Conformation of hindered piperidines: Spectroscopic evidence for contribution of boat conformations

Abstract

High resolution ¹H and ¹³C NMR resonance assignments and conformational assignments were carried out for four t(3)-benzyl-r(2),c(6)-bis(aryl)piperidin-4-ones 1–4 and their four N-nitroso-t(3)-benzyl-r(2),c(6)-bis(aryl)piperidin-4-ones 5–8. In addition to conventional 1D NMR methods, 2D shift-correlated NMR techniques (¹H-¹H COSY and ¹H-¹³C COSY) were used for signal assignments. At room temperature the t(3)-benzyl-r(2),c(6)-bis(aryl)piperidin-4-ones 1–4 exist in only one isomeric form whereas their N-nitroso derivatives 5–8 exist in two isomeric forms. The preferred conformations of both the isomeric forms of nitrosamines were determined by comparison of the spectral data with those of the corresponding parent amines 1–4 and with the aid of substituent parameters. The results indicate the presence of an equilibrium mixture of boat forms B ₁ and B ₂ for Z isomers of 5–8. For the E isomers of 5–8, boat form B ₁ is predicted to be the major conformer. The piperidin-4-ones 1–4 exist in normal chair conformations with equatorial orientations of all the substituents.