Quaternary Ammonium Salts Interact with Enolates and Sulfonates via Formation of Multiple N+-C-H Hydrogen Bonding Interactions

We report herein sharp physical evidence, i.e., single-crystal X-ray diffraction and H-1-NMR spectral data, confirming that quaternary ammonium species interact with anions via a set of directional ion-dipole cooperative N+-C-H unusual H-bonding interactions and not via pure non-directional ionic electrostatic interactions. This finding, which has been often invoked by calculations, is herein substantiated by the preparation of two model compounds and an analysis of their X-ray crystal structures in the solid state and H-1-NMR spectra in solution. These observations are particularly pertinent for the rational design of novel catalyses and catalysts and providing guidance to an understanding of these species in solution and during asymmetric enantioselective catalysis.