Evidence for hydrogen-bond enhanced structural anomeric effects from the protonation of two aminals, 5-methyl-1,5,9-triazabicyclo[7.3.1]tridecane and 1,4,8,11-tetraazatricyclo[9.3.1.14,8]hexadecane

Abstract

The structure of the monoprotonated ion, 2, of 5-methyl-1,5,9-triazabicyclo[7.3.1]tridecane, 1, as a picrate shows an NMeH⁺ group engaged in a transannular hydrogen bond with one of the aminal nitrogen atoms and this, in turn, induces a small but measurable structural anomeric effect in the aminal functional group. Diprotonation of 1 in CF₃CO₂H yields an equilibrium mixture of two isomeric bicyclic dications which are interconverted via a monocyclic ion containing a H₂CN⁺ group and further protonation by CF₃SO₃H traps the iminium ion as a monocyclic trication. The structure of the monoprotonated ion, 4, of 1,4,8,11-tetraazatricyclo[9.3.1.1^4,8]hexadecane, 3, as a perchlorate shows the cation with effective C₂ symmetry, a nearly linear, and almost symmetrical, transannular hydrogen bond, and a more pronounced structural anomeric effect in both hexahydropyrimidine units. Diprotonation of 3 gives a dication, 5, and the structure of the bis(trifluoroacetate) shows measurable shortening of the aminal C–NR₂ bond in spite of the fact that the lone pair and the C–N⁺ bond are close to gauche. The structural effects in 2, 4, and 5 are well-reproduced by Becke–Perdew density functional theory calculations, and a natural bond orbital analysis of the parent system H₃N–H⁺· · ·NH₂CH₂NH₂, based on SCF and MP2 ab initio calculations is described.