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

Article abstract of DOI:10.1021/j100095a025

N-Ethyl-N-methyl-2-aminopropane (EMAP) is one of the simplest tertiary aliphatic amines that has a chiral center at nitrogen.Racemization occurs by inversion-rotation at the pyramidal nitrogen.For each EMAP enantiomer, additional conformational interconversions occur via isolated rotation about carbon-nitrogen bonds.The 1H and 13C<1H> dynamic NMR (DNMR) spectra of EMAP and a selectively deuterated derivative decoalesce into four subspectra at 95 K.The spectrum at 95 K is best rationalized in terms of five equilibrium conformations present at concentrations high enough to be NMR-detectable including two conformations that interchange rapidly even at 95 K.A two-letter designation is used to name the various conformations.The first letter defines the orientation of the ethyl methyl group (G denotes gauche to the lone pair and to the N-methyl group; G' denotes gauche to the lone pair and to 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 ethyl group; A denotes anti to the lone pair).The major subspectrum at 95 K is assigned to a family of G'G' and GG' conformations (59percent) that interconvert rapidly at 95 K.The other three subspectra are assigned to the GG (34percent), AA (5percent), and GA (2percent) conformations.Simulations of the DNMR spectra reveal a barrier to inversion-rotation at nitrogen (ΔG(excit.) = 7.5 kcal/mol at 160 K) that is higher than the barriers for a number of observable isolated rotations about the N-CH2 and N-CH bonds (ΔG(excit.) = 4.7-6.4 kcal/mol).A 5000-point optimized energy surface computed as a function of two dihedral angles by using Allinger's MM2(87) computer program, MM2(87) energy calculations for all the optimized equilibrium conformations of EMAP, and MM2(87)-calculated isolated rotation barriers show excellent agrreement with the DNMR data.