A computational study on the reaction mechanisms for the tautomeric equilibria of H-phosphonates

H-phosphonates are important building blocks in organic synthesis and serve as essential ligands for the extraction of actinides. Previous computational studies on the tautomerism of certain R₂P(O)H derivatives revealed that the interconversion between two tautomers becomes significant in the presence of catalysts at room temperature. However, a systematic and comprehensive study aimed at obtaining mechanistic insights into the reactivity of H-phosphonates remains lacking. In this study, the uncatalyzed and catalyzed reaction mechanisms for the tautomerization of dialkyl H-phosphonates are thoroughly investigated. Eleven probable tautomeric pathways are proposed and analyzed using density functional theory (DFT) for compounds such as phosphonic acid (Pn), dimethyl H-phosphonate (DMHP), dibutyl H-phosphonate (DBHP), di-sec-butyl H-phosphonate (DsBHP) and di-tert-butyl H-phosphonate (DtBHP). The analysis includes examining the electronic structures of the reactant phosphonates, product phosphites and transition states. The reaction free energy barrier (activation barrier) is evaluated, and the overall variation in the barrier height with increasing alkyl chain length is noted. Our calculations reveal that the most favorable pathway is catalyzed by three water molecules for DMHP, DBHP and DtBHP, and by four water molecules for Pn and DsBHP. Natural Bond Orbital (NBO) analysis demonstrates how the electron density of the PH, PO and OH bonds evolves from the reactant to the transition state (TS) and then to the product during the reaction. Additionally, Wiberg bond index analysis suggests that all reaction pathways are concerted and slightly asynchronous.