Quantum-chemical study of rearrangement of vinyl alcohol into acetaldehyde. Intramolecular sigmatropic [1,3] shift of hydrogen

Abstract

The rearrangement mechanism of vinyl alcohol into acetaldehyde in the isolated molecule has been studied with respect to two possible different ways: the rearrangement in the plane of the molecule and in the plane perpendicular to the molecule. According to the Woodward-Hoffmann rules the former way is considered to be an allowed antarafacial [1,3] shift, and the latter way is a forbidden suprafacial shift. The reaction pathways have been studied by the method of reaction coordinate at the level of the semiempirical CNDO/2 and MINDO/3 methods by the technique of complete optimization. The reaction coordinates used for the study of reaction pathways in the plane of the molecule were the lengths of the bond being split between oxygen and hydrogen and that being formed between carbon and hydrogen and the angle formed by the three mentioned atoms (OHC). The reaction coordinates used for the perpendicular plane were the bond length between hydrogen and carbon (being formed) and those describing the way above the bonds. All the reaction pathways have to cross barriers, which means that the suprafacial shift is forbidden, and the antarafacial shift is geometrically difficult. Surprisingly the barrier of the suprafacial shift is lower than that of the antarafacial one, which has been explained by interaction of the migrating hydrogen atoms with bonds. This explanation is supported by the bond distance between the migrating hydrogen and central carbon atom in the transition state as well as by formation of hydrogen bond between oxygen, migrating hydrogen and double bond, which is indicated by the charge transfer connected with a considerable dipole moment. Existence of the relatively high barriers indicates that a spontaneous hydrogen shift in vinyl alcohol is impossible.