Hydrofluorination of Alkynes: From (E) to (Z)

Access to fluorinated dienes through hydrofluorination of 2-En-4-ynoates

The hydrofluorination of enynoates has been developed for the synthesis of fluorinated dienoates. Using a pyridinium tetrafluoroborate salt that is easily prepared on large scale, this approach enabled the direct conversion of these substrates to fluorinated targets through a vinyl cation mediated process. This approach was applied to a range of aryl-substituted enynoates to deliver the (Z)-configured products with high levels of stereo- and regioselectivity. Mechanistic studies were conducted to provide insights into the stereochemical outcome and reaction efficiency under different reaction conditions.

Synthetic Strategy for Unsymmetrical α-Fluoro-α'-aryl Ketones

α-Fluoro-α'-aryl ketones are crucial in pharmaceuticals and agrochemicals. However, synthesizing unsymmetrical α-fluoro-α'-aryl ketones poses regioselective challenges. This study presents a one-pot aryl-oxy-fluorination method for synthesizing such unsymmetrical fluoro-aryl ketones. Using ynamide, aryl boronic acid, and F-source under Pd-catalysis, this method efficiently produces a wide range of valuable α-fluoro-α'-aryl ketones with potential applications. Through a combination of control experiments and DFT studies, we proposed a reaction mechanism involving in situ acetic acid formation.

Gold-catalyzed hydrofluorination of terminal alkynes using potassium bifluoride (KHF2)

A gold-catalyzed hydrofluorination of terminal alkynes is reported. The salient features of this study showcase the use of potassium bifluoride, KHF2, as the fluorinating agent in conjunction with a fluorinated solvent, hexafluoroisopropanol. The reaction is mediated by a well-defined precursor, [Au(IPr)(OH)] with an acid activator. A wide range of functionalized terminal alkynes can be converted to the corresponding monofluoroalkenes in up to 97% yield.

Chloride Induced S-C bond selective cleavage of disulfides to access unsymmetrical β-fluorodisulfides

Selective S-C bond cleavage of disulfides presents a significant challenge due to the fact that S-S bonds are weaker than S-C bonds. In this study, we present a novel chloride-induced Selectfluor radical cation process for converting readily available symmetrical disulfides into unsymmetrical β-fluorodisulfides through selective S-C bond cleavage. Mechanistic investigations and DFT calculations suggest the involvement of a chlorinated disulfide radical, which subsequently reacts with alkenes to form β-fluorodisulfides via the atom transfer radical addition (ATRA) mechanism. Furthermore, this method exhibits broad functional group tolerance, enabling the synthesis of various target products in moderate to good yields.

gem-Difluorination of carbon-carbon triple bonds using Brønsted acid/Bu4NBF4 or electrogenerated acid

gem-Difluorination of carbon-carbon triple bonds was conducted using Brønsted acids, such as Tf2NH and TfOH, combined with Bu4NBF4 as the fluorine source. The electrochemical oxidation of a Bu4NBF4/CH2Cl2 solution containing alkyne substrates could also give the corresponding gem-difluorinated compounds (in-cell method). The ex-cell electrolysis method was also applicable for gem-difluorination of alkynes.

Determination of the Hammett Acidity of HF/Base Reagents

Harnessing the acidity of HF/base reagents is of paramount importance to improve the efficiency and selectivity of fluorination reactions. Yet, no general method has been reported to evaluate their acidic properties, and experimental designs are still relying on a trial-and-error approach. We report a new method based on 19F NMR spectroscopy which allows highly sensitive measures and short-time analyses. Advantageously, the basic properties of the indicators can be determined upstream by DFT calculations, affording a simple yet robust semiempirical approach. In particular, the indicators used in this study were rationally designed to fit on the conceptually appealing and commonly used Hammett scale. This method has been applied to commercially available and recently developed HF/base reagents.

Transition-Metal-Free Approach for Z-Vinyl Fluorides by Hydrofluorination of Alkynes bearing SF4 and SF5 Groups

The synthesis of vinyl fluorides plays a crucial role in various scientific disciplines, including pharmaceutical and materials sciences. Herein, we present a direct and stereoselective hydrofluorination method for the synthesis of Z isomers of vinyl fluorides from alkynes containing unexplored SF5 and SF4 groups. Our strategy employed tetrabutylammonium fluoride (TBAF) as a fluorine source. It demonstrates high compatibility with aryls, biaryls, heteroaryls, and tert-alkyl groups, allowing facile incorporation of SF5 and SF4 groups across the triple bond without any transition-metal catalysts. This approach avoids the potential decomposition of the SF5 or SF4 units via coordination with transition metals or acidic protic sources. Remarkably, this transformation proceeded at room temperature without any additional additives, providing the Z isomer of vinyl fluorides in excellent yield and high selectivity. The presence of a water molecule as a hydrate in TBAF is essential for efficient conversion. This methodology opens new avenues for the synthesis of enchanting SF5 - and SF4 -containing fluorinated vinylic scaffolds, thereby providing advanced opportunities for novel drug discovery and fluorinated polymers.

An amorphous Lewis-acidic zirconium chlorofluoride as HF shuttle: C-F bond activation and formation

An exceptional HF transfer reaction by C-F bond activation of fluoropentane and a subsequent hydrofluorination of alkynes at room temperature is reported. An amorphous Lewis-acidic Zr chlorofluoride serves as heterogeneous catalyst, which is characterised by an eightfold coordination environment at Zr including chlorine atoms. The studies are seminal in establishing sustainable fluorine chemistry.