Asymmetric α-electrophilic difluoromethylation of β-keto esters by phase transfer catalysis

Advances in the Synthetic Utility of Difluorocarbene Generated from TMSCF3 (Ruppert-Prakash Reagent) and Its Derivatives

Organofluorine compounds are of pivotal significance particularly, in drug and agrochemical industries and different strategies have been designed for their synthesis. The last two decades witnessed the emergence of difluorocarbene as an efficient synthetic tool, providing easy access to organofluorine compounds. This review summarises the reactions of difluorocarbene generated from Ruppert-Prakash reagent (TMSCF3) and its derivatives TMSCF2Cl and TMSCF2Br. Among the various fluorination techniques available, the difluorocarbene chemistry offers a cost effective and easy procedure, opening avenue to a large number of organofluorine compounds. This review details the developments in the utility of difluorocarbene generated from TMSCF3 and its derivatives, till date.

Asymmetric phase-transfer catalysis

Over the past three decades, chiral phase-transfer catalysts (PTCs) have emerged as highly successful organocatalysts in a diverse range of asymmetric reactions. A substantial number of chiral PTCs have now already been discovered and utilized in dependable routes to enantioenriched products. These extend beyond the classical cationic PTCs with the emergence of anionic phase-transfer catalysis and hydrogen-bonding phase-transfer catalysis providing new asymmetric synthetic approaches. Nevertheless, the application level of chiral PTCs in both academic and industrial processes is below our expectation. This Review highlights the notable advances in chiral PTCs, including challenges, limitations and efforts to overcome them. Following this, the potential for sustainable chiral PTCs is described with a focus on using photocatalysed, flow and electrochemical synthesis.

Highly Enantioselective Decarboxylative Difluoromethylation

Organofluorine molecules that contain difluoromethyl groups (CF2H) at stereogenic centers have gained importance in pharmaceuticals due to the unique ability of CF2H groups to act as lipophilic hydrogen bond donors. Despite their potential, the enantioselective installation of CF2H groups into readily available starting materials remains a challenging and underdeveloped area. In this study, we report a nickel-catalyzed decarboxylative difluoromethylation reaction that converts alkyl carboxylic acids into difluoromethylated products with exceptional enantioselectivity. This Ni-catalyzed protocol exhibits broad functional group tolerance and is applicable for synthesizing fluorinated bioisosteres of biologically relevant molecules.

Visible Light-Induced Radical Cascade Difluoromethylation/Cyclization of Unactivated Alkenes: Access to CF2H-Substituted Polycyclic Imidazoles

An efficient and mild protocol for the visible light-induced radical cascade difluoromethylation/cyclization of imidazoles with unactivated alkenes using easily accessible and bench-stable difluoromethyltriphenylphosphonium bromide as the precursor of the -CF2H group has been developed to afford CF2H-substituted polycyclic imidazoles in moderate to good yields. This strategy, along with the construction of Csp3-CF2H/C-C bonds, is distinguished by mild conditions, no requirement of additives, simple operation, and wide substrate scope. In addition, the mechanistic experiments have indicated that the difluoromethyl radical pathway is essential for the methodology.

The Revival of Enantioselective Perfluoroalkylation - Update of New Synthetic Approaches from 2015-2022

Over the last years, methods devoted to the synthesis of asymmetric molecules bearing a perfluoroalkylated chain have been limited in number. Among them, only a few can be used on a large variety of scaffolds. This microreview aims at summarizing these recent advances in enantioselective perfluoroalkylation (-CF3 , -CF2 H, -Cn F2n+1 ) and highlights the need for new enantioselective methods to easily synthesize chiral fluorinated molecules which would be useful for the pharmaceutical and agrochemical industries. Some perspectives are also mentioned.

Transition-Metal-Free Controllable Single and Double Difluoromethylene Formal Insertions into C-H Bonds of Aldehydes with TMSCF2 Br

We have developed a new strategy for controllable single and double difluoromethylene (CF₂ ) formal insertions into C-H bonds of aldehydes with nearly full selectivity under transition-metal-free conditions. The key to the success of controllable CF₂ insertions lies in the well-defined formation of 2,2-difluoroenolsilyl ether and 2,2,3,3-tetrafluorocyclopropanolsilyl ether intermediates using difluorocarbene reagent TMSCF₂ Br (TMS=trimethylsilyl). These two intermediates can react with various electrophiles including proton sources and various halogenation reagents, allowing for the access to diverse arrays of ketones containing difluoromethylene (CF₂ ) and tetrafluoroethylene (CF₂ CF₂ ) units. The first synthesis of relatively stable 2,2,3,3-tetrafluorocyclopropanolsilyl ethers has been achieved, which offers a new platform to explore other unknown chemical space.

Access to enantioenriched molecules with diverse fluorinated tetrasubstituted stereocenters using hydroxy as a kinetic resolution auxiliary group

We describe in this paper that using secondary OH as the kinetic resolution auxiliary group, a series of previously unavailable fluorinated fully-substituted carbon molecules can be obtained with excellent level of enantioselectivities.