A mechanistic MEDT study of the competitive catalysed [4+2] and [2+2] cycloaddition reactions between 1-methyl-1-phenylallene and methyl acrylate: the role of Lewis acid on the mechanism and selectivity

The selectivity and the nature of the mechanism of the competitive Lewis acid catalysed [4+2]/[2+2] cycloaddition reactions of 1-methyl-1-phenylallene (MPA) with methylacrylate (MA) have been theoretically studied within the Molecular Electron Density Theory using DFT methods at the B3LYP/6-31G(d) theoretical level. DFT reactivity indices indicate that MPA is a strong nucleophile and the LA-MA complex is a strong electrophile. The coordination of LA to MA enhances the reaction rate and increases the asynchronicity of the [4+2] CA reaction, changes the nature of the mechanism from one step to stepwise for the [2+2] CA reaction and increases the polar character of these cycloaddition reactions, which become demands a relatively low activation energy. Analysis of different energy profiles indicates that these competitive LA-catalysed CA reactions favour the formation of a mixture of meta regioisomers in both types of cycloaddition, in which the [4+2] cycloadducts were obtained in majority amount, in agreement with the experiment. Analysis based on Electron Localisation Function topological shows that the favoured [4+2] CA reaction takes place through a non-concerted two-stage one-step mechanism.