Enantioselective fluorination mediated by cinchona alkaloid derivatives/Selectfluor combinations: Reaction scope and structural information for N-fluorocinchona alkaloids

Cinchona-alkaloid/Selectfluor combinations efficiently fluorinate a variety of carbonyl compounds in a highly enantioselective manner to furnish chiral α-fluorocarbonyl compounds. The DHQB/Selectfluor combination is effective for the enantioselective fluorination of indanones and tetralones 1 in up to 91% ee. The first enantioselective syntheses of chiral derivatizing reagents 3 was accomplished with high ee and in high chemical yields by the DHQDA/Selectfluor combination. 3-Fluorooxindoles 7 were prepared with ee up to 83% using the (DHQ)₂AQN/Selectfluor or the (DHQD)₂PYR/Selectfluor combination. Since the combinations are conveniently prepared in situ from readily available reagents, the present system represents a practical method for enantioselective fluorination. X-ray crystallography and ¹H NMR analyses of the cinchona alkaloids/Selectfluor combination have established that the species that mediate this novel reaction are N-fluoroammonium cinchona alkaloid tetrafluoroborates, which adopt open conformations.