Ab initio and DFT studies on the mechanism of ring-opening reactions of 4H-1-benzopyran-4-one with hydroxide ion

Abstract

The mechanism of the first steps of the degradation of 4H-1-benzopyran-4-one in alkaline media, i.e., nucleophilic addition of hydroxide ion at the C2 carbon of 4H-1-benzopyran-4-one followed by ring-opening, is investigated by ab initio [HF/6-31+G(d) and MP2/6-31+G(d)] and density functional theory [B3LYP/6-31+G(d)] methods. In addition, bulk solvent effects are estimated by the polarised continuum (overlapping spheres) model (PCM) and the polarizable conductor PCM model (CosmoPCM). Depending on the level of theory used three reaction coordinates are found: path A (1→TS1→2→TS2A→3A), path B (1→TS1→2→TS2B→3B) and path C (1→TS1→2→TS2A→3C). The ring-opening step passing through TS2B has a lower activation energy than the step proceeding through TS2A. These second barriers are higher compared with the barrier of the first step. Therefore fission of the γ-pyrone ring (2→TS2B→3B) seems to be the rate-determining step in the gas phase. Contrary to these gas phase results, addition of hydroxide ion to C2 (reactants→TS1→2) is the rate-determining step for the reaction in aqueous solution, in agreement with experimental results.