Photoredox‐Coupled F‐Nucleophilic Addition: Allylation of gem ‐Difluoroalkenes

Cobalt-Catalyzed Selective Unsymmetrical Dioxidation of gem-Difluoroalkenes

gem-Difluoroalkenes represent valuable synthetic handles for organofluorine chemistry; however, most reactions of this substructure proceed through reactive intermediates prone to eliminate a fluorine atom and generate monofluorinated products. Taking advantage of the distinct reactivity of gem-difluoroalkenes, we present a cobalt-catalyzed regioselective unsymmetrical dioxygenation of gem-difluoroalkenes using phenols and molecular oxygen, which retains both fluorine atoms and provides β-phenoxy-β,β-difluorobenzyl alcohols. Mechanistic studies suggest that the reaction operates through a radical chain process initiated by Co(II)/O₂/phenol and quenched by the Co-based catalyst. This mechanism enables the retention of both fluorine atoms, which contrasts most transition-metal-catalyzed reactions of gem-difluoroalkenes that typically involve defluorination.

F--Free Deoxyhydrotrifluoromethylation of α-Keto Esters with Ph3P+CF2CO2-: Synthesis of α-CF3-Substituted Esters

Trifluoromethylated compounds are usually obtained via trifluoromethylation reaction by the use of CF₃SiMe₃ and NaSO₂CF₃, Umemoto's and Togni's reagents. Here, an external fluorine anion-free direct deoxyhydrotrifluoromethylation of α-keto esters with a difluoromethylating reagent has been achieved, in which the employment of water can promote the dissociation of the CF₂ group to form a CF₃ moiety, which provides the successful transformation. The current protocol demonstrates one of the most practical approaches to generate α-trifluoromethyl esters with a broad substrate scope and high functional group compatibility, in which it is applicable to late-stage modification of biologically active compounds and can be readily scaled up. Mechanistic investigation reveals that an in situ-generated gem-difluoroalkene intermediate is decomposed by water, giving rise to acid fluoride and HF.

Palladium-Catalyzed Fluoroarylation of gem-Difluoroenynes to Access Trisubstituted Trifluoromethyl Allenes

Various new transformations of gem-difluoroalkenes leading to trifluoromethyl substituted compounds have been well established in the past years. However, the development of new transformations of gem-difluoroenynes lags much behind. Herein is reported the fluoroarylation of 1,1-difluoro-1,3-enynes with aryl halides in the presence of silver fluoride affording trisubstituted trifluoromethyl allenes under the catalysis of palladium. The reaction features mild conditions, high functional-group tolerance, and high regioselectivity.

Visible-Light-Mediated Manganese-Catalyzed Allylation Reactions of Unactivated Alkyl Iodides

Herein, we report a protocol for visible-light-mediated allylation reactions between unactivated alkyl iodides and allyl sulfones under mild conditions with catalysis by inexpensive and readily available Mn₂(CO)₁₀. This protocol is compatible with a wide array of sensitive functional groups and has a broad substrate scope with regard to both alkyl iodides and allyl sulfones.

Recent Advances in Transition Metal-Catalyzed Functionalization of gem-Difluoroalkenes

gem-Difluorinated alkenes are readily accessible building blocks that can undergo functionalization to provide a broad spectrum of fluorinated and non-fluorinated products. Herein, we review recent (since 2017) transition metal-catalyzed transformations of these specialized alkenes and summarize general reactivity patterns of these reactions. Many transition metal-catalyzed reactions undergo net C-F bond functionalization reactions to deliver monofluorinated products. These reactions typically proceed through β-fluoro alkylmetal intermediates that readily eliminate a β-fluoride to deliver monofluoroalkene products. A second series of reactions exploit coinage metal fluorides to add F- to the gem-difluorinated alkene, and further functionalization delivers trifluoromethyl-containing products. In stark contrast, few transition metal-catalyzed reactions proceed in net "fluorine-retentive processes" to deliver difluoromethylene-based products.

Construction of trifluoromethylated quaternary stereocenters via p-quinone methides

Development of a new synthetic method for the construction of quaternary centers with a CF₃ group was realized by way of 1,6-addition of various nucleophiles to highly reactive δ-trifluoromethylated p-quinone methides generated in situ.

Recent advances in three-component difunctionalization of gem-difluoroalkenes

Three-component difunctionalization of gem-difluoroalkenes via the generation and transformation of a α-fluoroalkylated carbanion, a carbon–metal species, a radical, and a carbocation intermediate.

Chemo- and regioselective synthesis of polysubstituted 2-aminothiophenes by the cyclization of gem-dibromo or gem-dichloroalkenes with β-keto tertiary thioamides

A facile and practical method for the synthesis of 2,3,4-trisubstituted 2-aminothiophenes by the cyclization of gem-dibromoalkenes or gem-dichloroalkenes with β-keto tertiary thioamides has been developed. The cyclization reaction proceeded chemoselectively and regioselectively under metal-catalyst-free conditions, providing various structurally diverse 2,3,4-trisubstituted N,N'-dialkyl 2-aminothiophenes in good to excellent yields.

A novel photoredox-active group for the generation of fluorinated radicals from difluorostyrenes

A 4-tetrafluoropyridinylthio group was suggested as a new photoredox-active moiety. The group can be directly installed on difluorostyrenes in a single step by the thiolene click reaction. It proceeds upon visible light catalysis with 9-phenylacridine providing various difluorinated sulfides as radical precursors. Single electron reduction of the C–S bond with the formation of fluoroalkyl radicals is enabled by the electron-poor azine ring. The intermediate difluorinated sulfides were involved in a series of photoredox reactions with silyl enol ethers, alkenes, nitrones and an alkenyl trifluoroborate.

A new photoredox-active group was applied for the generation of fluorinated radicals from difluorostyrenes under blue light irradiation.

Photocatalytic hydroacylation of trifluoromethyl alkenes

Hydrofunctionalization of trifluoromethyl alkenes is highly challenging, because the nucleophilic addition is oftentimes followed by β-F elimination in this case. By the use of tetrabutylammonium decatungstate (TBADT) as a hydrogen-atom-transfer (HAT) photocatalyst for acyl C-H activation, we successfully avoid the β-F elimination in the hydroacylation of trifluoromethyl alkenes with aldehydes. This light (390 nm) promoted reaction provides a facile and efficient access to various β-CF3 ketones in complete regiocontrol with high functionality tolerance and 100% atom economy.

Photoredox-catalyzed oxo-amination of aryl cyclopropanes

Cyclopropanes represent a class of versatile building blocks in modern organic synthesis. While the release of ring strain offers a thermodynamic driving force, the control of selectivity for C–C bond cleavage and the subsequent regiochemistry of the functionalization remains difficult, especially for unactivated cyclopropanes. Here we report a photoredox-coupled ring-opening oxo-amination of electronically unbiased cyclopropanes, which enables the expedient construction of a host of structurally diverse β-amino ketone derivatives. Through one electron oxidation, the relatively inert aryl cyclopropanes are readily converted into reactive radical cation intermediates, which in turn participate in the ensuing ring-opening functionalizations. Based on mechanistic studies, the present oxo-amination is proposed to proceed through an S_N2-like nucleophilic attack/ring-opening manifold. This protocol features wide substrate scope, mild reaction conditions, and use of dioxygen as an oxidant both for catalyst regeneration and oxygen-incorporation. Moreover, a one-pot formal aminoacylation of olefins is described through a sequential cyclopropanation/oxo-amination.

The ring-opening and functionalization of electronically unbiased cyclopropanes is highly challenging to achieve in a regioselective fashion. Here, the authors report a mild photoredox-coupled oxoamination of electronically unactivated aryl cyclopropanes with simple azaarenes and molecular oxygen.

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.

Efficient photocaging of a tight-binding bisubstrate inhibitor of cAMP-dependent protein kinase

PKA bisubstrate inhibitor photocaging resulted in an over 5 orders of magnitude affinity difference between the photocaged and the active inhibitor.