Unveiling the high reactivity of experimental pseudodiradical azomethine ylides within molecular electron density theory

Abstract

The [3+2] cycloaddition (32CA) reactions of N-methyl azomethine ylide (AY) with styrene, benzaldehyde and methyl 2-formyl-benzoate (MFB) were studied within molecular electron density theory (MEDT), at the ωB97X-D/6-311G(d) computational level, in order to characterize the reactivity of an experimental pseudodiradical TAC for the first time. ELF topological analysis indicates that AY presents a pseudodiradical structure. Analysis of CDFT reactivity indices allows classifying AY as a supernucleophile; while styrene is classified as a moderate electrophile, benzaldehyde and MFB are classified as strong electrophiles. The 32CA reaction with MFB is the most favorable one with a relatively low activation Gibbs free energy of 6.9 kcal mol⁻¹, being irreversible and completely endo stereo- and chemo-selective towards the carbonyl group, a behavior predicted by the analysis of the Parr functions. The bonding evolution theory (BET) study indicates that while the 32CA reaction of AY with styrene is characterized as a pdr-type 32CA reaction, the one involving benzaldehyde follows a pmr-type mechanism prompted by the presence of the carbonyl group. The present MEDT study describes in detail the tunable high reactivity of one of the few experimentally available pseudodiradical TACs, showing that the mechanism of 32CA reactions can be modified not only by changing the electronic structure of TACs through proper substitution but also by the nature of their opposing ethylene derivative.