Stereodynamics of N - Allyl - N - methyl - 2 - aminopropane . 1 H and 13 C { 1 H } DNMR Studies . Molecular Mechanics Calculations

N-Allyl-N-methyl-2-aminopropane (AMAP) is a relatively simple tertiary amine that has a chiral center at the pyramidal nitrogen. Racemization occurs by inversion-rotation at nitrogen. For each AMAP enantiomer, conformational exchanges occur via isolated rotation about carbon-nitrogen and carbon-carbon bonds. The 1H and 13C{1H} dynamic NMR (DNMR) spectra of AMAP show a higher temperature decoalescence due to slowing inversion-rotation at nitrogen (∆G ) 7.4 kcal/mol at 162 K) and a more complex decoalescence at lower temperatures due to slowing isolated rotation about carbon-nitrogen bonds (∆G ) 5.1-5.5 kcal/ mol at 100-110 K). A well-defined 1H NMR spectrum at 100 K is simulated accurately by invoking the presence of five equilibrium conformations including two conformers that interconvert rapidly at 100 K. A two-letter designation is used to name the various conformations. The first letter defines the orientation of the vinyl group with respect to the lone pair (G denotes gauche to the lone pair and to the N-methyl group; G′ denotes gauche to the lone pair and the isopropyl group; A denotes anti to the lone pair). The second letter defines the orientation of the isopropyl methine proton (G denotes gauche to the lone pair and to the N-methyl group; G′ denotes gauche to the lone pair and to the allyl group; A denotes anti to the lone pair). The major subspectrum at 100 K is assigned to a family of GG′ and G′G′ conformations (59 %) that interconvert rapidly at 100 K. Other subspectra at 100 K are assigned to the GG (36 %), GA (≈3 %), and AA (≈2 %) forms. The presence of dominant GG′, G′G′, and GG conformers is confirmed by the 13C{1H} NMR spectrum at 100 K. Conformational energies and internal rotation barriers calculated by using Allinger’s MM2(87) molecular mechanics computer program show good agreement with the NMR data. AMAP shows stereodynamics very similar to N-ethyl-N-methyl-2-aminopropane.