Transition-metal-catalyzed C-H bond activation as a sustainable strategy for the synthesis of fluorinated molecules: an overview

The last decade has witnessed the emergence of innovative synthetic tools for the synthesis of fluorinated molecules. Among these approaches, the transition-metal-catalyzed functionalization of various scaffolds with a panel of fluorinated groups (XR_F, X = S, Se, O) offered straightforward access to high value-added compounds. This review will highlight the main advances made in the field with the transition-metal-catalyzed functionalization of C(sp²) and C(sp³) centers with SCF₃, SeCF₃, or OCH₂CF₃ groups among others, by C-H bond activation. The scope and limitations of these transformations are discussed in this review.

Directed Palladium Catalyzed C-H (Ethoxycarbonyl)difluoromethylthiolation Reactions

The unprecedented Pd-catalyzed (ethoxycarbonyl)difluoromethylthiolation reaction of various unsaturated derivatives was studied. In the presence of the (ethoxycarbonyl)difluoromethylsulfenamide reagent I and under mild reaction conditions (60 °C), both 2-(hetero)aryl and 2-(α-aryl-vinyl)pyridine derivatives were smoothly functionalized with this methodology (37 examples, up to 87 % yield). Moreover, the synthetic interest of this fluorinated moiety was further showcased by its conversion into various original fluorinated residues. Finally, a plausible mechanism for this transformation was suggested.

Palladium-Catalyzed Regio- and Diastereoselective Olefinic C-H Difluoromethylthiolation at Room Temperature

Herein, we developed palladium-catalyzed regio- and diastereoselective difluoromethylthiolation of acrylamides to form the Z-isomer product at room temperature. Using 8-aminoquinoline as a directing group, this protocol resulted in a high efficiency under mild reaction conditions and showed good functional group tolerances, which opens a novel synthetic methodology for accessing SCF₂H-containing skeletons. Moreover, mechanistic studies were conducted to obtain insights into the reaction mechanism, and post-functionalization of the product reactions was performed.

Desulfonylation via Radical Process: Recent Developments in Organic Synthesis

As the "chemical chameleon", sulfonyl-containing compounds and their variants have been merged with various types of reactions for the efficient construction of diverse molecular architectures by taking advantage of their incredible reactive flexibility. Currently, their involvement in radical transformations, in which the sulfonyl group typically acts as a leaving group via selective C-S, N-S, O-S, S-S, and Se-S bond cleavage/functionalization, has facilitated new bond formation strategies which are complementary to classical two-electron cross-couplings via organometallic or ionic intermediates. Considering the great influence and synthetic potential of these novel avenues, we summarize recent advances in this rapidly expanding area by discussing the reaction designs, substrate scopes, mechanistic studies, and their limitations, outlining the state-of-the-art processes involved in radical-mediated desulfonylation and related transformations. With a specific emphasis on their synthetic applications, we believe this review will be useful for medicinal and synthetic organic chemists who are interested in radical chemistry and radical-mediated desulfonylation in particular.

Difluoromethylthiolator: A Toolbox of Reagents for Difluoromethylthiolation

Recently, the strategic installation of a fluorine atom or a fluoroalkyl group site-selectively at the specific position of the target molecule has become a routine approach and daily practice for medicinal chemists in their endeavor to fine tune the structure of the lead compound to improve its physicochemical properties such as the cell membrane permeability and metabolic stability. Among many fluoroalkyl groups, the difluoromethylthio group (-SCF₂H) has attracted recent intense attention. Largely due to the weak acidity of the proton in the difluoromethylthio group, the difluoromethylthio group is generally considered to be a lipophilic hydrogen-bonding donor and a bioisostere of the hydroxy/thio group that might interact with the heteroatom of the enzyme via a hydrogen bond to improve the binding selectivity of the drug molecule. Besides, the difluoromethylthio group is less lipophilic, less electron-withdrawing, and less stable to the acidic or basic environment than its analogue trifluoromethylthio group (-SCF₃), making it easier to regulate the metabolic stability of drug molecules. These beneficial effects render the difluoromethylthio group one of the most favorable functional groups in drug design; consequently, there is an urgent need to develop new strategies for the efficient introduction of the difluoromethylthio group into small molecules under mild conditions. Over the last few decades, several different approaches to the preparation of difluoromethylthiolated compounds have been developed, including the difluoromethylation of thiolated substrates with an electrophilic/nucleophilic difluoromethylating reagent or the insertion of a difluoromethyl carbene into the S-H bond of the thiols. In contrast, we adopt an alternative approach to the preparation of difluoromethylthiolated compounds by late-stage direct difluoromethylthiolation of the specific substrates with a difluoromethylthiolating reagent. With this aim in mind, in the last 6 years we have successfully developed a toolbox of reagents that are capable of the direct introduction of the difluoromethylthio group into the target molecules, including nucleophilic difluoromethylthiolating reagent [(SIPr)AgSCF₂H] I, electrophilic difluoromethylthiolating reagent PhthSCF₂H II, three optically pure difluoromethylthiolating reagents camphorsultam-SCF₂H III, radical difluoromethylthiolating reagent PhSO₂SCF₂H IV, and reagent PhSO₂SCFClH V that could be used for the preparation of ¹⁸F-labeled [¹⁸F]ArSCF₂H. These reagents reacted with a broad range of substrates to get access to difluoromethylthiolated compounds efficiently, thus providing medicinal chemists a powerful weapon for the direct introduction of the difluoromethylthio group into promising molecules during the search for new drugs.

Design and Use of Electrophilic Thiocyanating and Selenocyanating Reagents: An Interesting Trend for the Construction of SCN- and SeCN-Containing Compounds

Organothiocyanate and organoselenocyanate compounds are of paramount importance in organic chemistry as they are key intermediates to access sulfur- and selenium-containing compounds. Therefore, among the different synthetic pathways to get SCN- and SeCN-containing molecules, original methodologies using electrophilic reagents have recently been explored. This Minireview will showcase the recent advances that have been made. In particular, the design of several electrophilic sources and their applications for the thiocyanation and the selenocyanation of various classes of compounds will be highlighted and discussed.

Pd-Catalyzed Directed Thiocyanation Reaction by C-H Bond Activation

The Pd‐catalyzed directed thiocyanation reaction of arenes and heteroarenes by C−H bond activation was achieved. In the presence of an electrophilic SCN source, this original methodology offered an efficient tool to access a panel of functionalized thiocyanated compounds (21 examples, up to 78 % yield). Post‐functionalization reactions further demonstrated the synthetic utility of the approach by converting the SCN‐containing molecules into value‐added scaffolds.