors, facilitate C-S bond construction under mild conditions through single-electron transfer events upon visible light irradiation. This review highlights the utilization of various sulfur-containing substrates, including diacetoxybenzenesulfonyl (DABSO), sulfonic acids, sodium sulfinates, sulfonyl chlorides, and thiophenols, in EDA-promoted sulfonylation and thiolation reactions, covering the works published since 2017 to date. These reactions offer novel, environmentally friendly pathways for the synthesis of sulfur-containing compounds.

Construction of 3,6-Difluoropyridones via a Double Defluorinative [3 + 3] Annulation of α-Fluoro-α-sulfonylacetamides with 2-CF3-Alkenes

A remarkably simple and efficient double defluorinative [3 + 3] annulation approach involving N-phenyl-α-fluoro-α-phenylsulfonylacetamide and 2-CF3-alkenes to access N-phenyl-3,6-difluoropyridone derivatives has been achieved. The key to the success of this single-step synthesis of difluoropyridones is the strategic utilization of 2-CF3-alkenes for consecutive allylic and vinylic substitution reactions and a desulfonylation cascade. We could also show that these difluoropyridones serve as a versatile platform for C-6-selective defluorinative functionalizations.

Catalyst-Free Radical Carbosulfonylation of Enamides with Indoles, Aryldiazonium Tetrafluoroborates, and DABCO·(SO2)2

Herein, we have developed a catalyst-free four-component carbosulfonylation of enamides involving indoles, DABCO·(SO2)2, and aryldiazonium tetrafluoroborates for the preparation of various β-amidosulfone products in moderate to excellent yields. This approach features mild reaction conditions, high step-efficiency, and broad substrate scope, which provides a green and efficient strategy for carbosulfonyl difunctionalization of enamides. Based on the results of mechanism studies, a radical tandem reaction process is proposed for the transformation.

Electron Donor-Acceptor Complex Enabled Cyclization/Sulfonylation Cascade of N-Heterocycles with Thianthrenium Salts

We report a visible-light-promoted cyclization/sulfonylation cascade of N-heterocycles with thianthrenium salts using DABSO as the SO2 surrogate. This method features excellent functional group tolerance, wide substrate scope, and late-stage elaboration of bioactive relevant molecules. Mechanistic investigations reveal that the photoactive electron donor-acceptor (EDA) complexes between thianthrenium salts and DABCO are capable of the generation of aryl radicals, which induce the following SO2 insertion by attacking DABSO, thus triggering the key radical cyclization step.

Photoredox-Catalyzed Sequential Decarboxylative/Defluorinative Aminoalkylation of CF3-Alkenes with N-Arylglycines

A photoredox-catalyzed sequential decarboxylative/defluorinative aminoalkylation of CF3-alkenes with N-arylglycines is described. This metal-free and redox-neutral protocol provided efficient access to the monofluoroalkenyl-1,5-diamines in good yields with excellent functional group compatibility. Mechanistic studies revealed that the reaction proceeds via a radical pathway with the gem-difluoroalkenyl amine as an intermediate.

Transition-Metal-Free Hydrodefluoroamination of Trifluoromethyl Enones for the Synthesis of α-Fluoroenamides

A three-component strategy was developed to enable hydrodefluoroamination of β-trifluoromethyl enones by selectively activating two C(sp3)-F bonds in the trifluoromethyl group. The method involved a sequence of carbonyl reduction, hydrodefluorination, and defluoroamination under transition-metal-free conditions. Synthetically useful (E)-stereospecific α-fluoroenamides were obtained in good yields with diverse functional group tolerance, which could be easily transformed into valuable organofluorides and heterocycles. The carbonyl auxiliary exerts both electronic and steric impacts on the CF3-alkenes, allowing for controllable and selective defluorination.

Synthesis of constrained bicycloalkanes through bibase-promoted brook rearrangement/radical-polar crossover cyclization

Highly constrained bicyclic scaffolds are ubiquitous and attracting increasing interest in pharmaceutical and biotechnology discoveries owing to the enhanced activities. Herein, we report a protocol to access highly substituted constrained bicycloalkanes from readily accessible α-silyl alcohols and olefins through a bibase-promoted Brook rearrangement/radical-polar crossover cyclization (RPCC) process. Of note, the practical procedure features broad substrate scope and good group tolerance under mild and operationally simple conditions, using an inexpensive organic photocatalyst. Gram-scale preparation and diverse synthetic transformations demonstrate opportunities to rapidly construct molecular complexity. Mechanistic studies have indicated that the transformation involves a bibase-promoted radical transfer rearrangement addition/radical-polar crossover cyclization relay sequence, which differs from traditional solitary RPCC reactions.

Reversible Radical Addition Guides Selective Photocatalytic Intermolecular Thiol-Yne-Ene Molecular Assembly

In modern organic chemistry, harnessing the power of multicomponent radical reactions presents both significant challenges and extraordinary potential. This article delves into this scientific frontier by addressing the critical issue of controlling selectivity in such complex processes. We introduce a novel approach that revolves around the reversible addition of thiyl radicals to multiple bonds, reshaping the landscape of multicomponent radical reactions. The key to selectivity lies in the intricate interplay between reversibility and the energy landscapes governing C-C bond formation in thiol-yne-ene reactions. The developed approach not only allows to prioritize the thiol-yne-ene cascade, dominating over alternative reactions, but also extends the scope of coupling products obtained from alkenes and alkynes of various structures and electron density distributions, regardless of their relative polarity difference, opening doors to more versatile synthetic possibilities. In the present study, we provide a powerful tool for atom-economical C-S and C-C bond formation, paving the way for the efficient synthesis of complex molecules. Carrying out our experimental and computational studies, we elucidated the fundamental mechanisms underlying radical cascades, a knowledge that can be broadly applied in the field of organic chemistry.

Picking Two out of Three: Defluorinative Annulation of Trifluoromethyl Alkenes for the Synthesis of Monofluorinated Carbo- and Heterocycles

The increasing demand of organofluorine compounds in medicine, agriculture, and materials sciences makes sophisticated methods for their synthesis ever more necessary. Nowadays, not only the C-F bond formation but also the selective C-F bond cleavage of readily available poly- or perfluorine-containing compounds have become powerful tools for the effective synthesis of organofluorine compounds. The defluorinative cross-coupling of trifluoromethyl alkenes with various nucleophiles or radical precursors in an SN 2' manner is a convergent route to access gem-difluoroalkenes, which in turn react with nucleophiles or radical precursors via an SN V-type reaction. If the SN V reactions occur intramolecularly, the dual C-F bond cleavage of trifluoromethyl alkenes allows facile assembly of monofluorinated cyclic skeletons with structural complexity and diversity. In this personal account, we summarized the advances in this field on the basis of coupling and cyclization partners, including binucleophiles, alkynes, diradical precursors and radical precursors bearing a nucleophilic site. Accordingly, the annulation reactions can be achieved by base-mediated sequential SN 2'/SN V reactions, transition metal catalyzed or mediated reactions, photoredox catalysis, and the combination of photocatalytic reactions with SN V reaction. In the context of seminal works of others in this field, a concise summary of the contributions of the authors is also offered.

Photoredox-Catalyzed Defluorinative Carboxylation of gem-Difluorostyrenes with Formate Salt

Herein, we present a transition-metal-free, easy handling protocol for regioselective carboxylation of gem-difluorostyrenes with sodium formate as the C1 source. 30 examples of α-fluoracrylates were obtained in yields of 30 to 80% under these conditions. A defluorinative monofluorovinyl intermediate and consecutive photoinduced electron transfer mechanism were proposed after mechanism investigation.

Electrocatalysis-Enabled Defluorinative Cross-Coupling of gem-Difluoroalkenes with Aldehydes and Ketones

An electrochemical defluorinative cross-coupling of gem-difluoroalkenes with carbonyl compounds was described, by which highly stereoselective monofluoroalkene allyl alcohols were synthesized. The reaction tolerates a broad range of functional groups and has successfully been applied to synthesize complex molecules. Mechanistic studies indicate that the reaction starts from electron reduction of gem-difluoroalkenes to generate radical negative ions, which undergo β-fluoride elimination and subsequent reduction to form anions. These anions are subsequently trapped by carbonyl compounds to furnish target products.

Visible-Light-Mediated Three-Component Decarboxylative Coupling Reactions to Synthesize 1,4-Diol Monoethers

An efficient approach for 1,2-difunctionalization of aromatic olefins and the synthesis of functionalized 1,4-diols monoethers has been established via a photoinduced three-component reaction of an α-alkoxycarboxylic acid, an aromatic olefin, and an aldehyde. The reaction proceeds by photoinduced oxidative decarboxylation of the carboxylic acid followed by the addition of the α-alkoxyalkyl radical to the olefin, one-electron reduction of the addition radical, and the nucleophilic attack of the resulting carbanion to the aldehyde. Besides the convenient one-pot protocol of the three-component reaction, this method offers several other advantages, including good functional group tolerance for the three substrates, gentle reaction conditions, and ease of scaling up. The reaction mechanism has been investigated through free radical trapping experiment and isotope labeling experiments.

Combining Hydrodefluorination and Defluorophosphorylation for Chemo- and Stereoselective Synthesis of gem-Fluorophosphine Alkenes

A chemo-, regio-, and stereoselective reaction of trifluoromethyl enones, phenylsilane, and phosphine oxides through a sequential hydrodefluorination and defluorophosphorylation relay is developed for the synthesis of distinctive gem-fluorophosphine alkenes. This multicomponent reaction occurred under transition-metal-free conditions with good functional group tolerance. Moreover, the preinstalled carbonyl auxiliary is important for tuning the reactivity of β-trifluoromethyl enones, thereby enabling controllable and selective functionalization of two fluorine atoms in trifluoromethylated enones.

Radical-Polar Crossover Enabled Triple Cleavage of CF2Br2: A Multicomponent Tandem Cyclization to 3-Fluoropyrazoles

The elaboration of step-economy and catalytic approaches for accessing diverse fluorinated heterocyclics is highly desirable. Described herein is a radical-polar crossover enabled three-component cyclization to polysubstituted fluoropyrazoles by using CF2Br2 as a novel C1F1 synthon. Mechanistic experiments revealed that the in situ generation of the reactive intermediate gem-difluorovinylimine ion is the key to this transformation. This protocol unlocks the novel reactivity of CF2Br2 and adds significant synthetic values to fluorine chemistry.

Photoredox radical/polar crossover enables C-H gem-difunctionalization of 1,3-benzodioxoles for the synthesis of monofluorocyclohexenes

A photocatalytic C-H gem-difunctionalization of 1,3-benzodioxoles with two different alkenes for the synthesis of highly functionalized monofluorocyclohexenes is described. Using 4CzIPN as the photocatalyst, the direct single electron oxidation of 1,3-benzodioxoles allows their defluorinative coupling with α-trifluoromethyl alkenes to produce gem-difluoroalkenes in a redox-neutral radical polar crossover manifold. The C-H bond of the resultant γ,γ-difluoroallylated 1,3-benzodioxoles was further functionalized via radical addition to electron-deficient alkenes using a more oxidizing iridium photocatalyst. The capture of in situ generated carbanions by an electrophilic gem-difluoromethylene carbon and consecutive β-fluoride elimination afford monofluorocyclohexenes. The synergistic combination of multiple termination pathways of carbanions enables rapid incorporation of molecular complexity via stitching simple and readily accessible starting materials together.

Controllable Synthesis of Trifluoromethyl- or gem-Difluorovinyl-containing Analogues of Neonicotinoids by the Reaction of α-(Trifluoromethyl)styrenes with 2-Nitroimino-imidazolidine

A simple and straightforward addition or defluorination of α-(trifluoromethyl)styrenes with 2-nitroimino-imidazolidine (2a), 2-(nitromethylene)imidazolidine (2b), 2-cyanoimino-thiazolidine (2c), and (E)-1-methyl-2-nitroguanidine (2d), in a controlled manner, was developed. The hydroamination of α-(trifluoromethyl)styrenes with 2a, 2b, 2c, and 2d was completed in the presence of DBN at room temperature within 0.5-6 h, affording structurally diverse β-trifluoromethyl-β-arylethyl analogues of neonicotinoids in moderate to good yields. The γ,γ-difluoro-β-arylallyl analogues of neonicotinoids were also successfully synthesized via defluorination of α-(trifluoromethyl)styrenes, with 2a and 2c using NaH as base at an elevated temperature together with a prolonged reaction time of 12 h. The method features simple reaction setup, mild reaction conditions, broad substrate scope, high functional group compatibility, and easy scalability.