Exploiting propane-1,3-diimines as building blocks for superbases: a DFT study

Abstract

The DFT calculations are employed to construct propane-1,3-diimine as an essential building block to design organic superbases in the gas and acetonitrile phases. This molecular framework is versatile in terms of anchoring different functional groups to achieve higher basicities. Some of these organic superbases showed very high pK_a values of ∼40. The influence of iminophosphorane substituents on the propane-1,3-diimine scaffold 4 showed a remarkable enhancement in the basicity, which leads to an increase in the pK_a by ∼20 units. The same molecular scaffold also leads to the design of a chiral organic superbase, which keeps a close resemblance to the known ligands for asymmetric synthesis. This newly designed chiral superbase exhibits the highest proton affinity and pK_a compared to other reported chiral superbases. The topographical analysis of the molecular electrostatic potential (MESP) has been carried out for some selected compounds at the B3LYP/6-311+G** level of theory. The MESP analyses and steric strain calculations revealed the importance of intrinsic basicity of nitrogen centers and the cation strain effect. To the best of our knowledge, this is the first time that the combination of MESP and strain calculations is employed to segregate the intrinsic basicity of reactive centers and the structural factors responsible for determining the basicities of such organic superbases. The cation resonance effect also plays an important role in enhancing the basicities of the systems studied here. Intramolecular and intermolecular proton transfer barriers have been calculated to examine the kinetic activity of these diimines.