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

Abstract

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 delta(C) and delta(H) shifts also supported the ring delocalization for the P and As unsaturated heterocycle oxides (delta(C) approximately 165 and delta(H) approximately 9 ppm). In addition, these values for 1H-azirine oxide resembled standard C=C double bond values (delta(C) approximately 130 and delta(H) approximately 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, DeltaH(). The C=C bond influence in the MR was slight, with the same DeltaH 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.

Description of Electron Delocalization via the Analysis of Molecular Fields

“…Molina et al analyzed the aromaticity of threemembered heterocycle (N, P, and As) oxides using the CPDEDs and ELF(r). 40 They found a high electron delocalization only for the unsaturated phosphorus and arsenic oxide derivatives. Negative hyperconjugative effects in the N-O bond formation explain the nonaromaticity of the amine oxide derivatives.…”

“…Molina et al analyzed the aromaticity of three-membered heterocycle (N, P, and As) oxides using the CPDEDs and ELF( r ) . They found a high electron delocalization only for the unsaturated phosphorus and arsenic oxide derivatives.…”