Study of Ground State Interactions of Enantiopure Chiral Quaternary Ammonium Salts and Amides, Nitroalkanes, Nitroalkenes, Esters, Heterocycles, Ketones and Fluoroamides

Abstract

Chiral phase-transfer catalysis provides high level of enantiocontrol, however no experimental data showed the interaction of catalysts and substrates. ¹ H NMR titration was carried out on Cinchona and Maruoka ammonium bromides vs. nitro, carbonyl, heterocycles, and N-F containing compounds. It was found that neutral organic species and quaternary ammonium salts interacted via an ensemble of catalyst ⁺ N-C-H and (sp² )C-H, specific for each substrate studied. The correspondent BArF salts interacted with carbonyls via a diverse set of ⁺ N-C-H and (sp² )C-H compared to bromides. This data suggests that BArF ammonium salts may display a different enantioselectivity profile. Although not providing quantitative data for the affinity constants, the data reported proofs that chiral ammonium salts coordinate with substrates, prior to transition state, through specific C-H positions in their structures, providing a new rational to rationalize the origin of enantioselectivity in their catalyses.

Keywords: NMR spectroscopy; heterocycles; hydrogen bonds; organocatalysis; phase-transfer catalysis.