Fast, highly enantioselective, and sustainable fluorination of 4-substituted pyrazolones catalyzed by amide-based phase-transfer catalysts

We developed the amide-based Cinchona alkaloids-catalyzed enantioselective fluorination of 4-substituted pyrazolones; the products with tetrasubstituted fluorine-attached chiral center were afforded in high yields (up to 99%) with excellent enantioselectivities (up to 98% ee). Our method is highlighted by low catalyst loading (0.05-0.5 mol%), short reaction times (30 minutes) and broad substrate scopes (35 examples). Furthermore, we carried out this asymmetric fluorination under sustainable conditions in 10 cycles, and the optically pure product (>99.5% ee) could be easily obtained by simple extraction and recrystallization on a multigram scale. Controlled experiments and DFT studies showed that the multiple hydrogen bonding interactions and the steric hindrance at the ortho-position in the benzene ring of the catalyst were the main reasons for the high enantioselectivity and reactivity. With a simple operational protocol and low catalyst loading, this transformation provides a practical method for the preparation of chiral fluorine-containing compounds.