Effects of intramolecular hydrogen bond and electron delocalization on the basicity of proton sponges and superbases with benzene, pyridine, pyrazine and pyrimidine scaffolds

Abstract

There are several classes of organic superbases whose basicity increases because of the formation of intramolecular hydrogen bond or/and of enhancement of electron delocalization in their protonated structures. In this work, effects of these two factors on the basicity of π-π and n-π conjugated nitrogen bases (mainly imines) are studied theoretically using the B3LYP/6-311++G(d,p) method. Comparison of proton affinities (PA) and gas phase basicities (GB) of investigated bases shows that change of electron delocalization after protonation in one ring enhances PA even by 30–40 kcal mol−1. Comparison of PAs of some proton sponges and “proton sponge-like” structures reveals that formation of a single hydrogen bond enhances PA by about 10 kcal mol−1. During this work, a large number of new superbases were designed and their basicities were assessed in the gas phase (with PA between 270 and 295 kcal mol−1) and acetonitrile (with pKa between 30 and 40).