A bonding evolution theory study of the mechanism of [3+2] cycloaddition reactions of nitrones with electron-deficient ethylenes†
The mechanism of zw-type [3+2] cycloaddition (32CA) reactions of nitrones with electron-deficient (ED) ethylenes has been studied using density functional theory (DFT) methods at the MPWB1K/6-31G(d) level of theory. An exploration of the potential energy surfaces associated with the four competitive reactive channels of the 32CA reaction between C-phenyl-N-methyl nitrone and acrolein indicates that the cycloaddition reaction takes place through a one-step mechanism. This cycloaddition reaction presents a moderate meta regioselectivity and a complete endo stereoselectivity, which is diminished in dichloromethane. Analysis of the DFT reactivity indices of the reagents allows an explanation of the participation of nucleophilic nitrones in zw-type 32CA reactions towards ED ethylenes. A bonding evolution theory (BET) study of the two endo regioisomeric reactive channels allows establishing the molecular mechanism of these relevant 32CA reactions. Both regioisomeric channels topologically take place along eight differentiated phases. While the formation of the C–C single bond follows Domingo's recently proposed model, the formation of the O–C single bond takes place at the short distance of 1.6 Å through the donation of some electron density of the oxygen lone pairs of the nitrone to the β-conjugated carbon atom of acrolein. BET supports the non-concerted nature of these zw-type 32CA reactions and makes it possible to reject the pericyclic mechanism proposed for them.