Comparative studies on the enantioselective fluorination of oxindoles with structurally modified N-fluorobenzenesulfonimides

N-Fluoro Ammonium Salts of Cinchona Alkaloids in Enantioselective Electrophilic Fluorination

From 2000, our two research groups independently and simultaneously designed and developed a novel family of electrophilic fluorinating reagents based on the use of Cinchona alkaloids. The chiral N-fluoro ammonium salts demonstrated the highest efficiency compared to prior art in enantioselective electrophilic fluorination for a wide range of substrates. In this account, we tell our respective stories, how the same idea germinated in our laboratories, the characterization of the chiral reagents, the use in stoichiometric quantity then the development of a catalytic version, the application to the synthesis of chiral fluorinated molecules of pharmaceutical interest, and finally the exploitation of our reagents by other teams and for other applications.

Development of N-F fluorinating agents and their fluorinations: Historical perspective

This review deals with the historical development of all N-F fluorinating agents developed so far. The unique properties of fluorine make fluorinated organic compounds attractive in many research areas and therefore fluorinating agents are important. N-F agents have proven useful by virtue of their easy handling. This reagent class includes many types of N-F compounds: perfluoro-N-fluoropiperidine, N-fluoro-2-pyridone, N-fluoro-N-alkylarenesulfonamides, N-fluoropyridinium salts and derivatives, N-fluoroquinuclidium salts, N-fluoro-trifluoromethanesulfonimide, N-fluoro-sultams, N-fluoro-benzothiazole dioxides, N-fluoro-lactams, N-fluoro-o-benzenedisulfonimide, N-fluoro-benzenesulfonimide, 1-alkyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane salts, N-fluoropyridinium-2-sulfonate derivatives, 1-fluoro-4-hydroxy-1,4-diazoniabicyclo[2.2.2]octane salts, N-fluorodinitroimidazole, N-fluoro-trichloro-1,3,5-triazinium salt, N-F ethano-Tröger’s base derivatives, N-fluoro-methanesulfonimide, N-fluoro-N-arylarenesulfonamides, bisN-F salts such as N,N’-difluorobipyridinium salts and N,N’-difluoro-1,4-diazoniabicyclo[2.2.2]octane salts, and their many derivatives and analogs, including chiral N-F reagents such as optically active N-fluoro-sultam derivatives, N-fluoro-alkaloid derivatives, DABCO-based N-F derivatives, and N-F binaphthyldisulfonimides. The synthesis and reactions of these reagents are described chronologically and the review also discusses the relative fluorination power of each reagent and their mechanisms chronicling developments from a historical perspective.

Bench-Stable Electrophilic Fluorinating Reagents for Highly Selective Mono- and Difluorination of Silyl Enol Ethers

Efficient methods for the synthesis of fluorinated compounds have been intensively studied, recently. Development of practical fluorinating reagents is indispensable for this purpose. Herein, bench-stable electrophilic fluorinating reagents were synthesized as N-fluorobenzenesulfonimide (NFSI) substitutes. Reagents obtained by replacing one of the NFSI sulfonyl groups with an acyl group led to the highly selective monofluorination of silyl enol ethers with suppression of undesired overreaction, that is, difluorination. On the other hand, reagents bearing electron-withdrawing substituents at NFSI benzenesulfonyl groups efficiently facilitated the difluorination of silyl enol ethers under base-free conditions. Thus, both mono- and difluorinated target materials were prepared from the same substrate.

Reactivities of electrophilic N-F fluorinating reagents

Electrophilic fluorination represents one of the most direct and useful methods available for the selective introduction of fluorine into organic compounds. Electrophilic fluorinating reagents of the N-F class have revolutionised the incorporation of fluorine atoms into both pharmaceutically- and agrochemically-important substrates. Since the earliest N-F reagents were commercialised in the 1990s, their reactivities have been investigated using qualitative and, more recently, quantitative methods. This review discusses the different experimental approaches employed to determine reactivities of N-F reagents, focussing on the kinetics studies reported in recent years. We make critical evaluations of the experimental approaches against each other, theoretical approaches, and their applicability towards practical problems. The opportunities for achieving more efficient synthetic electrophilic fluorination processes through kinetic understanding are highlighted.

A quantitative reactivity scale for electrophilic fluorinating reagents

Through kinetic studies we provide a quantitative reactivity scale for ten electrophilic fluorination reagents.

Electrophilic N–F fluorination agents underpin the introduction of fluorine in aliphatic systems across drug and academic research. The choice of N–F reagent is currently determined through empirical experimentation in the absence of quantitative values for electrophilicities. Here we report an experimentally-determined kinetic reactivity scale for ten N–F fluorinating reagents, including Selectfluor™, NFSI, Synfluor™ and several N-fluoropyridinium salts, in CH₃CN. The reactivity scale, which covers eight orders of magnitude, employs para-substituted 1,3-diaryl-1,3-dicarbonyl derivatives to measure relative and absolute rate constants. The para-substituted 1,3-diaryl-1,3-dicarbonyl scaffold delivers a convenient, sensitive spectrophotometric reporter of reactivity that also led to the discovery of a unique form of tautomeric polymorphism.

Modern Approaches for Asymmetric Construction of Carbon-Fluorine Quaternary Stereogenic Centers: Synthetic Challenges and Pharmaceutical Needs

New methods for preparation of tailor-made fluorine-containing compounds are in extremely high demand in nearly every sector of chemical industry. The asymmetric construction of quaternary C-F stereogenic centers is the most synthetically challenging and, consequently, the least developed area of research. As a reflection of this apparent methodological deficit, pharmaceutical drugs featuring C-F stereogenic centers constitute less than 1% of all fluorine-containing medicines currently on the market or in clinical development. Here we provide a comprehensive review of current research activity in this area, including such general directions as asymmetric electrophilic fluorination via organocatalytic and transition-metal catalyzed reactions, asymmetric elaboration of fluorine-containing substrates via alkylations, Mannich, Michael, and aldol additions, cross-coupling reactions, and biocatalytic approaches.

Catalytic asymmetric synthesis of hetero-substituted oxindoles

The construction of chiral disubstituted oxindoles is an intriguing challenge for organic chemists.

Transition-Metal-Free Fluoroarylation of Diazoacetamides: A Complementary Approach to 3-Fluorooxindoles

An efficient transition-metal-free fluoroarylation reaction of N-aryl diazoacetamides with NFSI (N-fluorobenzenesulfonimide) is described. This reaction directly provides 3-fluorooxindole derivatives in yields of 67-93% with high selectivity via a carbene-free process under mild reaction conditions.

Synthesis of 3-fluoroalkenyl-3-trifluoromethyl-2-oxindoles by the reaction of indoline-2,3-diones with difluoromethylene phosphabetaine

An unprecedent reaction of indoline-2,3-diones and (triphenylphosphonio)difluoroacetate (PDFA) afforded novel 3-fluoroalkenyl-3-trifluoromethyl-2-oxindoles in moderate to excellent yields. These products could be transformed into other trifluoromethylated oxindole derivatives.

An in situ directing group strategy for chiral anion phase-transfer fluorination of allylic alcohols

An enantioselective fluorination of allylic alcohols under chiral anion phase-transfer conditions is reported. The in situ generation of a directing group proved crucial for achieving effective enantiocontrol. In the presence of such a directing group, a range of acyclic substrates underwent fluorination to afford highly enantioenriched α-fluoro homoallylic alcohols. Mechanistic studies suggest that this transformation proceeds through a concerted enantiodetermining transition state involving both C-F bond formation and C-H bond cleavage.