Enone-dienol tautomerism of but-2-enal and substituent effect: A theoretical study

The effect of substituents on the enone-dienol tautomerism of but-2-enal by [1,5]-H shift has been investigated systematically. All stationary point structures and energies of substituted but-2-enal are obtained at the levels of B3LYP/6-31G* and B3LYP/6-311++G(d,p). It is shown that the substituents in the 2-substituted series exert more significant effects on the relative energies of dienol with respect to enone and the energy barriers for enolization than those in the 3- or 4-substituted series. In general, the relative energies and energy barriers could be reduced by the electron-withdrawing substituents BH₂, CN, NO, since the structure of dienol and the corresponding transition state could be stabilized more than those of enone, but increased by the electron-donating substituents in the order NH₂<Cl<OH<F, and is little influenced by CH₃ due to its weak electron-donating ability. The perfluorinated dienol form of but-2-enal is also examined, it can be manifested that destabilization of the dienol and transition state by fluorine are responsible for the remarkable increase in the relative energy and the energy barrier. It is also shown by study that the enone-dienol tautomeric equilibrium of OC-CC-O are solvent-dependent to a certain extent.