Stereo dynamics of N-isopropyl-N-methylpropargylamine. Dynamic NMR studies. Molecular mechanics calculations

N-Isopropyl-N-methylpropargylamine (N-isopropyl-N-methyl-2-propyn-1-amine; IMPA), is chiral at the pyramidal nitrogen. Racemization occurs via an inve rsion-rotation process. Both C-13{1H} and H-1 dynamic NMR (DNMR) spectra de coalesce in response to slowing inversion-rotation (DeltaG(double dagger) = 7.7 +/- 0.1 kcal/mol). While aspects of the DNMR spectra suggest the prese nce of minor conformations, the spectrum at 100 K shows a strong preference for one conformation. The NMR data suggest that the preferred conformation has both the isopropyl methine proton and the ethynyl group anti to the lo ne pair. Both isopropyl methyl groups are gauche to the lone pair. This con formational preference is in significant contrast to N-ethyl-N-methyl-2-ami nopropane in which the population of that conformation having the ethyl met hyl group and the isopropyl methine proton both anti to the lone pair is on ly 5% at 95 K. The NMR data, supported by molecular mechanics (MMX) calcula tions, suggest a special stabilization for the ethynyl group being oriented anti to the lone pair.