Synthesis of Ring-Fluorinated Thiophene Derivatives Based on Single C-F Bond Activation of CF3-Cyclopropanes: Sulfanylation and 5-endo-trig Cyclization

Sequential C-F Bond Transformation of the Difluoromethylene Unit in Perfluoroalkyl Groups: A Combination of Fine-Tuned Phenothiazine Photoredox Catalyst and Lewis Acid

A sequential process via photoredox catalysis and Lewis acid mediation for C-F bond transformation of the CF2 unit in perfluoroalkyl groups has been achieved to transform perfluoroalkylarenes into complex fluoroalkylated compounds. A phenothiazine-based photocatalyst promotes the defluoroaminoxylation of perfluoroalkylarenes with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) under visible light irradiation, affording the corresponding aminoxylated products. These products undergo a further defluorinative transformation with various organosilicon reagents mediated by AlCl3 to provide highly functionalized perfluoroalkyl alcohols. Our novel phenothiazine catalyst works efficiently in the defluoroaminoxylation. Transient absorption spectroscopy revealed that the catalyst regeneration step is crucial for the photocatalytic aminoxylation.

Fluorine-Activated and -Directed Allene Cycloadditions with Nitrile Oxides and Imine Oxides: Synthesis of Ring-Fluorinated Isoxazole Derivatives

In this study, 1,1-difluoroallenes underwent a regioselective [2+3] cycloaddition with nitrile oxides and imine oxides in the presence of a AuCl catalyst. (E)-4-Alkylidene-5,5-difluoroisoxazolines and -isoxazolidines were obtained in regioselective and diastereoselective manners by employing aurated difluoroallylic cation intermediates. The synthesized 5,5-difluoroisoxazolines were readily aromatized through dehydrofluorination or allylic fluorine substitution to provide 5-fluoroisoxazoles.

Defluorinative Transformation of (2,2,2-Trifluoroethyl)arenes Catalyzed by the Phosphazene Base t-Bu-P2

In this study, we demonstrated that 1-tert-butyl-2,2,4,4,4-pentakis(dimethylamino)-2λ5,4λ5-catenadi(phosphazene) (t-Bu-P2) catalyzes the defluorinative functionalization reactions of (2,2,2-trifluoroethyl)arenes with alkanenitriles to produce monofluoroalkene products. The reaction proceeds through HF elimination from a (2,2,2-trifluoroethyl)arene to form a gem-difluorostyrene intermediate, which is followed by nucleophilic addition of an alkanenitrile and elimination of a fluoride anion. The catalysis is compatible with a variety of functional groups.

Synthesis of Indole-Fused Dihydrothiopyrano Scaffolds via (3 + 3)-Annulations of Donor-Acceptor Cyclopropanes with Indoline-2-Thiones

A new methodology for the synthesis of N-haloindole-fused dihydrothiopyrano derivatives via (3 + 3)-annulation of donor-acceptor cyclopropanes (DACs) with indoline-2-thiones in the presence of Sc(OTf)₃ as a Lewis acid catalyst has been developed. This protocol provides a variety of indole-fused dihydrothiopyrano molecules in good to excellent yields, which architecturally resemble other indole-fused tricyclic molecules having potential medicinal value. In addition, we have described a detailed reaction mechanism and transformation of the furnished product into N-fused thiazino indole molecule.

Two-Step Synthesis of 2-Trifluoromethylated and 2-Difluoromethylated Benzoheteroles Starting from HFO-1224yd(Z) and HFO-1233yd(Z)

An efficient two-step method for synthesizing 2-(trifluoromethyl)- and 2-(difluoromethyl)benzoheteroles bearing various substituents was developed. Commercially available HFO-1224yd(Z) or HFO-1233yd(Z) underwent the Suzuki-Miyaura coupling with arylboronic acids (acid esters) bearing a nucleophilic moiety at the ortho position to yield the corresponding β-fluoro-β-(trifluoromethyl)- or β-fluoro-β-(difluoromethyl)styrenes, respectively. Treatment of the obtained styrenes with potassium phosphate induced nucleophilic 5-endo-trig cyclization to provide the corresponding 2-trifluoromethylated or 2-difluoromethylated indoles and benzofurans, as well as benzothiophenes.

New trifluoromethylated sesquiterpenoids: synthesis, rearrangement, and biological activity

The tandem isomerization and trifluoromethylation reactions are a simple, convenient, and atom-economical strategy for the synthesis of various products in high yields from simple substrates.

Construction of Thienothiophene and Thienofuran Ring Systems via Ring Expansion of Difluorothiiranes Generated from Dithioesters

The reaction of aryl thiophene-2-carbodithioates or thiophene-3-carbodithioates with difluorocarbene generated from BrCF₂CO₂Li/molecular sieves 4A produced arylsulfanylated 2,2-difluoro-3-thienylthiiranes. In the presence of lithium ion, the thiirane intermediates underwent ring expansion followed by HF elimination, leading to fluorinated thieno[3,2-b]thiophenes or thieno[2,3-b]thiophenes. The reactions of the oxygen analogues, aryl furancarbodithioates, also proceeded to afford the corresponding thieno[3,2-b]furans. Intramolecular fluorine substitution in the produced arylsulfanyl(fluoro)thienofurans allowed for another thiophene ring construction, leading to the synthesis of fused pentacyclic thienothienofurans.

Insertion of Diazo Esters into C-F Bonds toward Diastereoselective One-Carbon Elongation of Benzylic Fluorides: Unprecedented BF3 Catalysis with C-F Bond Cleavage and Re-formation

Selective transformation of C-F bonds remains a significant goal in organic chemistry, but C-F insertion of a one-carbon-atom unit has never been established. Herein we report the BF₃-catalyzed formal insertion of diazo esters as one-carbon-atom sources into C-F bonds to accomplish one-carbon elongation of benzylic fluorides. A DFT calculation study revealed that the BF₃ catalyst could contribute to both C-F bond cleavage and re-formation. This elongation provided α-fluoro-α,β-diaryl esters with a high level of diastereoselectivity. Various benzylic fluorides and diazo esters were applicable. The synthetic utility of this method was demonstrated by the synthesis of a fluoro analogue of a compound that is used as a transient receptor and potential canonical channel inhibitor.

C-F bond activation under transition-metal-free conditions

The unique properties of fluorine-containing organic compounds make fluorine substitution attractive for the development of pharmaceuticals and various specialty materials, which have inspired the evolution of diverse C-F bond activation techniques. Although many advances have been made in functionalizations of activated C-F bonds utilizing transition metal complexes, there are fewer approaches available for nonactivated C-F bonds due to the difficulty in oxidative addition of transition metals to the inert C-F bonds. In this regard, using Lewis acid to abstract the fluoride and light/radical initiator to generate the radical intermediate have emerged as powerful tools for activating those inert C-F bonds. Meanwhile, these transition-metal-free processes are greener, economical, and for the pharmaceutical industry, without heavy metal residues. This review provides an overview of recent C-F bond activations and functionalizations under transition-metal-free conditions. The key mechanisms involved are demonstrated and discussed in detail. Finally, a brief discussion on the existing limitations of this field and our perspective are presented.

Modern approaches towards the synthesis of geminal difluoroalkyl groups

This review will cover the importance of and most recent approaches toward geminal difluoroalkyl groups. Transition metal-mediated, photochemical, organocatalytic, and other methods as well as their mechanistic implications will be discussed, with special emphasis on applications to biologically-relevant compounds.

Recent advance in the C–F bond functionalization of trifluoromethyl-containing compounds

The C–F bond is the strongest single bond in organic compounds.