Understanding the reaction mechanism of the Lewis acid (MgBr2)-catalysed [3+2] cycloaddition reaction between C-methoxycarbonyl nitrone and 2-propen-1-ol: a DFT study

The mechanism of the non-catalysed and the MgBr2-catalysed [3+2] cycloaddition (32CA) reactions between C-methoxycarbonyl nitrone and 2-propen-1-ol has been theoretically investigated within the molecular electron density theory using DFT methods at the B3LYP/6-31G(d) computational level. Analysis of DFT reactivity indices allows explaining the role of the MgBr2 Lewis acid in the catalysed 32CA reaction. The 32CA reaction between C-methoxycarbonyl nitrone and 2-propen-1-ol takes place with a relative high activation enthalpy, 13.5 kcal mol−1, as a consequence of the non-polar character of this zw-type 32CA reaction. Coordination of the MgBr2 LA to C-methoxycarbonyl nitrone accelerates the corresponding zw-type 32CA reaction by taking place through a polar mechanism and with lower activation enthalpy, 8.5 kcal mol−1. Both 32CA reactions, which take place through a one-step mechanism, are completely meta regioselective and present low exo stereoselectivity, which increases in the catalysed process. Energy and non-covalent interaction analyses at the transition-state structures indicate that the formation of an intramolecular H–Br hydrogen bond in the catalysed process could be responsible for the exo selectivity experimentally observed.