Hydrogen Bond Directed Photocatalytic Hydrodefluorination: Overcoming Electronic Control

Photoredox Catalyzed Single C-F Bond Activation of Trifluoromethyl Ketones: A Solvent Controlled Divergent Access of gem-Difluoromethylene Containing Scaffolds

Selective defluorinative functionalization of trifluoromethyl ketones is a long-standing challenge owing to the exhaustive mode of the process. To meet the demands for the installation of the gem-difluoromethylene unit for the construction of the molecular architectures of well-known pharmaceuticals and agrochemicals, a distinct pathway is thereby highly desirable. Here, a protocol is introduced that allows the divergent synthesis of gem-difluoromethylene group containing tetrahydrofuran derivatives and linear ketones via single C-F bond activation of trifluoromethyl ketones using visible-light photoredox catalysis in the presence of suitable olefins as trapping partner. The choice of appropriate solvent and catalyst plays a significant role in controlling the divergent behavior of this protocol. Highly reducing photo-excited catalysts are found to be responsible for the generation of α,α-difluoromethyl ketone (DFMK) radicals as the key intermediate via a SET process. This protocol also results in a high diastereoselectivity towards the formation of partially fluorinated cyclic ketal derivatives with simultaneous construction of one C-C and two C-O bonds. State-of-the-art DFT calculations are performed to address the origin of diastereoselectivity as well as the divergence of this protocol.

Late-Stage Modification of Drugs via Alkene Formal Insertion into Benzylic C-F Bond

C-F insertion of carbon-atom units is underdeveloped although it poses significant potential applications in both drug discovery and development. Herein, we report a photocatalytic protocol for late-stage modification of trifluoromethyl aromatic drugs involving formal insertion of abundant alkene feedstocks into a benzylic C-F bond selectively. This redox-neutral transformation features mild conditions and extraordinary functional group tolerance. Preliminary studies are consistent with this transformation involving a radical-polar crossover pathway. Additionally, it offers an alternative strategy for difunctionalization of alkenes via quenching of the carbocation intermediate with nucleophiles other than external fluoride.

Fluoro, Trifluoromethyl, and Trifluoroacetyl Substituent Effects on Cycloaddition Reactivities: Computations and Analysis

The importance of fluoro and trifluoromethyl substituents in drug effectiveness prompted the computational exploration of fluorine-containing substituents in valuable synthetic cycloadditions. Diels-Alder or 1,3-dipolar cycloaddition reactions of typical reactants, cyclopentadiene, N-phenyldiazoacetamide, tetrazine, and N-phenylsydnone involving fluorine-containing substituents (F, CF₃, and COCF₃) were studied with M06-2X density functional theory. Inductive and conjugative effects influence normal and inverse electron-demand reactions differently. These results provide a guide to the design and use of cycloadditions for the introduction of fluoro and trifluoromethyl substituents in synthetic processes.

Chemodivergent Csp3─F bond functionalization and cross-electrophile alkyl-alkyl coupling with alkyl fluorides

The widespread use of fluorinated organic compounds in the health, agrochemical, and materials sciences is sustained by a steadily growing pool of commercially available fine chemicals. The synthetic utility of the increasingly ubiquitous Csp³─F bond, however, remains to be fully exploited, which is often a difficult task because of its paramount stability and chemical inertness. Here, we demonstrate chemodivergent activation of monofluoroalkyl compounds toward either nucleophilic or electrophilic intermediates. This is accomplished under conditions that are compatible with several reaction types and many functional groups, which drastically widens the current scope of organofluorine chemistry and sets the stage for carbon-carbon and carbon-heteroatom bond formations, stereoselective construction of bisoxindole alkaloid scaffolds via in situ Umpolung, and cross-electrophilic coupling methodology. The selective generation of either nucleophilic or electrophilic species and the possibility of doing so simultaneously or, alternatively, switching molecular polarity enable previously unidentified synthetic opportunities that recognize alkyl fluorides as chemodivergent building blocks.

HFIP-Assisted Single C-F Bond Activation of Trifluoromethyl Ketones using Visible-Light Photoredox Catalysis

A visible light photoredox catalytic method for the selective cleavage of single strong C-F bond in trifluoromethyl ketones is reported. Single electron reduction of trifluoromethyl ketones generates difluoromethyl radicals which can be engaged in intermolecular C-C bond formation with N-methyl-N-arylmethacrylamides to furnish fluorine-containing oxindole derivatives in good yields. The reaction shows excellent chemoselectivity with good functional group tolerance under mild conditions. 1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) as a solvent plays a critical role for the selective single C-F bond cleavage. High-level DFT calculations are depicted to shed light on the mechanism.

Recent advances in radical-based C-F bond activation of polyfluoroarenes and gem-difluoroalkenes

The direct employment of polyfluoroarenes and gem-difluoroalkenes as building blocks is regarded as one of the most effective and straightforward strategies for the introduction of fluorine-containing moieties into organic skeletons. Accordingly, radical chemistry has gradually become a mild and powerful method for the activation of their C-F bonds. The radical-based transformations of polyfluoroarenes and gem-difluoroalkenes can be primarily categorized into three types based on the specific intermediates involved: (1) multifluoroaryl radical anions, (2) monofluoroalkenyl radicals and (3) other radicals. Compared with the more established multifluoroaryl radical anion pathway, the monofluoroalkenyl radical-involved cross-coupling reaction can proceed through C-radical cross-coupling, radical addition/elimination or the hydrogen atom transfer process. For the presented examples in this review, the typical reaction modes, substrate scope, radical-involved mechanisms, and late-stage applications in the modification of bioactive molecules are discussed, aiming to provide a comprehensive overview of the recent advances of the radical-based transformations of polyfluoroarenes and gem-difluoroalkenes.

Hydrogen-bonded nickel(I) complexes

A series of nickel(ii) tris(2-pyridylmethyl)amine (TPA) complexes featuring appended hydrogen bonds (H-bonds) to halides (F, Cl, Br) was synthesized and charcterized. Reduction to the nickel(i) state provided access to an unusual nickel(i) fluoride complex stabilized by H-bonds, enabling structural and spectroscopic characterization.

Ruthenium-Catalyzed Hydrodefluorination with Silane as the Directing and Reducing Group

We describe herein an efficient and selective Ru-catalyzed intramolecular HDF directed by a silyl group, which is readily installed, and removable and transformable following the HDF reaction. The hydrosilyl group in polyfluoroaryl silane acts not only as the directing group but also as the internal reductant, enabling precise control of the ortho-selectivity and avoiding overdefluorination. Mechanistic studies reveal a plausible catalytic cycle involving a Ru(IV)-aryne intermediate.

Organic synthesis of fixed CO2 using nitrogen as a nucleophilic center

In this review, recent progress in the application of CO2 as an electrophilic reagent and nitrogen as a nucleophilic center under different catalytic conditions in organic synthesis is summarized. The used catalytic methods in the reactions of CO2 and nitrogen are classified as metal catalysis, metal-free catalysis, photocatalysis and electrocatalysis. Various catalytic conditions have been used to solve the problems of thermodynamic properties and stability of CO2. The transformation mechanisms of these reactions are discussed.

Organophotoredox Hydrodefluorination of Trifluoromethylarenes with Translational Applicability to Drug Discovery

Molecular editing such as insertion, deletion, and single atom exchange in highly functionalized compounds is an aspirational goal for all chemists. Here, we disclose a photoredox protocol for the replacement of a single fluorine atom with hydrogen in electron-deficient trifluoromethylarenes including complex drug molecules. A robustness screening experiment shows that this reductive defluorination tolerates a range of functional groups and heterocycles commonly found in bioactive molecules. Preliminary studies allude to a catalytic cycle whereby the excited state of the organophotocatalyst is reductively quenched by the hydrogen atom donor, and returned in its original oxidation state by the trifluoromethylarene.

Visible-Light-Induced Selective Defluoroborylation of Polyfluoroarenes, gem-Difluoroalkenes, and Trifluoromethylalkenes

Fluorinated organoboranes serve as versatile synthetic precursors for the preparation of value-added fluorinated organic compounds. Recent progress has been mainly focused on the transition-metal catalyzed defluoroborylation. Herein, we report a photocatalytic defluoroborylation platform through direct B-H activation of N-heterocyclic carbene boranes, through the synergistic merger of a photoredox catalyst and a hydrogen atom transfer catalyst. This atom-economic and operationally simple protocol has enabled defluoroborylation of an extremely broad scope of multifluorinated substrates including polyfluoroarenes, gem-difluoroalkenes, and trifluoromethylalkenes in a highly selective fashion. Intriguingly, the defluoroborylation protocol can be transition-metal free, and the regioselectivity obtained is complementary to the reported transition-metal-catalysis in many cases.

Enantioselective copper-catalysed defluorosilylation of trifluoro-methylated alkenes with silylboronates

An efficient method for the construction of gem-difluoroallylsilanes with high enantiomeric excess via a copper-catalysed defluorosilylation of trifluoromethylated alkenes with silylboronates is described.

Selective C-F bond carboxylation of gem-difluoroalkenes with CO2 by photoredox/palladium dual catalysis

The catalytic C-F bond carboxylation of organofluorines with CO2 gas remains a challenging problem in synthetic chemistry. Here, we describe a selective defluorinative carboxylation of gem-difluoroalkenes through photoredox/palladium dual catalysis. The C-F bond activation is enabled by single electron reduction through photoredox catalysis to generate a fluorovinyl radical, which subsequently participates in an unprecedented palladium-catalyzed carboxylation. This novel C-F functionalization proved applicable to a wide range of substituted gem-difluoroalkenes, providing a rapid access to valuable α-fluoroacrylic acids.

Progress and prospects in the use of photocatalysis for the synthesis of organofluorine compounds

Data on the synthesis of fluorinated organic compounds by photocatalysis are systematically considered and analyzed. The attention is focused on the mechanisms of photocatalytic reactions and the selectivity problem.

The bibliography includes 173 references.

Umpolung cross-coupling of polyfluoroarenes with hydrazones via activation of C–F bonds

An umpolung strategy for the cross-coupling of polyfluoroarenes with various substituted hydrazones to construct C(sp²)–C(sp³) bonds is developed.

Unusual structural changes as a result of weathering benzofuran-based diarylethenes in simulated sunlight

Two unusual products are observed when benzofuran-based diartylethenes are weathered in simulated sunlight.

Prenyl Praxis: A Method for Direct Photocatalytic Defluoroprenylation

The prenyl fragment is the quintessential constituent of terpenoid natural products, a diverse family which contains numerous members with diverse biological properties. In contrast, fluorinated and multifluorinated arenes make up an important class of anthropogenic molecules which are highly relevant to material, agricultural, and pharmaceutical industries. While allylation chemistry is well developed, effective prenylation strategies have been less forthcoming. Herein, we describe the photocatalytic defluoroprenylation, a powerful method that provides access to "hybrid molecules" that possess both the functionality of a prenyl group and fluorinated arenes. This approach involves direct prenyl group transfer under very mild conditions, displays excellent functional group tolerance, and includes relatively short reaction times (<4 h), which is the fastest photocatalytic C-F functionalization developed to date. Additionally, the strategy can be extended to include allyl and geranyl (10 carbon fragment) transfers. Another prominent finding is a reagent-dependent switch in regioselectivity of the major product from para to ortho C-F functionalization.

Identifying the potential of pulsed LED irradiation in synthesis: copper-photocatalysed C–F functionalisation

The results of a proof-of-concept study demonstrate for the first time that pulsed LED irradiation enhances the rate of product formation and the yield of a visible light-mediated photoredox-catalysed reaction.

Selective Single C(sp3)-F Bond Cleavage in Trifluoromethylarenes: Merging Visible-Light Catalysis with Lewis Acid Activation

The conversion of easily available trifluoromethylarenes into aryldifluoromethyl compounds, which are valuable motifs in the pharmaceutical chemistry, is highly atom- and step-economical. However, the single C(sp³)-F bond cleavage of ArCF₃ is a great challenge because of the chemical inertness of the C(sp³)-F bond and the difficult selectivity control of monodefluorination. We report here the first example of single C(sp³)-F functionalization of trifluoromethylarenes via visible-light catalysis merged with Lewis acid activation. The method allows good chemoselectivity control and shows good functional group tolerance. Mechanistic studies suggest an in situ-generated borenium cationic species as the key intermediate for C(sp³)-F bond cleavage in this reaction.