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