A density functional theory study on the [3 + 2] cycloaddition of N-(p-methylphenacyl)benzothiazolium ylide and 1-nitro-2-(p-methoxyphenyl) ethene: the formation of two diastereomeric adducts via two different mechanisms

Abstract

The [3 + 2] cycloaddition reaction of N-(p-methylphenacyl)benzothiazolium ylide (NBY) and 1-nitro-2-(p-methoxyphenyl) ethane (NME), experimentally investigated by Yan et al., was studied theoretically. They reported that the reaction proceeds via a stepwise mechanism and produces two diastereomeric adducts in a 4:1 ratio. For the study of the reaction, two regioselective attacks were considered between the reagents and their theoretical parameters were calculated. In excellent agreement with the experimental outcomes, the Fukui and Parr functions reactivity indices analysis as well as the energetic results indicated that among the two studied regioselective attacks, one which leads to the formation of two experimentally reported adducts, is more favorable than the other. The molecular mechanism of the studied reactions was characterized using the IRC, QTAIM and Wiberg bond indices analyses and the results suggested that two diastereomeric adducts are generated via two different mechanisms. The major adduct is produced via a two-stage one-step mechanism without the formation of any stable intermediate, whereas the minor one is generated through a stepwise mechanism along with the formation of a stable zwitterionic intermediate. The analysis of global electron density transfer showed that the reactions are polar and electron density fluxes from NBY toward NME. It was found from molecular electrostatic potential map that at the more favorable transition state, approach of reactants locates the oppositely charged regions over each other resulting in attractive forces between the two fragments. Analysis of the frontier molecular orbitals indicated that the HOMO orbital of NBY is also a frontier effective-for-reaction molecular orbital.