Two Sterically Encumbered 1,3,2-Dioxaphospholanes – Reactions, Comparison of Crystal Structures and Computational Explanations^†

3,4,5,6-Tetrachlorobenzo-3-(2,4,6-tri-tert-butylphenyl)-1,3,2-dioxaphospholane (2) and benzo-3-(2,4,6-tri-tert-butylphenyl)-1,3,2-dioxaphospholane (4), in which the reactive P^III-center lies close to the sterically demanding Mes* group (Mes* = 2,4,6-tri-tert-butylphenyl), were prepared from Mes*–Br and the corresponding P-chloro-phospholane. Compounds 2 and 4 reacted with various oxidants, azides, MeSO₃CF₃ or [(tht)AuCl] (tht = tetrahydrothiophene) to give the expected products. All crystal structures of the products display a strongly distorted Mes* system with a boat conformation of the phenyl ring and appreciable out-of-plane deviations of phosphorus and the ortho-tert-butyl groups to opposite sides of the ring. Quantum chemical calculations at the DFT (density functional theory) level of theory were used in order to discriminate between intra- and intermolecular forces, which are responsible for these distortions.