CF2H Radical Research Articles

Pd-Catalyzed 1,4-Difluoromethylative Functionalization of 1,3-Dienes Using Freon-22.

We report a visible-light-driven, palladium-catalyzed 1,4-difluoromethylative functionalization of conjugated dienes using chlorodifluoromethane (ClCF2H, Freon-22) as a cost-effective difluoromethyl source. The excited palladium catalyst efficiently reduces the C-Cl bond, which generates a CF2H radical, followed by regioselective SN2' substitution to afford 1,4-difunctionalized products. This versatile, redox-neutral method accommodates diverse nucleophiles and exhibits broad functional group compatibility, making it suitable for late-stage functionalization in drug discovery and offering a direct route to difluoromethylated molecules.

Cu-Electrocatalysis Enables Vicinal Bis(difluoromethylation) of Alkenes: Unraveling Dichotomous Role of Zn(CF2H)2(DMPU)2 as Both Radical and Anion Source.

The difluoromethyl group (CF2H) serves as an essential bioisostere in drug discovery campaigns according to Lipinski's Rule of 5 due to its advantageous combination of lipophilicity and hydrogen bonding ability, thereby improving the ADME properties. However, despite the high prevalence and importance of vicinal hydrogen bond donors in pharmaceutical agents, a general synthetic method for doubly difluoromethylated compounds in the vicinal position is absent. Here we describe a copper-electrocatalyzed strategy that enables the vicinal bis(difluoromethylation) of alkenes. By leveraging electrochemistry to oxidize Zn(CF2H)2(DMPU)2-a conventionally utilized anionic transmetalating source, we paved a way to utilize it as a CF2H radical source to deliver the CF2H group in the terminal position of alkenes. Mechanistic studies revealed that the interception of the resultant secondary radical by a copper catalyst and subsequent reductive elimination is facilitated by invoking the Cu(III) intermediate, enabling the second installation of the CF2H group in the internal position. The utility of this electrocatalytic 1,2-bis(difluoromethylation) strategy has been highlighted through the late-stage bioisosteric replacement of pharmaceutical agents such as sotalol and dipivefrine.

Photo‐Induced Difluoromethylation‐Cyclization of Indoles Using Tetrazole Sulfone Reagent Under Visible Light

AbstractPolycyclic indoles are prevailed in natural products, pharmaceuticals and biologically active compounds. Herein is described photo‐induced difluoromethylation‐cyclization of N‐alkene tethered indoles to synthesis of tetrahydropyrido[1,2‐a]indole using 5‐((difluoromethyl)sulfonyl)‐1‐phenyl‐1H‐tetrazole as a CF2H radical source. This photochemical reaction tolerates an array of functional groups like ester, cyano, ketone, halide, ether, aldehyde and heteocycles. Mechanistic experiments indicated that the difluoromethyl radical and the single electron transfer were involved.

Why •CF2H is nucleophilic but •CF3 is electrophilic in reactions with heterocycles

Radical substitution is a useful method to functionalize heterocycles, as in the venerable Minisci reaction. Empirically observed regiochemistries indicate that the CF2H radical has a nucleophilic character similar to alkyl radicals, but the CF3 radical is electrophilic. While the difference between •CH3 and •CF3 is well understood, the reason that one and two Fs make little difference but the third has a large effect is puzzling. DFT calculations with M06-2X both reproduce experimental selectivities and also lead to an explanation of this difference. Theoretical methods reveal how the F inductive withdrawal and conjugative donation alter radical properties, but only CF3 becomes decidedly electrophilic toward heterocycles. Here, we show a simple model to explain the radical orbital energy trends and resulting nucleophilicity or electrophilicity of fluorinated radicals.

The 18F‐Difluoromethyl Group: Challenges, Impact and Outlook

AbstractThe difluoromethyl functionality has proven useful in drug discovery, as it can modulate the properties of bioactive molecules. For PET imaging, this structural motif has been largely underexploited in (pre)clinical radiotracers due to a lack of user‐friendly radiosynthetic routes. This Minireview provides an overview of the challenges facing radiochemists and summarises the efforts made to date to access 18F‐difluoromethyl‐containing radiotracers. Two distinct approaches have prevailed, the first of which relies on 18F‐fluorination. A second approach consists of a 18F‐difluoromethylation process, which uses 18F‐labelled reagents capable of releasing key reactive intermediates such as the [18F]CF2H radical or [18F]difluorocarbene. Finally, we provide an outlook for future directions in the radiosynthesis of [18F]CF2H compounds and their application in tracer radiosynthesis.

The 18F-Difluoromethyl Group: Challenges, Impact and Outlook.

The difluoromethyl functionality has proven useful in drug discovery, as it can modulate the properties of bioactive molecules. For PET imaging, this structural motif has been largely underexploited in (pre)clinical radiotracers due to a lack of user-friendly radiosynthetic routes. This Minireview provides an overview of the challenges facing radiochemists and summarises the efforts made to date to access 18F-difluoromethyl-containing radiotracers. Two distinct approaches have prevailed, the first of which relies on 18F-fluorination. A second approach consists of a 18F-difluoromethylation process, which uses 18F-labelled reagents capable of releasing key reactive intermediates such as the [18F]CF2H radical or [18F]difluorocarbene. Finally, we provide an outlook for future directions in the radiosynthesis of [18F]CF2H compounds and their application in tracer radiosynthesis.

Difluoromethylated Difunctionalization of Alkenes under Visible Light.

Difluoromethylated compounds usually act as bioisosteres for alcohol functional groups and show unique physicochemical and biological properties. The cyano-difluoromethylation of alkenes using 5-((difluoromethyl)sulfonyl)-1-phenyl-1H-tetrazole as a CF2H radical difluoromethyl precursor was developed to afford nitriles including a CF2H group. A low-cost, stable, easily handled 5-((difluoromethyl)sulfonyl)-1-methyl-1H-tetrazole (DFSMT) was synthesized and applied as the radical CF2H reagent. Using DFSMT as the radical CF2H precursor, the oxyl-difluoromethylation of alkenes was developed to obtain difluoromethylated ether products. All of the reactions showed good functional group tolerability. Initial mechanistic experiments indicated that the CF2H radical was involved as the key active intermediate.

Copper-Catalyzed Visible-Light-Induced Allylic Difluoromethylation of Unactivated Alkenes Using Difluoroacetic Acid.

We herein describe a straightforward allylic difluoromethylation reaction of unactivated alkenes. Compared to cross-couplings of prefunctionalized allylic substrates for the construction of allylic CF2H bonds, this reaction employs readily available alkenes as substrates under mild conditions. Difluoroacetic acid is used as an inexpensive and easy-to-handle source of CF2H radical under visible light irradiation with PIDA. The copper catalyst plays an important role of diverting the reaction pathway toward allylic difluoromethylation as opposed to previously found hydrodifluoromethylation.

Photocatalytic radical cascade cyclization of N‐(o–cyanobiaryl) acrylamides: access to CF2H‐functionalized pyrido[4,3,2‐gh] phenanthridines

AbstractHere we report a simple and efficient method for the preparation of CF2H‐substituted pyrido[4,3,2‐gh]phenanthridines under photoredox conditions. The reactions are triggered by addition of a CF2H radical to a carbon–carbon double bond to form three different types of bonds via a cascade radical difluoromethylation/cyano insertion/cyclization. This difluoromethylated transformation exhibits good functional group compatibility, leading to the construction of the desired ring‐fused phenanthridine derivatives in moderate to excellent yields.

Radical hydrodifluoromethylation of unsaturated C−C bonds via an electroreductively triggered two-pronged approach

Due to its superior ability in controlling pharmaceutical activity, the installation of difluoromethyl (CF2H) functionality into organic molecules has been an area of intensive research. In this context, difluoromethylation of C−C π bonds mediated by a CF2H radical have been pursued as a central strategy to grant access to difluoromethylated hydrocarbons. However, early precedents necessitate the generation of oxidative chemical species that can limit the generality and utility of the reaction. We report here the successful implementation of radical hydrodifluoromethylation of unsaturated C−C bonds via an electroreductively triggered two-pronged approach. Preliminary mechanistic investigations suggest that the key distinction of the present strategy originates from the reconciliation of multiple redox processes under highly reducing electrochemical conditions. The reaction conditions can be chosen based on the electronic properties of the alkenes of interest, highlighting the hydrodifluoromethylation of both unactivated and activated alkenes. Notably, the reaction delivers geminal (bis)difluoromethylated products from alkynes in a single step by consecutive hydrodifluoromethylation, granting access to an underutilized 1,1,3,3-tetrafluoropropan-2-yl functional group. The late-stage hydrodifluoromethylation of densely functionalized pharmaceutical agents is also presented.

Organocatalytic Three-Component Acyldifluoromethylation of Vinylarenes via N-Heterocyclic Carbene-Catalyzed Radical Relay.

We herein describe an N-hetercyclic carbene-catalyzed three-component acyldifluoromethylation of vinylarenes, aldehydes, and NaSO2CF2H. This organocatalytic approach provides a practical route for the synthesis of pharmaceutically relevant α-aryl-β-difluormethyl ketones without the need for transition metals or photocatalysts. The late-stage acyldifluoromethylation of drug analogues was also demonstrated. The reaction design employs NaSO2CF2H as the source of the CF2H radical in the presence of an oxidant for the radical relay.

Copper-catalyzed carbo-difluoromethylation of alkenes via radical relay

Organic molecules that contain alkyl-difluoromethyl moieties have received increased attention in medicinal chemistry, but their synthesis in a modular and late-stage fashion remains challenging. We report herein an efficient copper-catalyzed radical relay approach for the carbo-difluoromethylation of alkenes. This approach simultaneously introduces CF2H groups along with complex alkyl or aryl groups into alkenes with regioselectivity opposite to traditional CF2H radical addition. We demonstrate a broad substrate scope and a wide functional group compatibility. This scalable protocol is applied to the late-stage functionalization of complex molecules and the synthesis of CF2H analogues of bioactive molecules. Mechanistic studies and density functional theory calculations suggest a unique ligand effect on the reactivity of the Cu-CF2H species.

A radical chlorodifluoromethylation protocol for late-stage difluoromethylation and its application to an oncology candidate

Radical fluoroalkylation is a powerful synthetic tool for the late-stage incorporation of fluorinated moieties into organic molecules, which is widely used in the development of pharmaceuticals and agrochemicals. Here, we report an efficient radical chlorodifluoromethylation protocol with sodium chlorodifluoromethanesulfinate, which is complementary to the existing late-stage difluoromethylation strategies. CF2Cl radical is a suitable surrogate for accessing the CF2H group while possessing completely different electronic properties compared to the CF2H radical. This method is chemoselective and regioselective for the chlorodifluoromethylation of (hetero)arenes and electron-rich alkenes and shows good functionality, tolerance, and generality on scope. The preparation of the sodium chlorodifluoromethanesulfinate is thoroughly investigated and can be scaled up to hundreds of kilograms. The method is successfully implemented on the synthesis of an oncology candidate compound 1.

Visible‐Light‐Induced Radical Difluoromethylation of Alkynoates by [bis(Difluoroacetoxy)iodo]benzene to Yield 3‐Difluoromethylated Coumarins

AbstractA visible‐light‐induced radical difluoromethylation of alkynoates using [bis(difluoroacetoxy)iodo]benzene as the CF2H radical precursor was developed for the first time. 3‐Difluoromethyl‐substituted coumarins were synthesized via a radical difluoromethylation/cyclization/ester migration cascade. The mild and catalyst‐free conditions, as well as the good functional group tolerance, render this protocol an alternative and green strategy for the synthesis of 3‐difluoromethyl‐substituted coumarins.

Unlocking the direct photocatalytic difluoromethylation of C[double bond, length as m-dash]N bonds.

The current study presents a direct CF2H radical addition to C[double bond, length as m-dash]N bonds predicated on the photocatalytic activation of commercially available zinc difluoromethanesulfinate. The mild conditions in place lead to impressive structural diversity, as quinoxalinones and dibenzazepines, among others, are successfully functionalized.

Keto-Difluoromethylation of Aromatic Alkenes by Photoredox Catalysis: Step-Economical Synthesis of α-CF2H-Substituted Ketones in Flow

A step-economical method for synthesis of α-CF2H-substituted ketones from readily available alkene feedstocks has been developed. Radical difluoromethylation of aromatic alkenes combining DMSO oxidation and photoredox catalysis is a key to the successful transformation. Electrochemical analysis, laser flash photolysis (LFP), and density functional theory (DFT) calculations reveal that N-tosyl-S-difluoromethyl-S-phenylsulfoximine serves as the best CF2H radical source among analogous sulfone-based CF2H reagents. The present photocatalytic keto-difluoromethylation has been applied to flow synthesis and easily scaled up to gram-scale synthesis within a reasonable reaction time. Furthermore, potentials of the α-CF2H-substituted ketones for useful synthetic intermediates are shown; e.g., synthesis of the CF2H-containing α-hydroxyamide with the same carbon skeleton as that of the anticonvulsant active CF3-analogue, is disclosed. Additionally, mechanistic studies are also discussed in detail.

Progress in Difluoroalkylation of Organic Substrates by Visible Light Photoredox Catalysis.

The selective incorporation of fluorinated motifs, in particular CF2FG (FG=a functional group) and CF2H groups, into organic compounds has attrracted increasing attention since organofluorine molecules are of the utmost importance in the areas of nuclear imaging, pharmaceutical, agrochemical, and material sciences. A variety of synthetic approaches has been employed in late‐stage difluoroalkylation reactions. Visible light photoredox catalysis for the production of CF2FG and CF2H radicals has provided a more sustainable alternative to other conventional radical‐triggered reactions from the viewpoint of safety, cost, availability, and “green” chemistry. A wide range of difluoroalkylating reagents has been successfully implemented in these organic transformations in the presence of transition metal complexes or organic photocatalysts. In most cases, upon excitation via visible light irradiation with fluorescent light bulbs or blue light‐emitting diode (LED) lamps, these photocatalysts can act as both reductive and oxidative quenchers, thus enabling the application of electron‐donor or electron‐acceptor difluoroalkylating reagents for the generation of CF2FG and CF2H radicals. Subsequent radical addition to substrates and additional organic transformations afford the corresponding difluoroalkylated derivatives. The present review describes the distinct strategies for the transition metal‐ and organic‐photocatalyzed difluoroalkylation of a broad range of organic substrates by visible light irradiation reported in the literature since 2014.

Electrochemical Difluoromethylarylation of Alkynes.

An unprecedented radical difluoromethylarylation reaction of alkynes has been developed by discovering a new difluoromethylation reagent, CF2HSO2NHNHBoc. This air-stable and solid reagent can be prepared in one step from commercially available reagents CF2HSO2Cl and NH2NHBoc. The CF2H radical, generated through ferrocene-mediated electrochemical oxidation, participates in an unexplored alkyne addition reaction followed by a challenging 7-membered ring-forming homolytic aromatic substitution step to afford fluorinated dibenzazepines.

Radical Difluororomethylation of Thiols with Difluoromethylphosphonium Triflate under Photoredox Catalysis.

A convenient, visible light-induced radical difluoromethylation of aryl-, heteroaryl-, and alkylthiols with difluoromethyltriphenylphosphonium triflate was developed to afford various difluoromethyl thioethers in moderate to excellent yields. The key reaction features include the use of a readily available CF2H radical source, mild reaction conditions, and excellent chemoselective thiol-difluoromethylation.

Recent Developments in Trifluoromethylation or Difluoroalkylation by Use of Difluorinated Phosphonium Salts

AbstractRecent advances in trifluoromethylation or difluoroalkylation reactions by use of difluorinated phosphonium salts are summarized. In the presence of base, water, photocatalyst, fluoride or under the heating conditions, the difluorinated phosphonium salts can produce difluorocarbene, the CF2H radical or the trifluoromethyl anion, which are major intermediates for the difluoroalkylation or trifluoromethylation reaction and give moderate to excellent yields of the desired products in most cases.magnified image