The reactions of N-vinyliminophosphoranes. Part 14. A short new synthesis of [n](2,4)pyridinophane ring system (n = 9-6): Static and dynamic structural studies of [7]- and [6](2,4)pyridinophanes

A short new synthesis of the [n](2,4)pyridinophane ring system (n = 9-6) consists of allowing N-vinyl- and N-(1-phenylvinyl)iminophosphoranes to react with cyclic α,β-unsaturated ketones. Structural studies of the compounds prepared were based on spectroscopic measurements and MNDO calculations. The ¹H and ¹³C NMR spectra at various temperatures showed dynamic behaviour for the oligomethylene chains of [7]- and [6]-(2,4)pyridinophane derivatives (8c,d). The energy barriers ΔG _c^‡ of the bridge flipping are 12-13 kcal mol ^-1 (T_c, 20°C) for (8c) and 21-22 kcal mol^-1 kcal mol^-1 (T_c, 150°C) for (8d). The lower-energy process of the oligomethylene chain in (8d) is the pseudorotation with E _a = 10.3 ± 0.2 kcal mol^-1. ΔH ^‡ = 9.8 ± 0.2 kcal mol^-1, and ΔS‡ = -4.8 cal mol^-1 deg^-1. Two stable conformations of the hexamethylene bridge of (8d) were unambiguously determined by low-temperature NMR. The strain of the [n](2,4)pyridinophane ring system was found to increase as the chain length becomes shorter. Remarkable deformation of the pyridine rings of (8c,d) was suggested by the geometrical optimization by MNDO calculation and the red shift of the UV spectrum.