On the reaction of (vinylimino)phosphoranes and related compounds. Novel synthesis and properties of [n](2,4)pyridinophanes and [n](2,4)quinolinophanes (n = 9–7)

Abstract

A short, new synthesis of [n](2,4)pyridinophanes 13a–c (n = 9–7) and 14a–c (n = 9–7), as well as 15a (n = 9) and 16a (n = 9) consists of the reaction of 3-aminocyclohex-2-enone and several β-amino enones with cycloalk-2-enones in an enamine-alkylation process, subsequent condensation of the amino group with a carbonyl function, and dehydrogenation of the product in the presence of a dehydrogenating agent (10% Pd/C). Cyclohexenone-annulated [n](2,4)pyridinophanes 13a–c, which have a quinoline skeleton, were converted conveniently to a series of [n](2,4)quinolinophanes 26a–c (n = 9–7), including the known [8](2,4)quinolinophane 26b. ¹H NMR spectroscopy at various temperatures clarified the dynamic behavior of the heptamethylene chain of [7](2,4)pyridinophanes 13c, 14c, and [7](2,4)quinolinophane 26c. The energy barriers (ΔG ^‡_c) for bridge flipping are 11.3 kcal mol^–1 (T_c –10 °C) for 13c, 11.7 kcal mol^–1 (T_c 0 °C) for 14c, and 12.2 kcal mol^–1 (T_c –5 °C) for 26c. The heptamethylene chain of 26c does not flex easily as compared with that of 13c and 14c. The deformation of the pyridine ring of 13a–c and 26a–c is also suggested by the red shifts of the UV spectra and by the ¹H NMR spectra. The base strength of 26a–c seems to be almost independent of the size of the methylene bridge, although pK_a-values of the protonated form of 26a–c become slightly larger when the methylene bridge becomes shorter.