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[9.3.1.1 (4,8)]hexadecane

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 transan nular hydrogen bond with one of the aminal nitrogen atoms and this, in turn , induces a small but measurable structural anomeric effect in the aminal f unctional group. Diprotonation of 1 in CF3CO2H yields an equilibrium mixtur e of two isomeric bicyclic dications which are interconverted via a monocyc lic ion containing a H2C=N+ group and further protonation by CF3SO3H traps the iminium ion as a monocyclic trication. The structure of the monoprotona ted ion, 4, of 1,4,8,11-tetraazatricyclo[9.3.1.1 (4,8)]hexadecane, 3, as a perchlorate shows the cation with effective C-2 symmetry, a nearly linear, and almost symmetrical, transannular hydrogen bond, and a more pronounced s tructural anomeric effect in both hexahydropyrimidine units. Diprotonation of 3 gives a dication, 5, and the structure of the bis(trifluoroacetate) sh ows measurable shortening of the aminal C-NR2 bond in spite of the fact tha t the lone pair and the C-N+ bond are close to gauche. The structural effec ts in, 4, and 5 are well-reproduced by Becke-Perdew density functional theo ry calculations, and a natural bond orbital analysis of the parent system H 3N-H+ ... NH2CH2NH2, based on SCF and MP2 ab initio calculations is describ ed.