Catalytic asymmetric fluorinations

The appearance of structurally diverse fluorinating reagents displaying a large spectrum of reactivity has been critical to the development of the catalytic asymmetric fluorination processes known to date. In this article, we discuss how this area of research emerged and which strategies have allowed for the successful development of both nucleophilic and electrophilic catalytic enantioselective fluorinations. We also present the fundamental understanding of catalytic activity and enantioselectivity for the most efficient processes and highlight the first synthetic application with the preparation of a complex fluorinated target.

Carbon−Fluorine Bond Activation Coupled with Carbon−Hydrogen Bond Formation α to Iridium: Kinetics, Mechanism, and Diastereoselectivity

Reactions of iridium(fluoroalkyl)hydride complexes CpIr(PMe(3))(CF(2)R(F))Y (R(F) = F, CF(3); Y = H, D) with LutHX (Lut = 2,6-dimethylpyridine; X = Cl, I) results in C-F activation coupled with hydride migration to give CpIr(PMe(3))(CYFR(F))X as variable mixtures of diastereomers. Solution conformations and relative diastereomer configurations of the products have been determined by (19)F{(1)H}HOESY NMR to be (S(C), S(Ir))(R(C), R(Ir)) for the kinetic diastereomer and (R(C), S(Ir))(S(C), R(Ir)) for its thermodynamic counterpart. Isotope labeling experiments using LutDCl/CpIr(PMe(3))(CF(2)R(F))H and CpIr(PMe(3))(CF(2)R(F))D/LutHCl) showed that, unlike a previously studied system, H/D exchange is faster than protonation of the alpha-CF bond, giving an identical mixture of product isotopologues from both reaction mixtures. The kinetic rate law shows a first-order dependence on the concentration of iridium substrate, but a half-order dependence on that of LutHCl; this is interpreted to mean that LutHCl dissociates to give HCl as the active protic source for C-F bond activation. Detailed kinetic studies are reported, which demonstrate that lack of complete diastereoselectivity is not a function of the C-F bond activation/H migration steps but that a cationic intermediate plays a double role in loss of diastereoselectivity; the intermediate can undergo epimerization at iridium before being trapped by halide and can also catalyze the epimerization of kinetic diastereomer product to thermodynamic product. A detailed mechanism is proposed and simulations performed to fit the kinetic data.

Carbon−Fluorine Bond Activation Coupled with Carbon−Carbon Bond Formation at Iridium. Confirmation of Complete Kinetic Diastereoselectivity at the New Carbon Stereocenter by Intramolecular Trapping Using Vinyl as the Migrating Group

The iridium(perfluoropropyl)(vinyl) complex CpIr(PMe(3))(n-C(3)F(7))(CH=CH(2)) (5) has been prepared. It has been characterized by X-ray crystallography, and its ground state conformation in solution has been determined by (19)F{(1)H} HOESY NMR studies. It reacts with the weak acid lutidinium iodide to afford the eta(1)-allylic complex CpIr(PMe(3))((Z)-CH(2)CH=CFC(2)F(5))I (6), which has also been characterized crystallographically. The mechanism of C-F bond activation and C-C bond formation leading to 6 has been elucidated in detail by studying the reaction of 5 with lutidinium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate [LutH(+)B(ArF)(4)(-)], containing a weakly coordinating counteranion. The main kinetic product of this reaction, determined by (19)F{(1)H} HOESY studies at -50 degrees C, is the endo-CpIr(PMe(3))(anti-eta(3)-CH(2)CHCFCF(2)CF(3))[B(ArF)(4)] diastereomer 9, along with a small amount of the exo-syn-isomer 8. Isomer 9 rearranges at -20 degrees C to its exo-anti isomer 7, and subsequently to the thermodynamically favored exo-syn-isomer 8, which has been isolated and crystallographically characterized. Complex 8 reacts with iodide to afford complex6. On the basis of the unambiguously defined kinetically controlled stereochemistry of 9 and 8, a detailed mechanism for the C-F activation/C-C coupling reaction is proposed, the principal conclusion of which is that C-F activation is completely diastereoselective.

Quinine/selectfluor combination induced asymmetric semipinacol rearrangement of allylic alcohols: an effective and enantioselective approach to α-quaternary β-fluoro aldehydes

A quinine/Selectfluor combination inducing rearrangement reaction of allylic alcohols was discovered, which involved a moderate yield with good enantioselective construction of alpha-quaternary carbon center and beta-fluoro aldehyde under base condition.

Selectfluor: Mechanistic Insight and Applications

The replacement of hydrogen atoms with fluorine substituents in organic substrates is of great interest in synthetic chemistry because of the strong electronegativity of fluorine and relatively small steric footprint of fluorine atoms. Many sources of nucleophilic fluorine are available for the derivatization of organic molecules under acidic, basic, and neutral conditions. However, electrophilic fluorination has historically required molecular fluorine, whose notorious toxicity and explosive tendencies limit its application in research. The necessity for an electrophilic fluorination reagent that is safe, stable, highly reactive, and amenable to industrial production as an alternative to very hazardous molecular fluorine was the inspiration for the discovery of selectfluor. This reagent is not only one of the most reactive electrophilic fluorinating reagents available, but it is also safe, nontoxic, and easy to handle. In this Review we document the many applications of selectfluor and discuss possible mechanistic pathways for its reaction.

Recent highlights in electrophilic fluorination with 1-chloromethyl-4-fluoro- 1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate)

Synthetic and structural aspects of organofluorine compounds continue to be the focal points of vigorous research activities, as evidenced by the appearance of a large number of publications. Among the various useful methodologies for the introduction of fluorine into organic molecules, electrophilic fluorination is a promising and exciting area of research. While a variety of electrophilic fluorinating reagents are available, currently 1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (Selectfluor) provides a remarkably straightforward and effective route. The breadth of applications realizable from this reagent in its role as a key electrophilic fluorinating reagent is highlighted here. This Account covers the literature for electrophilic fluorination reactions that employ Selectfluor during the period January 1999-January 2003.

Mild Electrophilic Trifluoromethylation of β-Ketoesters and Silyl Enol Ethers with 5-Trifluoro Methyldibenzothiophenium Tetrafluoroborate

Cyclic and acyclic beta-ketoesters were efficiently trifluoromethylated with 5-trifluoromethyldibenzothiophenium tetrafluoroborate in the presence of a phase-transfer catalyst to afford the corresponding alpha-substituted alpha-trifluoromethyl beta-ketoesters in good to excellent yields. In a second approach, 5-trifluoromethyldibenzothiophenium tetrafluoroborate and tetrabutylammonium difluorotriphenylstannate were used for efficient electrophilic trifluoromethylation of various silyl enol ethers leading to the corresponding alpha-trifluoromethyl ketones in good to high yields.

Enantioselective synthesis of BMS-204352 (MaxiPost) using N-fluoroammonium salts of cinchona alkaloids (F-CA-BF4)

The enantioselective synthesis of a potent Maxi-K potassium channel opener (BMS-204352) mediated by N-fluoroammonium salts of cinchona alkaloids is described. Two synthetic pathways were evaluated. An ee as high as 88% was achieved (>99% after a single recrystallisation).

Titanium-catalyzed stereoselective geminal heterodihalogenation of beta-ketoesters

[reaction: see text] beta-Ketoesters can be effectively monofluorinated with F-TEDA using CpTiCl(3) as a catalyst. With the use of this catalyst, the extent of the competing difluorination does not reach 10%. [TiCl(2)(TADDOLato)] complexes catalyze the one-pot enantioselective heterodihalogenation of beta-ketoesters with F-TEDA and NCS to afford alpha-chloro-alpha-fluoro-beta-ketoesters in moderate to good yields. The sequence of addition of the halogenating agents determines the sense of chiral induction.

Enantioselective fluorination mediated by N-fluoroammonium salts of cinchona alkaloids: first enantioselective synthesis of BMS-204352 (MaxiPost)

We have employed a cinchona alkaloid/Selectfluor-mediated enantioselective fluorination of the oxindole 2 to achieve the first enantioslective synthesis of BMS-204352 (MaxiPost, S-1), an effective opener of maxi-K channels. Fluorination occurred to produce S-1 with 84% ee using the bis-cinchona alkaloid (DHQ)(2)AQN. Recrystallization produced enantiomerically pure (>99% ee) product. Quinidine-mediated fluorination of 2 gave the (R)-antipode of 1 with 68% ee.

An efficient enantioselective fluorination of various beta-ketoesters catalyzed by chiral palladium complexes

Reflecting the importance of fluorinated organic compounds in medicinal chemistry, development of an efficient method for catalytic enantioselective fluorination is increasingly desirable. Using a novel palladium complex 2 (1-2.5 mol %), various beta-ketoesters including cyclic and acyclic substrates were fluorinated with excellent enantioselectivity in the range of 83-94% ee. It is environmentally advantageous that this reaction proceeds well in solvents such as EtOH, rather than usual organic solvents. Furthermore, the product was successfully tranformed into both alpha-fluoro beta-hydroxy and beta-amino acid derivatives, which should be extremely useful in developing novel drugs.