On the Aromaticity and Meisenheimer Rearrangement of Strained Heterocyclic Amine, Phosphine, and Arsine Oxides

A theoretical investigation (AIM and ELF analyses together with NMR chemical shifts) has been conducted for three-membered heterocycle (N, P, and As) oxides. An aromatic stabilization was found for the P and As rings. However, the N derivatives displayed a net negative hyperconjugation in the N−O bond formation, without ring aromaticity observed for their electronic properties. The calculated δC and δH shifts also supported the ring delocalization for the P and As unsaturated heterocycle oxides (δC ≈ 165 and δH ≈ 9 ppm). In addition, these values for 1H-azirine oxide resembled standard CC double bond values (δC ≈ 130 and δH ≈ 7 ppm). The different behavior for the N oxides was also observed in their Meisenheimer rearrangement (MR). All the reaction paths, yielding the corresponding hydroxyl structures, were exothermic (G2 method). However, the N derivatives showed the lowest values for activation enthalpy, ΔH ⧧. The CC bond influence in the MR was slight, with the same ΔH values for the saturated and unsaturated paths. This rearrangement for the P and As oxides yielded TSs closer to the reactives; however, the corresponding TSs resembled the products for the N-derivatives. The different reaction paths have been examined by their corresponding AIM and ELF analyses at the B3LYP/6-311G* level.