Intramolecular [3+2] Cycloaddition Reactions of Unsaturated Nitrile Oxides. A Study from the Perspective of Bond Evolution Theory (BET)

The reaction mechanism of the [3+2] intramolecular cycloaddition of 3,3-dimethyl-2-(prop-2-en-1-yloxy) and (prop-2-en-1-ylsulfanyl) nitrile oxides is analyzed using different DFT functionals with 6-311++G(d,p) basis set. The activation and the reaction energies for the cis and trans pathways are evaluated at the DFT, MP2 and CCSD(T) level of theory as well as their Gibbs free energy counterparts. It has shown that the trans regioisomers are both the thermodynamic and kinetic compounds, in agreement with experimental outcomes. For a deeper understanding of the reaction mechanism, a BET analysis along the reaction channel (trans and cis) has been carried out. This analysis reveals that the lone pair on the nitrogen atom is formed first, then the C-C bond and finally the O-C one. The global mechanism is similar for the two compounds and for the two pathways even if some small differences are observed, for instance in the values of the reaction coordinates of appearance of the different basins.