N-Halogeno compounds. Part 15. Synthesis of N-fluoroquinuclidinium salts via direct fluorination of quinuclidine-Lewis acid adducts, and a comparison of their "F+" transfer capabilities

Article abstract of DOI:10.1016/0022-1139(95)03356-4

Fluorine smoothly attacks quinuclidine-trifluoroborane, quinuclidine-pentafluorophosphorane, and quinuclidine-sulfur trioxide in acetonitrile at -35 °C to give the corresponding N-fluoroquinuclidinium salts NFQ⁺X^- (X^- =BF₄^-, PF₆^-, and FSO₃^- respectively; Q = quinuclidine). Like its tetrafluoroborate analogue (NFQ⁺BF₄^-), the hexafluorophosphate NFQ⁺PF₆^- can also be prepared by direct fluorination of quinuclidine in the presence of the appropriate sodium salt (NaPF₆). An alternative route to the tetrafluoroborate involves treatment of NFQ⁺F^- with boron trifluoride. A comparative study of site-specific electrophilic fluorination of methoxybenzene [ → 1-fluoro2-and 4-methoxybenzene], 2-hydroxynaphthalene ( → 1-fluoro-2-hydroxynaphthalene and 1,1-difluoro-2-oxo-1,2-dihydronaphthalene), 2-nitropropan-2-yl-lithium ( → 2-fluoro-2-nitropropane) and diethyl sodio (phenyl) malonate [ → diethyl fluoro(phenyl) malonate] with all of the NFQ⁺X^- salts mentioned above, plus the triflate (X^- = CF₃SO₃^-), revealed that the hexafluorophosphate and triflate are the most easily-handled and effective reagents.