2-Alkoxy- and 2-alkylthio-2-alkenals in the reactions of electrophilic and nucleophilic addition. DFT study and NBO analysis

A DFT approach at B3LYP/6-311+G∗∗ and M06/6-311+G∗∗ levels of theory, and natural bond orbital (NBO) analysis were used to investigate the electron distribution in 3-aryl(hetaryl)substituted 2-methoxy- and 2-methylthiopropenals to predict the possibility of electrophilic (Markovnikov-type) or nucleophilic (Michael-type) addition reactions depending on the nature of substituents. The dependence of the Cdouble bond; length as m-dashC bond length on the population ratio of its orbitals, Pimage/P(πC2double bond; length as m-dashC3), was used to evaluate the donor and acceptor effects of these substituents. The changes in energy values of the frontier orbitals showed that both the electrophilic and nucleophilic abilities of the molecules studied depended on their structure. The change of the polarization direction of the double bond (calculated as the difference of the natural charges formed on its atoms) under the effect of Cβ-substituent allows one to forecast the direction of the polar addition reactions for some representatives of 2-alkoxy- and 2-alkythiopropenals. The slightly different activation energies of hydration of 3-aryl(furyl)substituted 2-methoxypropenals for Markovnikov and Michael additions in acidic medium indicate to equal probability of these processes.