NMR structure determination of ion pairs derived from quinine:: A model for templating in asymmetric phase-transfer reductions by BH4- with implications for rational design of phase-transfer catalysts

The solution structures of ion pairs formed by quarternary ammonium ions derived from quinine alkaloid with small hard anions (BH4- or Cl-) in CDCl3 have been characterized by nuclear magnetic resonance methods. Structural observations have been correlated with the sense of asymmetric induction observed in the phase-transfer reduction of 9-anthryl trifluoromethyl ketone by borohydride (BH4-) when catalyzed by the quaternary N-benzylquinine ammonium ion. From interionic nuclear Overhauser effects (NOEs), it appears that the BH4- ion occupies two of the four trigonal pyramidal sites formed by substituents of the quarternary nitrogen of the catalyst cation. One of these sites is in close proximity to the cation's hydroxyl group that is strictly required for asymmetric induction in the model reaction, while the other site is near the vinyl group on the cation. The vinyl group does not appear to be important for determining the sense or extent of asymmetric induction. Using energy-minimized structures derived from NMR data, it was predicted that the N-(9-methyleneanthryl)quinine-quarternary ammonium catalyst would give improved asymmetric induction in the model reaction due to a preferred anion occupancy at the site near the hydroxyl group. An improvement in enantiomeric excess tee) is observed using the anthryl-modified catalyst, and NMR studies on the modified catalyst confirm the predicted change in anion binding site occupancies. The changes in site occupancies determined by NMR can be fitted to a simple kinetic model that correctly predicts the extent of change in ee.