All three N-methylated and N-protonated hydroxypyridinium BArF4– salt isomers were synthesized and their hydrogen bond donating abilities were investigated. DFT and G4 theory computations along with IR spectroscopic measurements were found to be effective methods for predicting the catalytic activities of these O–H and N–H Brønsted acids. A UV-vis titration approach for rapidly quantifying hydrogen bond donating ability revealed that carbon-hydrogen bonds also can participate in electrostatic interactions, but the presence of multiple equilibrium complexes results in a limitation of this method. In the methylated series of hydroxypyridines, the ortho and para isomers displayed modest rate enhancements relative to the meta derivative. Protonation introduces a new acidic site and the ortho hydroxypyridinium ion salt is a significantly more active catalyst than all of the other species examined. This is indicative of bidentate activation by the N–H and O–H acidic sites, and suggests a new design strategy for improving charge-enhanced catalysts.
Bibliographical noteFunding Information:
We would like to thank Ms. Grace Gast for her preliminary investigations on this project, Mr. Serdar Yalvac for his contributions toward developing the UV‐vis titration procedure, and Prof. Phillipe Buhlmann for helpful discussions and his guidance regarding titrations that result in multiple binding equilibria. Generous support from the National Science Foundation (CHE‐1665392) and the Minnesota Supercomputer Institute for Advanced Computational Research are also gratefully acknowledged.
© 2020 John Wiley & Sons, Ltd.
- Brønsted acid catalysis
- IR and UV-vis spectroscopy
- charge-activated acids