A molecular electron density theory study of the [3 + 2] cycloaddition reaction of nitrones with ketenes. Issue 7 (25th January 2017) - Ríos-Gutiérrez, Mar
Abstract : The zw -type 32CA reactions of nitrones with ketenes are controlled by the nucleophilic character of the nitrone and the electrophilic character of the ketene. They are chemo- and regio-selective and the use of electrophilic ketenes changes the mechanism from one-step to two-step. Abstract : The [3 + 2] cycloaddition (32CA) reaction between nitrones and ketenes has been studied within the Molecular Electron Density Theory (MEDT) at the Density Functional Theory (DFT) MPWB1K/6-311G(d, p) computational level. Analysis of the conceptual DFT reactivity indices allows the explanation of the reactivity, and the chemo- and regioselectivity experimentally observed. The particular mechanism of this 32CA reaction involving low electrophilic ketenes has been elucidated by using a bonding evolution theory (BET) study. It is determined that this reaction takes place in one kinetic step only but in a non-concerted manner since two stages are clearly identified. Indeed, the formation of the second C–O bond begins when the first O–C bond is already formed. This study has also been applied to predict the reactivity of nitrones with highly electrophilic ketenes. Interestingly, this study predicts a switch to a two-step mechanism due to the higher polar character of this zw -type 32CA reaction. In both cases, BET supports the non-concerted nature of the 32CA reactions between nitrones and ketenes.