A molecular electron density theory study to understand the interplay of theory and experiment in nitrone-enone cycloaddition

Abstract

[3 + 2] cycloaddition (32CA) reaction of C,N-diaryl nitrone with benzylidene acetone has been studied to analyse the mechanism, selectivity and polar character of this nitrone-enone cycloaddition. Topological analysis of the electron localization function (ELF) shows the absence of pseudoradical and carbenoid centre in the nitrone, which allows its classification as a zwitter-ionic (zw) type three atom component (TAC) and hence participation in zw- type cycloadditions is associated with high activation energy barriers. This 32CA reaction follows a one-step mechanism with asynchronous TSs. Endo/meta product is obtained as the major cycloadduct experimentally, which can be rationalized from its calculated lowest activation energy among the four possible reaction pathways. Global electron density transfer (GEDT) at the TSs predict the non-polar character of this 32CA reaction. Topological analysis of the ELF and QTAIM parameters was performed at the TSs. Finally, non-covalent interaction (NCI) gradient isosurfaces are computed to obtain a visualization of non-covalent interactions at the interatomic bonding regions. The experimental and theoretical aspects of [3+2] cycloaddition reactions of C,N-diaryl nitrone with benzylidene acetone is described. The reaction is meta/endo selective and follows one step mechanism with non-covalent interactions. The C-C and C-O bonds are generated through coupling of pseudoradical centers.