Novel synthesis, static and dynamic properties, and structural characteristics of 5-cyano[n](2,4)pyridinophane-6-ones (n = 9–6) and their chemical transformations

Abstract

A novel synthesis of 5-cyano[n](2,4)pyridinophane-6-ones 12a–d (n = 9, 8, 7, and 6) consists of allowing cyanoacetatoamide to react with cycloalk-2-enones. Their static and dynamic properties as well as structural characteristics are studied on the basis of their spectroscopic properties, cyclic voltammetry, and theoretical calculations. The ¹H and ¹³C NMR spectra at various temperatures have clarified the dynamic behavior of the methylene chains for [7](2,4)- and [6](2,4)pyridinophane-6-one derivatives 12c and 12d. The energy barrier (ΔG^‡) of the bridge flipping of 12c is estimated to be 12.0 kcal mol⁻¹ (T_c = 0 °C). On the other hand, compound 12d undergoes pseudorotation (conformational change of the methylene chain) at room temperature, and does not undergo bridge flipping even at 150 °C in DMSO-d₆. The energy barrier (ΔG^‡) of the pseudorotation of the methylene chain of 12d is found to be 10.5 kcal mol⁻¹ (T_c = −25 °C), and thus, two stable conformers of the hexamethylene bridge of 12d are determined as predicted by theoretical calculations. Deformation of the pyridone ring of 12d is also determined by X-ray crystallographic analysis. Furthermore, chemical transformations of 12a–c leading to 5-carbamoyl[n](2,4)pyridinophanes 15a–c are also accomplished successfully in moderate to good yields.