Single C−F Bond Cleavage of Trifluoromethylarenes with an ortho -Silyl Group

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.

Preparation and Functionalization of Mono- and Polyfluoroepoxides via Fluoroalkylation of Carbonyl Electrophiles

We outline a new synthetic method to prepare mono- and polyfluoroepoxides from a diverse pool of electrophiles (ketones, acyl chlorides, esters) and fluoroalkyl anion equivalents. The initially formed α-fluoro alkoxides undergo subsequent intramolecular ring closure when heated. We demonstrated the versatility of the method through the isolation of 16 mono- and polyfluoroepoxide products. These compounds provide unique entry points for further diversification via either fluoride migration coupled with ring opening, or defluorinative functionalization reactions, the latter of which can be used as a late-stage method to install select bioactive moieties. The reaction sequences described herein provide a pathway to functionalize the commonly observed products formed from 1,2-addition into carbonyl electrophiles.

Organoboron Reagent-Controlled Selective (Deutero)Hydrodefluorination

(Deuterium-labeled) CF₂ H- and CFH₂ -moieties are of high interest in drug discovery. The high demand for the incorporation of these fluoroalkyl moieties into molecular structures has witnessed significant synthetic progress, particularly in the (deutero)hydrodefluorination of CF₃ -containing compounds. However, the controllable replacement of fluorine atoms while maintaining high chemoselectivity remains challenging. Herein, we describe the development of a selective (deutero)hydrodefluorination reaction via electrolysis. The reaction exhibits a remarkable chemoselectivity control, which is enabled by the addition of different organoboron sources. The procedure is operationally simple and scalable, and provides access in one step to high-value building blocks for application in medicinal chemistry. Furthermore, density functional theory (DFT) calculations have been carried out to investigate the reaction mechanism and to rationalize the chemoselectivity observed.

Asymmetric Defluoroallylation of 4-Trifluoromethylpyridines Enabled by Umpolung C-F Bond Activation

Carbon-fluorine bond activation of the trifluoromethyl group represents an important approach to fluorine-containing molecules. While selective defluorinative functionalization reactions of CF₃ -containing substrates have been achieved by invoking difluorocarbocation, difluorocarboradical, or difluoroorganometallic species as the key intermediates, the transformations via fluorocarbanion mechanism only achieved limited success. Furthermore, the enantioselective defluorinative transformation of the CF₃ group remained a formidable challenge. Here we report a defluorinative functionalization reaction of 4-trifluoromethylpyridines involving difluoro(pyrid-4-yl)methyl anion as the key intermediate, which was developed based upon our previous studies on the N-boryl pyridyl anion chemistry. In particular, asymmetric defluoroallylation of 4-trifluoromethylpyridines and -pyrimidines could be achieved by using Ir-catalysis to forge a difluoroalkyl-substituted chiral center.

Photoredox-Catalyzed Defluorinative Functionalizations of Polyfluorinated Aliphatic Amides and Esters

Selective C-F bond functionalization of perfluoalkyl units has huge potential towards accessing functionalized organofluorinated compounds, but remains chanllenging due to the high C-F bond strength and inherent selectivity challenges. We report a new catalytic approach for the selective functionalization of the strong C-F bonds in polyfluorinated aliphatic esters and amides. This simple reaction proceeds in mild and operationally fashion with divergent conversions, including hydrodefluorination, defluoroalkylation, and defluoroalkenlylation, affording a diverse array of important partially fluorinated motifs. Straightforward downstream chemistry towards fluorinated alcohols, amines and drug derivatives highlights the potential of the protocol.

A Carbene Strategy for Progressive (Deutero)Hydrodefluorination of Fluoroalkyl Ketones

Hydrodefluorination is one of the most promising chemical strategies to degrade perfluorochemicals into partially fluorinated compounds. However, controlled progressive hydrodefluorination remains a significant challenge, owing to the decrease in the strength of C-F bonds along with the defluorination. Here we describe a carbene strategy for the sequential (deutero)hydrodefluorination of perfluoroalkyl ketones under rhodium catalysis, allowing for the controllable preparation of difluoroalkyl- and monofluoroalkyl ketones from aryl- and even alkyl-substituted perfluoro-alkyl ketones in high yield with excellent functional group tolerance. The reaction mechanism and the origin of the intriguing chemoselectivity of the reaction were rationalized by density functional theory (DFT) calculations.

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.

Selective C-F Bond Allylation of Trifluoromethylalkenes

Selective C-F bond functionalization of CF₃ group represents an appealing strategy for the incorporation of pharmaceutically privileged difluoromethylene moiety. Despite the recent significant advancement attained in the functionalization of Ar-CF₃ molecules, prescriptions amenable for alkenyl-CF₃ congeners remain sufficiently inadequate. Herein, we report a strategically novel protocol for the C-F bond elaboration of trifluoromethylalkene derivatives. By using readily available allyl metallics as nucleophilic coupling partner, the present reaction enables an expedient construction of structurally diversified CF₂ -bridged 1,5-dienes. Furthermore, the exquisite selectivity observed in this transformation is revealed to be based on the underlying mechanism that consists of a cascade of nucleophilic S_N 2' defluorinative allylation and electronically promoted Cope rearrangement.

Bipyridinium and Phenanthrolinium Dications for Metal-Free Hydrodefluorination: Distinctive Carbon-Based Reactivity

The development of novel Lewis acids derived from bipyridinium and phenanthrolinium dications is reported. Calculations of Hydride Ion Affinity (HIA) values indicate high carbon-based Lewis acidity at the ortho and para positions. This arises in part from extensive LUMO delocalization across the aromatic backbones. Species [C₁₀ H₆ R₂ N₂ CH₂ CH₂ ]²⁺ (R=H [1 a]²⁺ , Me [1 f]²⁺ , tBu [1 g]²⁺ ), and [C₁₂ H₄ R₄ N₂ CH₂ CH₂ ]²⁺ (R=H [2 a]²⁺ , Me [2 b]²⁺ ) were prepared and evaluated for use in the initiation of hydrodefluorination (HDF) catalysis. Compound [2 a]²⁺ proved highly effective towards generating catalytically active silylium cations via Lewis acid-mediated hydride abstraction from silane. This enabled the HDF of a range of aryl- and alkyl- substituted sp³ (C-F) bonds under mild conditions. The protocol was also adapted to effect the deuterodefluorination of cis-2,4,6-(CF₃ )₃ C₆ H₉ . The dications are shown to act as hydride acceptors with the isolation of neutral species C₁₆ H₁₄ N₂ (3 a) and C₁₆ H₁₀ Me₄ N₂ (3 b) and monocationic species [C₁₄ H₁₃ N₂ ]⁺ ([4 a]⁺ ) and [C₁₈ H₂₁ N₂ ]+ ([4 b]⁺ ). Experimental and computational data provide further support that the dications are initiators in the generation of silylium cations.

Functionalization of a Single C-F Bond of Trifluoromethylarenes Assisted by an ortho-Silyl Group Using a Trityl-Based All-in-One Reagent with Ytterbium Triflate Catalyst

Catalytic thiolation and azidation of a single C-F bond of trifluoromethylarenes were achieved assisted by an ortho-silyl group with all-in-one reagents to generate a trityl cation and nucleophiles. The reactions catalyzed by ytterbium triflate efficiently afforded a wide variety of difluoromethylenes avoiding the further C-F bond cleavage, by virtue of the mild conditions without the generation of a Brønsted acid.

Defluoroalkylation of sp3 C-F Bonds of Industrially Relevant Hydrofluoroolefins

A simple, one-pot procedure is reported for the selective defluoroalkylation of trifluoromethyl alkene derivatives with aldehydes and ketones. The reaction sequence allows construction of a new C-C bond in a highly selective manner from a single sp³ C-F bond of a CF₃ group in the presence of sp² C-F bonds. The scope incorporates industrially relevant fluorocarbons including HFO-1234yf and HFO-1234ze. No catalyst, additives or transition metals are required, rather the methodology relies on a recently developed boron reagent. Remarkably, the boron site of this reagent plays a dual role in the reaction sequence, being nucleophilic at boron in the C-F cleavage step (S_N 2') but electrophilic at boron en route to the carbon-carbon bond-forming step (S_E 2'). The duplicitous behaviour is underpinned by a hydrogen atom migration from boron to the carbon atom of a carbene ligand.

Picking One out of Three: Selective Single C-F Activation in Trifluoromethyl Groups

The introduction of a trifluoromethyl (CF₃ ) group into an organic molecule can modify its chemical behavior and lead to changes in its physicochemical and pharmacological properties. The CF₃ group is often chosen for its chemical inertness and stability, which are related to the strong C-F bonds. In recent years, the potential of gaining straightforward access to difluorinated compounds through selective single C-F activation in CF₃ groups has been unveiled. This review describes the latest methodologies for the synthesis of difluoromethylated arenes, difluoroalkenes and difluorinated aldol products of potential synthetic and/or biological interest, employing low-valent metals, transition metal catalysis, FLP and Lewis acid mediated transformations as well as photoredox catalysis.

Et3GeH versus Et3SiH: controlling reaction pathways in catalytic C–F bond activations at a nanoscopic aluminum chlorofluoride

Catalytic C–F activations at Lewis-acidic amorphous aluminum chlorofluoride (ACF) with Et₃GeH and Et₃SiH for reaction pathway control are presented.

Catalytic Enantioselective and Diastereoselective Allylic Alkylation with Fluoroenolates: Efficient Access to C3-Fluorinated and All-Carbon Quaternary Oxindoles

Synthetically versatile 3,3-disubstituted fluorooxindoles exhibiting vicinal chirality centers were obtained in high yields and with excellent enantio-, diastereo-, and regioselectivity through catalytic asymmetric fluoroenolate alkylation with allylic acetates. The reaction proceeds under mild conditions and can be scaled up without compromising the asymmetric induction. The unique synthetic usefulness of the products is highlighted by the incorporation of additional functionalities and the formation of 3-fluorinated oxindoles exhibiting an array of four adjacent centers of chirality. A new C-F bond functionalization path that provides unprecedented possibilities for the stereoselective generation of a chiral quaternary carbon center in the alkaloid scaffold is introduced.