Insight into the acidic behavior of oxazolidin-2-one, its thione and selone analogs through computational techniques

Abstract

Deprotonation thermochemistry of Oxazolidin-2-one (OXA), Oxazolidine-2-thione (OXA-S), and Oxazolidine-2-selone (OXA-Se) has been studied in order to find the most acidic site and relative acidities of these heterocyclics at various sites. The deprotonation enthalpies at MP2/6-311++G**//MP2/6-31+G* and B3LYP/6-31+G* levels, while the free energies for deprotonation process and pKa values at B3LYP/6-31+G* level both in gas and aqueous phase (using PCM continuum model) of the anions of the three heterocyclics have been computed at 298 K. Calculated aqueous phase pKa values of OXA vary by ~6–7 units from the experimental aqueous phase pKa values of OXA and its derivatives. The deprotonation at the nitrogen is favored in OXA over the carbon atoms in contrast to the OXA-S and OXA-Se where in the deprotonation at the carbon attached to the nitrogen is most preferred. Deprotonation at this carbon induces an important C–O bond rupture in OXA-S and OXA-Se promoting an energetically favored ring-opening process. The finding offers a rare case when C–H acidity is able to dominate over the N–H acidity. In order to explain the relative stabilities, relative acidities and deprotonation enthalpies various characteristics of these molecules as well as their anions such as molecular electrostatic potential surface (MEP), frontier molecular orbital (FMO) features, chemical hardness, softness have been governed. The three dimensional MEP maps and HOMO–LUMO orbitals encompassing these molecules yield a reliable relative stability and reactivity (in terms of acidity) map displaying the most probable regions for deprotonation. The differential distribution of the electrostatic potential over the neutral and anionic species of OXA, OXA-S, and OXA-Se molecules is authentically reflected by HOMO–LUMO orbitals and NBO charge distribution analysis. The lone pair occupancies, second order delocalization energies for orbital interactions and the distribution of atomic charges over the entire molecular framework as obtained from natural bond orbital (NBO) analysis are found to faithfully replicate the predictions from the MEP maps and HOMO–LUMO band gaps in respect of explaining the relative stabilities and acidities in most of the cases. Good linear correlations have been obtained between HOMO–LUMO gap and pKa values in the aqueous phase for OXA and OXA-S molecules.