Bromofluorination of Unsaturated Compounds using DMPU/HF as a Fluorinating Reagent

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.

From cyclic (alkyl)(amino)carbene (CAAC) precursors to fluorinating reagents. Experimental and theoretical study

Addition of anhydrous HF to the hydrochloride [^MeCAACH][Cl(HCl)_0.5] resulted in the formation of salts with high HF content. By stepwise removal of HF in vacuo, we selectively prepared [^MeCAACH][F(HF)₂] (3) and [^MeCAACH][F(HF)₃] (4). We also characterised a salt with [F(HF)₄]^- anions within the structure of [^MeCAACH][F(HF)_3.5] (5). Compounds with a lower content of HF were not accessible under vacuum conditions. ^MeCAAC(H)F (1) was selectively prepared by abstraction of HF from 3 with CsF or KF, while [^MeCAACH][F(HF)] (2) was prepared by mixing 3 and 1 in a 1 : 1 ratio. Compound 2 proved to be quite unstable as it tends to disproportionate into 1 and 3. This observation triggered our computational study, in which the structural relationships between CAAC-based fluoropyrrolidines and dihydropyrrolium fluorides were investigated using different DFT methods. The study showed that the results were very sensitive to the computational method used. For a correct description, the quality of the triple-ζ basis set was crucial. Surprisingly, the isodesmic reaction of [^MeCAACH][F] + [^MeCAACH][F(HF)₂] → [^MeCAACH][F(HF)] + [^MeCAACH][F(HF)] did not confirm the low thermodynamic stability of 2. Furthermore, the use of 3 as a nucleophilic fluorinating reagent was tested on a range of organic substrates, as it is the most stable compound in this series. It was found to have the potential to fluorinate benzyl bromides, 1- and 2-alkyl bromides, silanes and sulfonyls with good to excellent yields of the target fluorides.

Solventless and Metal-free Regioselective Hydrofluorination of Functionalized Alkynes and Allenes: An Efficient Protocol for the Synthesis of gem-Difluorides

The combination of two easily handled, highly acidic liquid HF complex reagents, DMPU-12HF and KHSO4-13HF, generated a highly acidic fluorination system that facilitated exclusive Markovnikov addition of HF to widely functionalized alkynes, including alkyne tethered drugs, or allenes to produce gem-difluorides with high atom economy, and with an easy workup.

Metal-free Regioselective Hydrochlorination of Unactivated Alkenes via a Combined Acid Catalytic System

A combined acid HCl/DMPU-acetic acid catalytic system was used in the hydrochlorination of a wide range of unactivated alkenes. This hydrochlorination strategy is remarkably greener than previous reported methods in terms of high atom efficiency, no toxic waste generated and metal-free process. The higher efficiency, compared with other commercially available HCl reagents, was augmented by the good regioselectivity and functionality tolerance found. A stepwise mechanism for this hydrochlorination process was proposed based on kinetic studies.

Hydrogen Bonding: Regulator for Nucleophilic Fluorination

The recent advances in nucleophilic fluorination, regulated through hydrogen bonding interactions are summarized. Two main categories of fluorine nucleophiles are discussed. Alkali-metal fluorides are widely used in various fluorination transformations because they are inexpensive and safe nucleophilic fluorine sources. But the non-controllable nucleophilicity and strong basicity of some of them cause undesired side reactions, which led to the introduction of hydrogen bonding to fine tune their nucleophilicity and basicity. In contrast, an HF-based fluorine nucleophile, HF/DMPU, is in some aspects superior to the conventional HF/pyridine (Olah's reagent) or HF/Et3 N because of the higher hydrogen bond basicity of DMPU. It has been used in several nucleophilic fluorinations such as fluorination of alkynes, fluoro-Prins reaction and fluorination of aziridines.