Gerd Borgen

CONFORMATIONS AND CONFORMATIONAL PROCESSES OF UNDECYLOLACTAM

The conformation of undecylolactam (1‐azacyclododecan‐2‐one) was investigated by 1H and 13C NMR spectroscopy in solutions between 25 and ‐140°C. Only the trans isomer is present. At low temperature, the solution was shown to contain two species, different with respect to the ring conformation. Two conformational processes were observed. The higher energy process was visible in the 1H but not in the 13C spectra, with a coalescence temperature of ca. ‐75°C and a free energy of activation of ΔG = 37 kJ mol−1. This process was identified as geminal exchange of methylene hydrogens. A lower energy process was visible in both the 1H and 13C NMR spectra with coalescence temperatures of ca. ‐87°C and a free energy of activation of ΔG = 35 kJ mol−1. Below ‐90°C all 13C resonances are split into a double set of lines in the proportion of about 1:2. Changes also take place in the 1H spectrum, but the resonances are broad and overlapping. This conformational process was identified as interconversion between the two trans forms of undecylolactam. Conformational candidates for the two trans conformers in solution are proposed.

The inherent instability of 1,3-dioxan and the conformation of 1,3,7,9-tetraoxacyclododecane

The conformation of 1,3,7,9-tetraoxacyclo-dodecane, formed by dimerization of 1,3-dioxan, is shown by n.m.r. spectroscopy to be ‘square,’ with both 1,3-dioxa-groupings across corners, and to possess a very high conformational barrier (11 kcal/mol).

The preferred conformation of 1,1,4,4-tetramethylcyclononane and some derivatives: low temperature nuclear magnetic resonance study

Low temperature n.m.r. spectra of a series of nine-membered-ring compounds indicate a rigid, unique conformation of C2 symmetry (D3 symmetry for the ring skeleton).

Conformational analysis of 14- and 16-membered rings

Cyclotetradecane has in all phases the diamond-lattice conformation, whereas cyclohexadecane uses the diamond-lattice conformation in the low-temperature solid phase, but exists as a conformer mixture above the transition point and in solution; the carbonyl and ether oxygen groups prefer the position β to two “corner” atoms.

Occlusion of solvent molecules by 20-membered rings

1,1,5,5,11,11,15,15-Octamethylcycloeicosane and its derivatives crystallize with, and retain, solvent molecules; no occlusion occurs with cycloeicosane itself and the analogous derivatives.