How Different Are Nitrile Oxides from Nitrones in zw-type [3 + 2] Cycloaddition Reactions? A Molecular Electron Density Theory Study.

Using the molecular electron density theory, the zw-type [3 + 2] cycloaddition (32CA) reactions of three benzonitrile oxides (BNOs) with several ethylenes, exhibiting different nucleophilic/electrophilic activation, have been studied to establish their poor reactivity. ELF topological analysis of the three BNOs reveals that the para substitution on the benzene ring does not affect the electron density distribution on the CNO frameworks. According to a DFT-based reactivity indices analysis, the nonsubstituted BNO is a moderate electrophile and a moderate nucleophile, which accounts for its poor reactivity in polar reactions. The activation energy analysis of the 32CA reactions of the nonsubstituted BNO allows us to establish that it reacts more effectively with strongly nucleophilic ethylenes than strongly electrophilic ones. Additionally, while the 32CA reactions with nucleophilic ethylenes are entirely ortho regioselective, those with electrophilic ethylenes are poor meta regioselective. A relative interacting atomic energy analysis of these 32CA reactions confirms the establishment of participation of these BNOs in zw-type 32CA reactions. The electrophilic BNO framework stabilization in 32CA reactions with supernucleophilic ethylenes is stronger than that of the ethylene framework in reactions with strong electrophilic ethylenes, which explains the higher reactivity of BNOs toward supernucleophilic ethylenes.