Quantum-Chemical Study of Keto–Enol Equilibrium and Global Electrophilicity of Hydroxymaleimide Derivatives

The energies of enol and keto forms for 36 3-hydroxymaleimide derivatives have been calculated by the DFT and DLPNO methods. The results clearly show that, with only a few exceptions, the enol form is energetically more favorable by 16–60 kJ mol^–1, and the energy difference depends on the substituent in the 4-position. Global electrophilic index has been calculated for all the compounds, which demonstrates that the keto form is generally more electrophilic, and electrophilicity depends on the substituent in the 4-position. Two possible structures of hydroxymaleimide anion have been evaluated; the deprotonation of the oxygen atom turns out to be the most energetically favorable.