Cation-pi interactions in iminium ion activation: correlating quadrupole moment & enantioselectivity

A linear correlation between quadrupole moment (Q(zz)) and enantioselectivity (es) advocates the notion that a cation-pi interaction is a contributing factor in the addition of uncharged nucleophiles to iminium salts derived from MacMillan's 1st generation catalyst. The quadrupole moment of the aryl shielding group is a useful qualitative parameter for predicting selectivity (Q(zz) < 0 -> high es).