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 - and -(2,4)pyridinophane derivatives (8c,d). The energy barriers ΔG c‡ of the bridge flipping are 12-13 kcal mol -1 (Tc, 20°C) for (8c) and 21-22 kcal mol-1 kcal mol-1 (Tc, 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.
|Number of pages||8|
|Journal||Journal of the Chemical Society, Perkin Transactions 1|
|Publication status||Published - 1990 Dec 1|
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