Stereodynamics of 2-(diethylamino)propane and 2-(dibenzylamino)propane. 1H and 13C{1H} DNMR studies. Molecular mechanics calculations

Article abstract of DOI:10.1021/ja00155a021

2-(Diethylamino)propane (DEAP) and 2-(dibenzylamino)propane (DBAP) possess similar molecular symmetries. Interconversion among the stable equilibrium conformations occurs by inversion-rotation at the pyramidal nitrogen and by isolated rotation about carbon-nitrogen bonds. In DEAP and DBAP, the fact that stable equilibrium conformations cannot have destabilizing syn-1,5 interactions between methyl or phenyl groups limits the number of equilibrium conformations that will be present at concentrations high enough to be NMR detectable. The ¹H and ¹³C{¹H} NMR spectra of DEAP at 100 K show two diastereomeric pairs of enantiomeric conformations. One pair of enantiomers has the isopropyl methine proton and both ethyl methyl groups gauche to the lone pair (75%). The other pair has the methine proton anti to the lone pair with the ethyl methyl groups respectively gauche and anti to the lone pair (25%). The barrier to inversion-rotation in DEAP (ΔG? = 6.4 kcal/mol) is higher than barriers to isolated rotation about carbon-nitrogen bonds (ΔG? = 5.3-5.7 kcal/mol). The ¹H and ¹³C{¹H} NMR spectra of DBAP at 100 K show just one pair of enantiomeric conformations that have the isopropyl methine proton and both phenyl groups gauche to the lone pair. There is no evidence in the NMR spectrum at 100 K for those conformations of DBAP that have a phenyl group anti to the lone pair. The barrier to inversion-rotation in DBAP (ΔG? = 6.4 kcal/mol) is higher than the barrier to racemization via isolated rotation about carbon-nitrogen bonds (ΔG? = 5.5 kcal/mol). Molecular mechanics calculations of conformational energies are in good agreement with the observed conformational preferences.