Nucleophilic monofluoroalkylation with fluorinated phosphonium salt toward carbonyl and imine compounds

Fluoride Anion Catalyzed Mukaiyama-Aldol Reaction: Rapid Access to α-Fluoro-β-hydroxy Esters

The Mukaiyama-aldol reaction is probably one of the most efficient strategies to prepare synthetically useful β-hydroxy carbonyl compounds. However, only several reported methods were concerned with the accesses to α-fluoro-β-hydroxy esters. Herein, we report a protocol for a fluoride anion-mediated Mukaiyama aldol reaction with low catalytic loading in a short reaction time to incorporate fluorine at the α position into β-hydroxy esters. The method shows good functional-group tolerance and scale-up potential, moreover, is applicable to the late-stage modification of natural products and small molecular drugs.

Catalytic stereoselective Mannich-type reactions for construction of fluorinated compounds

For its unique role in developing and designing new bioactive materials and healthcare products, fluoro-organic compounds have attracted remarkable interest. Along with ever-increasing demand for a wider availability of fluorine-containing structural units, a large diversity of methods has been introduced to incorporate fluorine atoms specially in a stereoselective fashion. Among them, catalytic Mannich reaction can proceed with a broad variety of reactants and open clear paths for the synthesis of versatile amine synthons in the synthesis of natural product and pharmaceutical molecules. This review provides an overview of the employment of catalytic asymmetric Mannich reactions in the synthesis of fluorine-containing amine compounds and highlights the conceivable distinct mechanisms.

Fluorinated Ylides/Carbenes and Related Intermediates from Phosphonium/Sulfonium Salts

Owing to the special effects of the fluorine element, including high electronegativity and small atomic radius, the incorporation of a fluorinated group into organic molecules may modify their physical, chemical, and biological properties. Fluorine-containing compounds have found widespread application in a variety of areas, and thus, the development of efficient reagents and methods for the incorporation of fluorinated groups has become a subject of significant interest.Described in this Account are our recent discoveries in the chemistry of fluorinated ylides/carbenes and related intermediates generated from phosphonium/sulfonium salts. Initially, we obtained the (triphenylphosphonio) difluoroacetate, Ph₃P⁺CF₂CO₂^- (PDFA), which was proposed as a reactive intermediate but had never been successfully synthesized. PDFA, shelf-stable and easy to prepare, is not only a mild ylide (Ph₃P⁺CF₂^-) reagent, but also an efficient difluorocarbene source. It can directly generate difluorocarbene, via the first generation of ylide Ph₃P⁺CF₂^-, simply under warming conditions without the need for any additive. Interestingly, difluorocarbene chemistry was then discovered by using PDFA as a reagent. Difluorocarbene can be oxidized to CF₂═O, can react with elemental sulfur to afford CF₂═S, and can be trapped by NaNH₂ or NH₃ to give CN^-. The development of these processes into synthetic tools allowed us to achieve various reactions, including the challenging ¹⁸F-trifluoromethylthiolation and cyanodifluoromethylation. It was found that a substituent on the cation of a phosphonium salt can be directly transferred as a nucleophile despite the cation's high electrophilicity. This transfer process is like an "umpolung" of the cation, which may provide more opportunities for the synthetic utilities of phosphonium salts. The investigation of this transfer process led us to find that iodophosphonium salts, active intermediates which can be easily generated, may efficiently promote deoxygenative functionalizations of aldehydes and alcohols. Dehydroxylative substitution of alcohols by this protocol permits the use of unprotected amines with higher pK_a values as nucleophiles, which is an attractive feature compared with the Mitsunobu reaction. On the basis of the ylide-to-carbene process (Ph₃P⁺CF₂^- → :CF₂), we further developed sulfonium salts as precursors of fluorinated ylides and fluorinated methyl carbenes. In particular, the studies on difluoromethylcarbene, remaining largely unexplored, may deserve more attention. The discoveries may find utility in the synthesis of biologically active fluorine-containing molecules.

The enantioselective addition of 1-fluoro-1-nitro(phenylsulfonyl)methane to isatin-derived ketimines

An asymmetric organocatalytic addition of fluorinated phenylsulfonylnitromethane to isatin-derived ketimines was developed. The reaction was efficiently catalyzed by a chiral tertiary amine, cinchonine. This methodology provides a new type of optically active compound with two adjacent quaternary carbon stereocenters in good yield (up to 96%), with moderate diastereoselectivity (up to 5.7 : 1 dr) and excellent enantioselectivity (up to 98/96% ee).