1H-Phosphepine-benzene phosphine valence tautomerizations: Impacts of substituents at ab initio and DFT levels

The main goal of this work is to find a more stable seven-membered ring 1H-phosphepine connected to its bicyclic benzene phosphine valence tautomer after passing through a nonplanar transition state. This challenge is met by probing the impacts of substituents (X = H, F, Cl, Br, CN, Me, CF3, NH2, and OMe) on the umbrella inversions of 1H-phosphepines, as well as their effects on two series of equilibria consisting of 2-X-benzene phosphines ⇌ 3-X-1H-phosphepines (series A) and 3-X-benzene phosphines ⇌ 4-X-1H-phosphepines (series B), all in the more stable endo form of pyramidal phosphorus atom. Among 17 tautomerization systems, scrutinized at B3LYP/6-311G∗ level, only the 3-methoxybenzene phosphine ⇌ 4-methoxy-1H-phosphepine system shows equilibrium shift to the right. In contrast to the tautomerizations, umbrella inversions of 1H-phosphepines are not much sensitive to the effects of substituents placed around their carbon skeletons. The magnetic (NICS) and structural criteria indicate partial aromaticity of 1H-phosphepines and high anti-aromaticity for their planar inversion transition states. Some justifications based on NBO data and calculated ΔEHOMO-LUMO are presented. Various inconsistencies occur between the ab initio and DFT data, making the equilibrium constants (Keqs) sensitive to the methods of calculation along the substituents effects.