A Theoretical Evaluation of the Michael-Acceptor Ability of Conjugated Nitroalkenes

Abstract

A theoretical evaluation of the relative Michael-acceptor abilities of a variety of substituted aromatic and aliphatic nitroalkenes is reported. Several global and local reactivity indices were evaluated with the incorporation of natural charge obtained from natural bond orbital (NBO) analysis. Natural charges at the carbon atom β to the NO2 group and the condensed Fukui functions derived by this method were quite consistent with the reactivity. The reactivity of nitroalkenes was further examined by kinetic experiments that were monitored by NMR spectroscopy. The isolated yields reported in the literature were also found to have a dependable correlation with these reactivity indices in the majority of cases. The computational methods employed in the calculation of natural charge, the condensed Fukui function, local softness and other reactivity parameters were chosen after evaluation of various methods for their ability to satisfactorily predict the NMR chemical shift of the 1H atom β and cis to the NO2 group (β-H) in nitroalkenes. Our approach appears suitable for predicting the relative Michael-acceptor abilities of various nitroalkenes, based on the β-carbon natural charge and the indices derived from it.