Visible-Light-Induced Deoxygenation/Defluorination Protocol for Synthesis of γ,γ-Difluoroallylic Ketones

Photocatalyzed 1,3-Bromodifluoroallylation of [1.1.1]Propellane with α-Trifluoromethylalkenes and KBr Salts

Herein we unveil a visible-light-driven transition-metal-free 1,3-bromodifluoroallylation of [1.1.1]propellane. This reactivity is harnessed through organophotocatalysis, providing practical synthetic pathways to 1-brominated-3-gem-difluoroallylic bicyclo[1.1.1]pentane derivatives, particularly derived from readily available α-trifluoromethylalkenes and inexpensive KBr salts utilized as precursors for bromine radicals. Mechanistic investigations reveal that bromide anions quench the excited state of the photocatalyst, leading to the formation of bromine radicals, which react in a strain-release radical addition process rather than hydrogen atom abstraction with [1.1.1]propellane.

Generation of alkyl and acyl radicals by visible-light photoredox catalysis: direct activation of C-O bonds in organic transformations

Alkyl and acyl radicals play a critical role in the advancement of chemical synthesis. The generation of acyl and alkyl radicals by activation of C-O bonds using visible-light photoredox catalysis offers a mild and environmentally benign approach to useful chemical transformations. Alcohols, carboxylic acids, anhydrides, xanthates, oxalates, N-phthalimides, and thiocarbonates are some examples of alkyl and acyl precursors that can produce reactive radicals by homolysis of the C-O bond. These radicals can then go through a variety of transformations that are beneficial for the construction of synthetic materials that are otherwise difficult to access. This study summarizes current developments in the use of organic photocatalysts, transition-metal photoredox catalysts, and metallaphotocatalysts to produce acyl and alkyl radicals driven by visible light.

Photoredox Enabled Defluorinative Benzylation of Trifluoromethyl Alkenes with Alkylarenes

Herein, we report a photoredox enabled defluorinative benzylation of trifluoromethyl alkenes with readily available alkylarenes, which provides convenient access to a series of structurally valuable benzylated gem-difluoroalkenes under mild reaction conditions. The synthetic value of this protocol has been demonstrated by the transformations of several substrates bearing drug moieties, gram-scale reactions, and various further derivatizations of the gem-difluoroalkene products. The preliminary mechanistic investigations suggest a reaction pathway with rate-determining benzyl C-H bond cleavage of toluene followed by benzylic radical formation.

Visible-Light-Induced Defluorinative α-C(sp3)-H Alkylation for the Synthesis of gem-Difluoroallylated α-Trifluoromethylamines

Herein, we describe a novel and efficient photoredox catalytic Cα radical addition/defluoroalkylation coupling reaction between α-trifluoromethyl alkenes and N-trifluoroethyl hydroxylamine. A series of gem-difluoroallylated α-trifluoromethylamines were synthesized by the Cα radical addition enabled by a 1,2-H shift of the in situ-generated N-trifluoroethyl radical. Notably, this protocol is distinguished by its mild conditions, easy operation, and excellent functional group tolerability.

Synthesis of gem-Difluorinated Isoxazoles via Palladium-Catalyzed Oxylallylation of Alkynone Oxime Ethers

A novel and reliable palladium-catalyzed oxylallylation of alkynone oxime ethers with fluorine-containing alkenes was accomplished. Using the bulk industrial chemical 3-bromo-3,3-difluoroprop-1-ene as the coupling partner, this synthetic methodology offers the first example for the assembly of structurally diverse gem-difluorinated isoxazole derivatives in moderate to good yields with high atom- and step-economy and excellent functional group compatibility. More importantly, this strategy allows for the direct combination of the isoxazole motifs and gem-difluoroalkene unit, which is not easy to obtain through a general synthetic strategy.

Nickel-Catalyzed Cross-Electrophile Ring Opening/gem-Difluoroallylation of Aziridines

Herein we report a nickel-catalyzed regioselective cross-electrophile ring opening reaction of sulfonyl-protected aziridines with trifluoromethyl-substituted alkenes as the gem-difluoroallylating agents, providing a new and efficient entry to prepare gem-difluorobishomoallylic sulfonamides. Moreover, the scaffold of 6-fluoro-1,2,3,4-tetrahydropyridine can be constructed starting from the ring opening products via NaH-mediated intramolecular defluorinative nucleophilic vinylic substitution.

Iron-Catalyzed Reductive Ring Opening/gem-Difluoroallylation of Cyclopropyl Ketones

By merging C-C and C-F bond cleavage, we developed a regioselective ring opening/gem-difluoroallylation of cyclopropyl ketones with α-trifluoromethylstyrenes, which proceeds under the catalysis of iron with the combination of manganese and TMSCl as the reducing agents, providing a new entry to the synthesis of carbonyl-containing gem-difluoroalkenes. Remarkably, the ketyl radical-induced selective C-C bond cleavage and the following generation of more-stable carbon-centered radicals enable complete regiocontrol of this ring opening reaction for various substitution patterns of the cyclopropane ring.

Transition metal-free photochemical C-F activation for the preparation of difluorinated-oxindole derivatives

The development of strategies for single and selective C-F bond activation represents an important avenue to overcome limitations in the synthesis of valuable fluorine-containing compounds. The synthetic and medicinal research communities would benefit from new routes that access such relevant molecules in a simple manner. Herein we disclose a straightforward and mechanistically distinct pathway to generate gem-difluoromethyl radicals and their installation onto N-arylmethacrylamides for the preparation of valuable difluorinated oxindole derivatives. To achieve operational simplicity, the use of a readily available benzenethiol as a photocatalyst under open-to-air conditions was developed, demonstrating the facile multigram preparation of the targeted fluorinated molecules. Additionally, dispersion-corrected density functional theory (DFT) and empirical investigations provide a new basis to support the proposed reaction pathway, indicating that arene thiolate is an efficient organophotocatalyst for this transformation.

Nickel-Catalyzed Unsymmetrical Bis-Allylation of Alkynes

The catalytic bis-allylation of alkynes is an important but challenging protocol to construct all-carbon tetra-substituted alkenes. Particularly, the catalytic unsymmetrical bis-allylation of alkynes remains as an underexplored task to date. We herein report an unprecedented unsymmetrical bis-allylation by simultaneously utilizing electrophilic trifluoromethyl alkene and nucleophilic allylboronate as the allylic reagents. With the aid of robust Ni⁰ /NHC catalysis, valuable skipped trienes can be obtained in high regio- and stereo-selectivities under mild conditions. Mechanistic studies indicate that the reaction may proceed through a β-fluorine elimination of a nickelacycle followed by a transmetalation step with allylboronate. The present method exhibits a good tolerance of various functional groups. Besides, the skipped triene products can undergo an array of elaborate transformations, which highlights the potential applications of this strategy.

Deaminative defluoroalkylation of α-trifluoromethylalkenes enabled by photoredox catalysis

Herein, we disclose a new photoredox-catalysed strategy to access gem-difluoroallylarenes from α-trifluoromethylalkenes with sterically hindered primary amines via C-N and C-F bond activation. This deaminative and defluorinative allylation is generally compatible with diverse functional groups and sterically hindered α-3° and 2° primary amines.

Synthesis of CF3-Substituted Alkylamines, 1,2-Bisazoles, and 1,4,5,6-Tetrahydro-1,2,4-triazines from Newly Designed Tetrazole-Activated Trifluoromethyl Alkenes

A new class of Michael acceptor, tetrazolyl-trifluoromethyl alkenes, has been discovered. They readily undergo Michael-type addition instead of addition-elimination reaction with aliphatic amines and azoles to furnish β-trifluoromethyl alkylamines and CF₃-substituted 1,2-bisazole derivatives, respectively. Additionally, some of the products are capable of engaging in microwave-assisted intramolecular denitrogenative annulation, leading to the formation of CF₃-substituted 1,4,5,6-tetrahydro-1,2,4-triazines that are otherwise difficult to access by other methodologies.

Advances in Catalytic C–F Bond Activation and Transformation of Aromatic Fluorides

The activation and transformation of C–F bonds in fluoro-aromatics is a highly desirable process in organic chemistry. It provides synthetic methods/protocols for the generation of organic compounds possessing single or multiple C–F bonds, and effective catalytic systems for further study of the activation mode of inert chemical bonds. Due to the high polarity of the C–F bond and it having the highest bond energy in organics, C–F activation often faces considerable academic challenges. In this mini-review, the important research achievements in the activation and transformation of aromatic C–F bond, catalyzed by transition metal and metal-free systems, are presented.

Divergent Synthesis of Fluorinated Alkenes, Allenes, and Enynes via Reaction of 2-Trifluoromethyl-1,3-enynes with Carbon Nucleophiles

Herein, inorganic base K₃PO₄ promoted divergent synthesis of CF₃-substituted allenes, cyclopentenes, alkynes, and fluorinated enynes via regioselective nucleophilic addition of carbon nucleophiles to 2-trifluoromethyl-1,3-enynes was developed. With the choice of different carbon nucleophiles, various fluorinated compounds could be obtained under K₃PO₄/DMF reaction system. When malononitriles were used as nucleophiles, CF₃-substituted allenes, cyclopentenes, and alkynes could be obtained, respectively. By using 1,3-dicarbonyl compounds as nucleophiles, ring-monofluorinated 4H-pyrans could be prepared, and 1,1-difluoro-1,3-enynes could be furnished with the participation of diethyl malonate. Moreover, these five kinds of fluorinated allenes, alkenes, and enynes are valuable building blocks.

Redox-neutral and metal-free synthesis of 3-(arylmethyl)chroman-4-ones via visible-light-driven alkene acylarylation

A metal- and aldehyde-free visible-light-driven photoredox-neutral alkene acylarylation with readily available cyanoarenes is described. A variety of 3-(arylmethyl)chroman-4-ones (i.e., homoisoflavonoids) and analogs are efficiently synthesized with good functional group tolerance. This mild protocol relies on a phosphoranyl radical-mediated acyl radical-initiated cyclization and selective radical-radical coupling sequence, and is also further highlighted by subsequent derivatization to chromone and 2H-chromene as well as its application in the three-component alkene acylarylation.

CF2Br2 as a Source for Difluoroolefination of 1,3-Enynes via N-Heterocyclic Carbene Catalysis

Commercially available CF₂Br₂ has been used as a convenient source for the rapid and reliable incorporation of the gem-difluorovinyl motif into an allene framework via an N-heterocyclic carbene catalyzed difluoroolefination of 1,3-enynes. The reaction proceeds through a cascade three-component radical relay/elimination process. This protocol is distinguished by its mild conditions, readily accessible starting materials, wide substrate scope, and ease of late-stage functionalization, thus unlocking an untraditional strategy to construct a new class of functionalized gem-difluorovinyl allenes.

Visible-Light-Induced C-F and C-N Bond Cleavage for the Synthesis of gem-Difluoroalkenes

Herein, we describe a novel and efficient photoredox catalytic radical addition/defluoroalkylation coupling reaction between primary amines and trifluoromethyl-substituted alkenes. A series of gem-difluoroalkenes were synthesized via C-N bond cleavage of α-3°, α-2°, and α-1° amines under visible light irradiation. This reaction is characterized by a broad substrate scope and good functional group tolerance.

MOF/POM hybrids as catalysts for organic transformations

Insertion of molecular metal oxides, e.g. polyoxometalates (POMs), into metal-organic frameworks (MOFs) opens up new research opportunities in various fields, particularly in catalysis. POM/MOF composites have strong acidity, oxygen-rich surface, and redox capacity due to typical characteristics of POMs and the large surface area, highly organized structures, tunable pore size, and shape are due to MOFs. Such hybrid materials have gained a lot of attention due to astonishing structural features, and hence have potential applications in organic catalysis, sorption and separation, proton conduction, magnetism, lithium-ion batteries, supercapacitors, electrochemistry, medicine, bio-fuel, and so on. The exceptional chemical and physical characteristics of POMOFs make them useful as catalysts in simple organic transformations with high capacity and selectivity. Here, the thorough catalytic study starts with a brief introduction related to POMs and MOFs, and is followed by the synthetic strategies and applications of these materials in several catalytic organic transformations. Furthermore, catalytic conversions like oxidation, condensation, esterification, and some other types of catalytic reactions including photocatalytic reactions are discussed in length with their plausible catalytic mechanisms. The disadvantages of the POMOFs and difficulties faced in the field have also been explored briefly from our perspectives.

Cooperative NHC and Photoredox Catalysis for the Synthesis of 1,4-Dicarbonyl Compounds via Diacylation of Alkenes

An intermolecular 1,2-diacylation of alkenes is disclosed via cooperative N-heterocyclic carbene and photoredox catalysis under the mediation of PPh₃ and Cs₂CO₃. This protocol provides a practical approach for construction of 1,4-dicarbonyl compounds toward novel diketone and pharmaceutical derivatives. Furthermore, the regioselective dicarbonyl compounds can be synthesized by adding acyl azolium salt. Mechanistic investigations suggest that the process was a critical radical/radical cross coupling of ketyl radicals with benzylic C-radicals.

Visible-light-induced defluorinative carbonylative coupling of alkyl iodides with α-trifluoromethyl substituted styrenes

A visible-light-mediated defluorinative carbonylative cross-coupling of alkyl iodides with α-trifluoromethyl styrenes has been developed. The reaction occurs at room temperature under blue light irradiation, and various gem-difluoroalkenes were obtained in moderate to good yields. Synthetic transformations of the obtained product were performed as well.

Photoredox Metal-Free Allylic Defluorinative Silylation of α-Trifluoromethylstyrenes with Hydrosilanes

We report an efficient strategy that combines organic photoredox and hydrogen atom transfer to deliver gem-difluoroallylsilanes via defluorinative silylation of α-trifluoromethylstyrenes using hydrosilanes as silicon sources. This protocol provides an environmentally friendly approach for the preparation of structurally diverse gem-difluoroallylsilanes with excellent functional group compatibility and renders it suitable for late-stage modification of bioactive and complex molecules.

Deoxygenative gem-difluorovinylation of aliphatic alcohols

An unprecedented deoxygenative gem-difluorovinylation of aliphatic alcohols using α-trifluoromethyl alkenes is achieved under photocatalytic conditions. Inexpensive Ph₃P acts as an efficient O-atom transfer reagent to facilitate the deoxygenation of alcohols for the generation of reactive alkyl radical species. Remarkable features of this reaction include mild conditions, simple operation and broad scope. The synthetic utility of this reaction was validated by the success of two-step one-pot reactions, scale-up synthesis and chemoselective monodeoxygenation of diols.

Regio- and Stereoselective Addition to gem-Difluorinated Ene-Ynamides: Access to Stereodefined Fluorinated Dienes

The first synthesis of gem-difluorinated ene-ynamides is presented via deprotonation of trifluoromethylated N-allenamides and δ extrusion of fluorine. These highly reactive building blocks, owing to their dual functional groups, offer a unique entry to difluorinated dienes and to stereodefined, monofluoro-substituted dienes. Stereoselective addition to the ynamide moiety led to difluorinated dienes. A stereocontrolled domino δ elimination reaction followed by an addition/elimination sequence from trifluoromethylated N-allenamides provided exclusively stereodefined monofluorinated ene-ynamides.

Iron-Catalyzed Allylic Defluorinative Ketone Olefin Coupling

In this protocol, we demonstrate our discovery that iron is able to efficiently catalyze the reductive allylic defluorinative ketyl olefin coupling reaction between α-trifluoromethyl alkenes and unactivated ketones. This operationally simple cross-electrophile reaction circumvents the use of pre-generated organometallics and allows for the synthesis of diverse functional-group-rich tertiary gem-difluorohomoallylic alcohols through a polarity-reversed strategy. Preliminary mechanistic studies support a mechanism that proceeds through a ketyl formation/olefin insertion/β-fluoro elimination sequence.

Nickel-Catalyzed Cross-Electrophile Coupling Reactions between Allylic Acetates and gem-Difluorovinyl Tosylate

A nickel-catalyzed cross-electrophile coupling of allylic acetates and gem-difluorovinyl tosylate is presented, which first achieves allylic gem-difluoroolefins via C(sp³)-C(sp²) cross-electrophile coupling. In addition, this protocol was performed under mild reaction conditions, affording a variety of allylic gem-difluorovinyl arenes in moderate to good yields. Moreover, both linear and branched allylic acetate could produce a linear cross-coupling product exclusively. Mechanistic studies reveal that the reaction involves two different Ni(0)/Ni(II) catalytic cycles.

Defluorinative C-C Bond-Forming Reaction of Trifluoromethyl Alkenes with gem-(Diborylalkyl)lithiums

We report the transition-metal-free defluorinative C-C bond-forming reaction of trifluoromethyl alkenes with gem-(diborylalkyl)lithiums. This synthetic strategy provides access to a variety of 4,4-difluoro homoallylic diboronate esters, which serve as versatile intermediates in the efficient preparation of valuable gem-difluoroalkene derivatives. Further synthetic modifications are conducted to demonstrate the synthetic utility of the obtained 4,4-difluoro homoallylic diboronate esters.

Recent advances in radical enabled selective Csp3-F bond activation of multifluorinated compounds

Fluorine-containing molecules have found broad applications in pharmaceutical and agrochemical industries as introducing fluorine into a molecule could significantly tune the biological activities of parent molecules. Thus, the synthesis of fluorine-containing molecules has received substantial attention over the past few decades. As a complementary strategy for the synthesis of fluorinated compounds through new C_sp³-F bonds formation, selective cleavage of inert C_sp³-F bonds from easily-available and cost-effective multifluorinated molecules, such as fluoroalkylaromatics, α-trifluoromethyl alkenes and α-multifluorinated carbonyl compounds, has been emerging as an attractive alternative to access fluorine-containing molecules. Moreover, the inherent nature of radical reactions offers the opportunity for the selective C_sp³-F functionalizations to occur under mild conditions. In this regard, the development of photoredox catalysis, transition-metal catalysis, or electrochemistry to enable radical species generation via selective C_sp³-F cleavage has gained broad attention and substantial progress has been made over recent years. This highlight summerizes the recent advances in the single-electron-transfer enabled selective functionalizations of C_sp³-F bonds in multifluorinated compounds via radical pathways.

Electrochemical synthesis of functionalized gem-difluoroalkenes with diverse alkyl sources via a defluorinative alkylation process

An electrochemical defluorinative alkylation of α-trifluoromethyl alkenes is described. This reaction enables the preparation of functionalized gem-difluoroalkenes with diverse alkyl sources including organohalides, NHP esters, and Katritzky salts.

Photochemical and electrochemical strategies in C–F bond activation and functionalization

The recent advances in photochemical or electrochemical C–F bond activation and functionalization have been summarized and discussed.

Metal-free photocatalytic intermolecular trifluoromethylation-gem-difluoroallylation of unactivated alkenes

An efficient, transition-metal-free, photocatalytic three-component intermolecular trifluoromethylation-gem-difluoroallylation of unactivated alkenes has been achieved.

Tunable photocatalytic oxysulfonylation and chlorosulfonylation of α-CF3 alkenes with sulfonyl chlorides

Tunable photoredox-catalyzed chlorosulfonylation and oxysulfonylation of α-trifluoromethylstyrenes with sulfonyl chloride were facilely achieved by simply manipulating the photocatalyst and solvent.

Visible-light-promoted defluorinative ring-opening gem-difluoroallylation of cycloalkanols using 1-trifluoromethyl alkenes

A photocatalytic redox-neutral defluorinative ring-opening coupling of cycloalkanols with α-trifluoromethyl alkenes for the synthesis of gem-difluoroalkenes bearing a distal carbonyl group was developed.

Nickel-Catalyzed Allylic Defluorinative Cross-Electrophile Coupling with Cycloalkyl Silyl Peroxides as the Alkyl Source

Herein we demonstrate the first successful application of cycloalkyl silyl peroxides (CSP) as an electrophilic coupling partner in the cross-electrophile coupling reaction. Diverse CSP are efficiently cross-coupled with an array of α-trifluoromethyl alkenes under the catalysis of nickel with the assistance of zinc as the reducing agent. This method allows the use of unstrained CSP as the carbonyl-containing alkyl source in the allylic defluorinative reaction, to access a variety of gem-difluoroalkenes bearing a pendent ketone moiety with high functionality tolerance.

Recent Advances in C-F Bond Cleavage Enabled by Visible Light Photoredox Catalysis

The creation of new bonds via C-F bond cleavage of readily available per- or oligofluorinated compounds has received growing interest. Using such a strategy, a myriad of valuable partially fluorinated products can be prepared, which otherwise are difficult to make by the conventional C-F bond formation methods. Visible light photoredox catalysis has been proven as an important and powerful tool for defluorinative reactions due to its mild, easy to handle, and environmentally benign characteristics. Compared to the classical C-F activation that proceeds via two-electron processes, radicals are the key intermediates using visible light photoredox catalysis, providing new modes for the cleavage of C-F bonds. In this review, a summary of the visible light-promoted C-F bond cleavage since 2018 was presented. The contents were classified by the fluorosubstrates, including polyfluorinated arenes, gem-difluoroalkenes, trifluoromethyl arenes, and trifluoromethyl alkenes. An emphasis is placed on the discussion of the mechanisms and limitations of these reactions. Finally, my personal perspective on the future development of this rapidly emerging field was provided.

Photoredox-Catalyzed Difunctionalization of Unactivated Olefins for Synthesizing Lactam-Substituted gem-Difluoroalkenes

Herein, the synthesis of lactam-substituted gem-difluoroalkenes has been developed through a photoredox-catalyzed radical cascade reaction. This developed photoredox-catalyzed, Brønsted base-assisted intramolecular 5-exo-trig cyclization/intermolecular radical addition/β-fluoride elimination reaction offers a simple method for producing lactam, carbamate, or urea-substituted gem-difluoroalkenes with good functional group tolerance and high yields.

Electroreductive Cross-Coupling of Trifluoromethyl Alkenes and Redox Active Esters for the Synthesis of Gem-Difluoroalkenes

An electroreductive access to gem-difluoroalkenes has been developed through the decarboxylative/defluorinative coupling of N-hydroxyphtalimides esters and α-trifluoromethyl alkenes. The electrolysis is performed under very simple reaction conditions in an undivided cell using cheap carbon graphite electrodes. This metal-free transformation features broad scope with good to excellent yields. Tertiary, secondary as well as primary alkyl radicals could be easily introduced. α-aminoacids L-aspartic and L-glutamic acid-derived redox active esters were good reactive partners furnishing potentially relevant gem-difluoroalkenes. In addition, it has been demonstrated that our electrosynthetic approach toward the synthesis of gem-difluoroalkenes could use an easily prepared Kratitsky salt as alkyl radical precursor via a deaminative/defluorinative carbofunctionalization of trifluoromethylstyrene.

Selective deoxygenative alkylation of alcohols via photocatalytic domino radical fragmentations

The delivery of alkyl radicals through photocatalytic deoxygenation of primary alcohols under mild conditions is a so far unmet challenge. In this report, we present a one-pot strategy for deoxygenative Giese reaction of alcohols with electron-deficient alkenes, by using xanthate salts as alcohol-activating groups for radical generation under visible-light photoredox conditions in the presence of triphenylphosphine. The convenient generation of xanthate salts and high reactivity of sequential C–S/C–O bond homolytic cleavage enable efficient deoxygenation of primary, secondary and tertiary alcohols with diverse functionality and structure to generate the corresponding alkyl radicals, including methyl radical. Moreover, chemoselective radical monodeoxygenation of diols is achieved via selective formation of xanthate salts.

The generation of alkyl radicals through deoxygenation of abundant alcohols via photoredox catalysis is of interest. In this study, the authors report a one-pot strategy for visible-light-promoted photoredox coupling of alcohols with electron-deficient alkenes, assisted by carbon disulfide and triphenylphosphine.