Fluorinated Vinylsilanes from the Copper-Catalyzed Defluorosilylation of Fluoroalkene Feedstocks

Electrochemical Synthesis of gem-Difluoro- and γ-Fluoro-Allyl Boronates and Silanes

The electrochemical synthesis of fluorinated allyl silanes and boronates was disclosed. The addition of electrogenerated boryl or silyl radicals onto many α-trifluoromethyl or α-difluoromethylstyrenes in an undivided cell allowed the formation of a large panel of synthetically useful gem-difluoro and γ-fluoroallyl boronates and silanes (64 examples, from 31 % to 95 % yield). In addition, a scale up of the reactions under continuous flow was showcased using an electrochemical reactor with promising volumetric productivity (688 g.L-1 .h-1 and 496 g.L-1 .h-1 ). Moreover, the synthetic utility of these building blocks was highlighted through versatile transformations. Finally, plausible reaction mechanisms were suggested to explain the formation of the products.

Nickel-Catalyzed Stereo- and Enantioselective Cross-Coupling of gem-Difluoroalkenes with Carbon Electrophiles by C-F Bond Activation

Stereo- and enantioselective cross-electrophile coupling involving C-F bond activation is reported. Treatment of gem-difluoroalkenes with racemic benzyl electrophiles in the presence of a chiral nickel complex using B₂ pin₂ as a stoichiometric reductant allows the construction of a C(sp² )-C(sp³ ) bond under mild conditions, affording a broad range of monofluoroalkenes bearing stereogenic allylic centers. Initial mechanistic studies indicate that a radical chain pathway may be operating, wherein the ester group in the gem-difluoroalkene promotes C-F bond activation through oxidative addition to a Ni species.

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.

Asymmetric Synthesis of 2,2-Difluorotetrahydrofurans through Palladium-Catalyzed Formal [3+2] Cycloaddition

The asymmetric synthesis of 2,2-difluorinated tetrahydrofurans was accomplished via enantioselective formal [3+2] cycloaddition catalyzed by palladium. The asymmetric reaction between gem-difluoroalkenes and racemic vinyl epoxides or vinylethylene carbonates resulted in the formation of enantioenriched 2,2-difluorotetrahydrofurans with an enantioselectivity up to 98 %. Notably, the reaction used the readily available (R)-BINAP as the ligand at a low loading and yielded a wide variety of difluorinated products in moderate to high yields. Both chiral diastereomers could be obtained in a single sequence.

Versatile Reaction Pathways of 1,1,3,3,3-Pentafluoropropene at Rh(I) Complexes [Rh(E)(PEt3 )3 ] (E=H, GePh3 , Si(OEt)3 , F, Cl): C-F versus C-H Bond Activation Steps

The reaction of the rhodium(I) complexes [Rh(E)(PEt₃)₃] (E=GePh₃ (1), H (6), F (7)) with 1,1,3,3,3‐pentafluoropropene afforded the defluorinative germylation products Z/E‐2‐(triphenylgermyl)‐1,3,3,3‐tetrafluoropropene and the fluorido complex [Rh(F)(CF₃CHCF₂)(PEt₃)₂] (2) together with the fluorophosphorane E‐(CF₃)CH=CF(PFEt₃). For [Rh(Si(OEt)₃)(PEt₃)₃] (4) the coordination of the fluoroolefin was found to give [Rh{Si(OEt)₃}(CF₃CHCF₂)(PEt₃)₂] (5). Two equivalents of complex 2 reacted further by C−F bond oxidative addition to yield [Rh(CF=CHCF₃)(PEt₃)₂(μ‐F)₃Rh(CF₃CHCF₂)(PEt₃)] (9). The role of the fluorido ligand on the reactivity of complex 2 was assessed by comparison with the analogous chlorido complex. The use of complexes 1, 4 and 6 as catalysts for the derivatization of 1,1,3,3,3‐pentafluoropropene provided products, which were generated by hydrodefluorination, hydrometallation and germylation reactions.

Experimental and Computational Studies of the Iron-Catalyzed Selective and Controllable Defluorosilylation of Unactivated Aliphatic gem-Difluoroalkenes

The first iron-catalyzed defluorosilylation of unactivated gem-difluoroalkenes was developed, delivering gem-disilylated alkenes and (E)-silylated alkenes with excellent efficiency. This protocol features good functional group compatibility and excellent regio- and stereoselectivity, enabling the late-stage silylation of biologically relevant compounds, thus providing good opportunities for applications in medicinal chemistry. Preliminary mechanistic studies and DFT calculations reveal that a nucleophilic addition and elimination of the second C-F bond might be involved in the disilylation catalytic system, demonstrating unusual reactivity characteristics of iron catalysis.

Copper-Catalyzed Defluorinative Borylation and Silylation of gem-Difluoroallyl Groups

Stereodefined (Z)-fluoroalkenes are bioisosteres of amides and synthetic precursors to value-added fluorinated compounds, but their stereoselective synthesis remains challenging. Herein, we report a copper-catalyzed formal SN2' defluorinative borylation of 3-substituted 3,3-difluoropropenes to form 3-fluoroallylboronic esters in high yields with excellent Z/E ratios. The primary 3-fluoroallylboronic esters undergo several synthetic sequences involving SE2' substitutions, SN2' substitutions, and sigmatropic rearrangements to provide tertiary allylic fluorides.

Activation of Aromatic C-F Bonds by a N-Heterocyclic Olefin (NHO)

A N-heterocyclic olefin (NHO), a terminal alkene selectively activates aromatic C-F bonds without the need of any additional catalyst. As a result, a straightforward methodology was developed for the formation of different fluoroaryl-substituted alkenes in which the central carbon-carbon double bond is in a twisted geometry.

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.

Copper-Catalyzed Triboration of Terminal Alkynes Using B2 pin2 : Efficient Synthesis of 1,1,2-Triborylalkenes

We report herein the catalytic triboration of terminal alkynes with B2 pin2 (bis(pinacolato)diboron) using readily available Cu(OAc)2 and Pn Bu3 . Various 1,1,2-triborylalkenes, a class of compounds that have been demonstrated to be potential matrix metalloproteinase (MMP-2) inhibitors, were obtained directly in moderate to good yields. The process features mild reaction conditions, a broad substrate scope, and good functional group tolerance. This copper-catalyzed reaction can be conducted on a gram scale to produce the corresponding 1,1,2-triborylalkenes in modest yields. The utility of these products was demonstrated by further transformations of the C-B bonds to prepare gem-dihaloborylalkenes (F, Cl, Br), monohaloborylalkenes (Cl, Br), and trans-diaryldiborylalkenes, which serve as important synthons and have previously been challenging to prepare.

An Efficient Approach to Regio- and Stereodefined Fully-Substituted Alkenylsilanes by Pd-Catalyzed Allenic C(sp3 )-H Oxidation

A highly efficient palladium-catalyzed functionalization of allenylsilanes to give regio- and stereodefined fully-substituted alkenylsilanes has been developed. This oxidative coupling reaction showed good functional group compatibility with exclusive regio- and stereoselectivity. The pending olefin on the silyl group was shown to be an indispensable element for the initial allenic C(sp³ )-H bond cleavage, and performs as the directing group to control the overall selectivity. The addition of substoichiometric amounts of Et₃ N was found to increase the reaction rate leading to a higher reaction yield. The reaction can be easily scaled up and applied for the late-stage functionalization of natural products and pharmaceutical compounds, including amino acids and steroid derivatives. The newly introduced functional groups include aryl, alkynyl, and boryl groups. The highly strained four-membered ring, silacyclobutene was obtained when B₂ pin₂ was employed as the coupling partner. Mechanistic studies, including kinetic isotope effects, showed that the allenic C(sp³ )-H bond cleavage is the rate-limiting step.

C-H and C-F Bond Activation Reactions of Fluorinated Propenes at Rhodium: Distinctive Reactivity of the Refrigerant HFO-1234yf

The reaction of [Rh(H)(PEt₃ )₃ ] (1) with the refrigerant HFO-1234yf (2,3,3,3-tetrafluoropropene) affords an efficient route to obtain [Rh(F)(PEt₃ )₃ ] (3) by C-F bond activation. Catalytic hydrodefluorinations were achieved in the presence of the silane HSiPh₃ . In the presence of a fluorosilane, 3 provides a C-H bond activation followed by a 1,2-fluorine shift to produce [Rh{(E)-C(CF₃ )=CHF}(PEt₃ )₃ ] (4). Similar rearrangements of HFO-1234yf were observed at [Rh(E)(PEt₃ )₃ ] [E=Bpin (6), C₇ D₇ (8), Me (9)]. The ability to favor C-H bond activation using 3 and fluorosilane is also demonstrated with 3,3,3-trifluoropropene. Studies are supported by DFT calculations.