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

• Published in: Theoretical chemistry accounts 2017, Vol.136 (1), p.1-12, Article 5
• Main Authors: Adjieufack, A. I., Ndassa, I. M., Mbadcam, J. Ketcha, Ríos-Gutiérrez, M., Domingo, L. R.
• Format: Article
• Language: English
• Subjects: Atomic/Molecular Structure and Spectra; Chemistry; Chemistry and Materials Science; Cycloaddition; Density functional theory; Electron density; Enthalpy; Hydrogen bonds; Inorganic Chemistry; Lewis acid; Organic Chemistry; Physical Chemistry; Reaction mechanisms; Regular Article; Selectivity; Stereoselectivity; Theoretical and Computational Chemistry
• ISSN: 1432-881X; 1432-2234
• DOI: 10.1007/s00214-016-2028-0

The mechanism of the non-catalysed and the MgBr 2 -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 MgBr 2 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 MgBr 2 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.