Siladifluoromethylation and Difluoromethylation onto C(sp(3)), C(sp(2)), and C(sp) Centers Using Ruppert-Prakash Reagent and Fluoroform

A metal-free strategy to construct fluoroalkyl-olefin linkages using fluoroalkanes

We present a metal-free strategy to access fluoroalkyl-olefin linkages from fluoroalkane precursors and vinyl-pinacol boronic ester (BPin) reagents. This reaction sequence is templated by the boron reagent, which induces C-C bond formation upon oxidation. We developed this strategy into a one-pot synthetic protocol using RCF2H precursors directly with vinyl-BPin reagents in the presence of a Brønsted base, which tolerated oxygen- and nitrogen-containing heterocycles, and aryl halogens. We also found that HCF3 (HCF-23; a byproduct of the Teflon industry) and CH2F2 (HCF-32; a low-cost refrigerant) are amenable to this protocol, representing distinct strategies to generate RCF2H and RCF3 molecules. Finally, we demonstrate that the vinyldifluoromethylene products can be readily derivatized, representing an avenue for late-stage modification after installing the fluoroalkyl unit.

Oxidant- and Base-Free, Copper-Catalyzed Difluoromethylation of Haloalkynes

We report herein a highly efficient copper-catalyzed protocol for the transformation of haloalkynes to the corresponding difluoromethylated alkynes. This scalable protocol exhibits a broad substrate scope and excellent functional group tolerance, enabling the late-stage difluoromethylation of bioactive molecules. Additionally, the strategy of utilizing the difluoromethylalkynes in gram-scale reactions and multiple transformations has proven to be highly valuable in synthetic chemistry.

Reactions Using Freons and Halothane as Halofluoroalkyl/Halofluoroalkenyl Building Blocks

In recent years, hydrofluorocarbon compounds such as chlorofluorocarbons, hydrochlorofluorocarbons, and 2-bromo-2-chloro-1,1,1-trifluoroethane (halothane) have been used as fluorine-containing building blocks to construct functional fluorine-containing compounds, e. g., polymers, liquid crystals, and medicines. Hydrofluorocarbons promote the formation of reactive fluoroalkyl or fluoroalkenyl species via anionic or radical processes, and these species can act as nucleophiles or electrophiles depending on the reaction conditions. Progress in fluorine chemistry using hydrofluorocarbons in the last 30 years is described in this review and diverse reactions are discussed, including the fluoroalkyl/alkenyl products and proposed mechanisms involved.

Copper-Catalyzed Synthesis of Difluoromethyl Alkynes from Terminal and Silyl Acetylenes

An efficient method for the direct C(sp)-H difluoromethylation of terminal alkynes and the desilylation-difluoromethylation of (trimethylsilyl)acetylenes is disclosed. The copper-catalyzed transformation provides access to a wide range of structurally diverse CF₂H alkynes in good yields, utilizing a (difluoromethyl)zinc reagent and an organic oxidant. The difluoromethylation of important synthons and API's is showcased. The synthetic utility of these (difluoromethyl)alkynes is demonstrated by selected cycloaddition reactions. Additionally, a slight modification to the reaction conditions allowed the selective preparation of a 2-difluoromethylindole.

Metal-Free C-H Difluoromethylation of Imidazoles with the Ruppert-Prakash Reagent

The reaction of trimethyl(trifluoromethyl)silane-tetrabutylammonium difluorotriphenylsilicate (CF₃SiMe₃-TBAT) with a series of imidazoles gives products of the formal difluorocarbene insertion into the C-H bond at the C-2 position (i.e., C-difluoromethylation). According to NMR spectra, the corresponding 2-(trimethylsilyl)difluoromethyl-substituted derivatives are likely formed as the intermediates in the reaction, and then, they slowly convert to 2-difluoromethyl-substituted imidazoles. Quantum chemical calculations of two plausible reaction mechanisms indicate that it proceeds through the intermediate imidazolide anion stabilized through the interaction with solvent molecules and counterions. In the first proposed mechanism, the anion reacts with difluorocarbene without an activation barrier, and then, the CF₂ moiety of the adduct attacks the CF₃SiMe₃ molecule. After the elimination of the CF₃ anion, 2-(trimethylsilyl)difluromethyl-substituted imidazole is formed. Another possible reaction pathway includes silylation of imidazolide anion at the N-3 atom, followed by the barrierless addition of difluorocarbene at the C-2 atom and then by 1,3-shift of the SiMe₃ group from N-3 to the carbon atom of the CF₂ moiety. Both proposed mechanisms do not include steps with high activation barriers.

Palladium-Catalyzed Aryldifluoromethylation of Aryl Halides with Aryldifluoromethyl Trimethylsilanes

Diaryl difluoromethanes are valuable targets for medicinal chemistry because they are bioisosteres of diaryl ethers and can function as replacements for diaryl methane, ketone, and sulfone groups. However, methods to prepare diaryl difluoromethanes are scarce, especially methods starting from abundant aryl halides. We report the Pd-catalyzed aryldifluoromethylation of aryl halides with aryldifluoromethyl trimethylsilanes (TMSCF2 Ar). The reaction occurs when the catalyst contains a simple, but unusual, dialkylaryl phosphine ligand that promotes transmetallation of the silane. Computational studies show that reductive elimination following transmetallation occurs with a low barrier, despite the fluorine atoms on the α-carbon, due to coordination of the difluorobenzyl π-system to palladium. The co-development of a cobalt-catalyzed synthesis of the silanes broadened the scope of the process including several applications to the synthesis biologically relevant diaryl difluoromethanes.

A Base-Promoted Reductive Coupling Platform for the Divergent Defluorofunctionalization of Trifluoromethylarenes

We report a trifluoromethylarene reductive coupling method that dramatically expands the scope of difluorobenzylic substructures accessible via C-F bond functionalization. Catalytic quantities of a Lewis base, combined with a disilane reagent in formamide solvent, promotes the replacement of a single trifluoromethyl fluorine atom with a silylated hemiaminal functional group. The reaction proceeds through a difluorobenzyl silane intermediate that can also be isolated. Together, these defluorinated products are shown to provide rapid access to over 20 unique difluoroalkylarene scaffolds.

Repurposing of F-gases: challenges and opportunities in fluorine chemistry

Fluorinated gases (F-gases) are routinely employed as refrigerants, blowing agents, and electrical insulators. These volatile compounds are potent greenhouse gases and consequently their release to the environment creates a significant contribution to global warming. This review article seeks to summarise: (i) the current applications of F-gases, (ii) the environmental issues caused by F-gases, (iii) current methods of destruction of F-gases and (iv) recent work in the field towards the chemical repurposing of F-gases. There is a great opportunity to tackle the environmental and sustainability issues created by F-gases by developing reactions that repurpose these molecules.

Organocatalytic difluorobenzylation of 1,2-diketones via mild cleavage of carbon–carbon bonds

Difluoroacetophenones (DFAPs) are developed as a class of novel and practical reagents for organocatalytic difluorobenzylation reactions.

Late-stage difluoromethylation: concepts, developments and perspective

This review describes the conceptual advances that have led to the multiple difluoromethylation processes making use of well-defined CF₂H sources.

Anchimerically Assisted Selective Cleavage of Acid-Labile Aryl Alkyl Ethers by Aluminum Triiodide and N,N-Dimethylformamide Dimethyl Acetal

Aluminum triiodide is harnessed by N,N-dimethylformamide dimethyl acetal (DMF-DMA) for the selective cleavage of ethers via neighboring group participation. Various acid-labile functional groups, including carboxylate, allyl, tert-butyldimethylsilyl (TBS), and tert-butoxycarbonyl (Boc), suffer the conditions intact. The method offers an efficient approach to cleaving catechol monoalkyl ethers and to uncovering phenols from acetal-type protecting groups such as methoxymethyl (MOM), methoxyethoxymethyl (MEM), and tetrahydropyranyl (THP) chemoselectively.

Stereospecific Cross-Coupling Reactions Provide Conformationally-Biased Arylalkanes with Anti-Leukemia Activity

A focused small library of carbamates and alcohols was prepared employing stereospecific Kumada-ring opening reactions of tetrahydropyrans. The core framework of the library members is acyclic and incorporates 1,3-substituents, to provide a conformational bias in avoiding syn-pentane interactions. A new compound with micromolar activity against MOLT-4, CCRF-CEM, and HL-60(TB) leukemia cell lines was identified from this series.

Highly stereoselective nickel-catalyzed difluoroalkylation of aryl ketones to tetrasubstituted monofluoroalkenes and quaternary alkyl difluorides

A nickel-catalyzed difluoroalkylation of α-C-H bonds of aryl ketones to furnish highly stereo-defined tetrasubstituted monofluoroalkenes or quaternary alkyl difluorides from secondary or tertiary ketones, respectively, has been established. Mechanistic investigations indicated that these C-H fluoroalkylations proceed via a Ni(i)/Ni(iii) catalytic cycle. An obvious fluorine effect was observed in the reaction, and this reaction has demonstrated high stereoselectivity, mild conditions, and broad substrate scopes, thus enabling the late-stage fluoroalkylation of bioactive molecules. This method offers a solution for expedient construction of monofluoroalkenes from readily available materials, and provides an efficient approach for the synthesis of bioactive fluorinated compounds for the discovery of lead compounds in medicinal chemistry.

Controlling the Cleavage of Carbon-Carbon Bonds To Generate α,α-Difluorobenzyl Carbanions for the Construction of Difluoromethylbenzenes

Controlling the cleavage of carbon-carbon bonds during a chemical reaction is a substantial challenge; however, synthetic methods that accomplish this objective produce valuable and often unexplored reactivity. We have designed a mild process to generate α,α-difluorobenzyl carbanions in the presence of potassium carbonate by exploiting the cleavage of C-C bonds during the release of trifluoroacetate. The initiating reagent is potassium carbonate, which represents an improvement over existing protocols that require a strong base. Fragmentation studies across substituted arenes and heteroarenes were conducted along with computational analyses to elucidate reactivity trends. Furthermore, the mildly generated α,α-difluorobenzyl carbanions from electron-deficient aromatics and heteroaromatic rings can react with aldehydes to create derivatives of difluoromethylbenzenes, which are valuable synthetic targets.

Cobalt-catalyzed difluoroalkylation of tertiary aryl ketones for facile synthesis of quaternary alkyl difluorides

The selective incorporation of gem-difluoroalkyl groups into biologically active molecules has long been used as an efficient strategy for drug design and discovery. However, the catalytic C(sp³)-CF₂ bond-forming cross-coupling reaction for selective incorporation of difluoromethylene group into diverse alkyl chains, especially more sterically demanding secondary and tertiary functionalized alkanes, still remains as a major challenge. Herein, we describe a cobalt-catalyzed difluoroalkylation of tertiary aryl ketones for facile synthesis of quaternary alkyl difluorides, which exhibited high efficiency, broad scope and mild conditions. The synthetic utility of this method is demonstrated by late-stage difluoroalkylation of donepezil, a well-known acetylcholinesterase inhibitor used to treat the Alzheimer's disease. Preliminary mechanistic investigations indicate that a difluoroalkyl radical is involved in a Co(I)/Co(III) catalytic cycle. This cobalt-catalyzed fluoroalkylation thus offers insights into an efficient way for the synthesis of fluoroalkylated bioactive molecules for drug discovery.

Oxidative Difluoromethylation of Tetrahydroisoquinolines Using TMSCF2SPh: Synthesis of Fluorinated Pyrrolo[2,1-a]isoquinolines and Benzo[a]quinolizidines

An efficient C1-difluoromethylation of tetrahydroisoquinolenes was achieved using TMSCF₂SPh as a difluoromethylating agent and 2,2,6,6-tetramethylpiperidine-1-oxoammonium tetrafluoroborate (TEMPO⁺BF₄^-) as an oxidant. The process provides an access to a variety of C1-difluoro(phenylsulfanyl)methylated tetrahydroisoquinoline adducts in good yields. These adducts were employed as key precursors for preparing fluorinated pyrrolo[2,1-a]isoquinoline and benzo[a]quinolizidines.

Domino cyclization/trifluoromethylation of 2-alkynylanilines using fluoroform-derived CuCF3: synthesis of 3-(trifluoromethyl)indoles

A domino cyclization/trifluoromethylation strategy for the construction of indole cores with concomitant installation of a CF₃ group is described.

Highly selective nickel-catalyzed gem-difluoropropargylation of unactivated alkylzinc reagents

In spite of the important applications of difluoroalkylated molecules in medicinal chemistry, to date, the reaction of difluoroalkylating reagents with unactivated, aliphatic substrates through a controllable manner remains challenging and has not been reported. Here we describe an efficient nickel-catalyzed cross-coupling of unactivated alkylzinc reagen\ts with gem-difluoropropargyl bromides. The reaction proceeds under mild reaction conditions with high efficiency and excellent regiochemical selectivity. Transformations of the resulting difluoroalkylated alkanes lead to a variety of biologically active molecules, providing a facile route for applications in drug discovery and development. Preliminary mechanistic studies reveal that an alkyl nickel intermediate [Ni(tpy)alkyl] (tpy, terpyridine) is involved in the catalytic cycle.

Difluoroalkylated compounds find wide application in medicinal chemistry. Here, the authors describe the mild nickel-catalyzed difluoroalkylation of aliphatic alkylzinc reagents with gem-difluoropropargylbromides and show its utility in a number of post-synthetic transformations.

Deprotonative Silylation of Aromatic C-H Bonds Mediated by a Combination of Trifluoromethyltrialkylsilane and Fluoride

A method for the deprotonative silylation of aromatic C-H bonds has been developed using trifluoromethyltrimethylsilane (CF₃SiMe₃, Ruppert-Prakash reagent) and a catalytic amount of fluoride. In this reaction, CF₃SiMe₃ is considered to act as a base and a silicon electrophile. This process is highly tolerant to various functional groups on heteroarenes and benzenes. Furthermore, this method can be applied to the synthesis of trimethylsilyl group-containing analogs of TAC-101, which is a bioactive synthetic retinoid with selective affinity for retinoic acid receptor α (RAR-α) binding. We also report further transformations of the silylated products into useful derivatives.