Relevance of torsional effects to the conformational equilibria of 1,5-diaza-cis-decalins:: A theoretical and experimental study

1,5-Diaza-cis-decalin populates two conformations in which the nitrogen atoms are either gauche IN-in) or anti IN-out) to one another. The equilibrium mixture of the two conformers depends on the substituents at the nitrogen atom, as well as the reaction conditions. Ab initio (HF/6-31G*, B3LYP/6-31+G'*) and molecular mechanics (Amber*) calculations have been performed to examine the possible role of stereoelectronics and steric effects in controlling the equilibrium of substituted 1,5-diaza-cis-decalins. In the present study, N,N'-diethyl- and N,N'-bistrifluoroethyl-1,5-diaza-cis-decalins have been synthesized, and the equilibrium mixtures have been measured using H-1 and C-13 NMR experiments. Steric effects appear to control the equilibria between the two conformational isomers of 1,5-diaza-cis-decalin while torsional effects appear to dominate the equilibria for the N,N'-dialkyl derivatives.