Eclipsed Conformation of the Exocyclic N−CH2 Bond in N-Neopentylpiperidines and the Stereodynamic Consequences As Studied by Dynamic NMR Spectroscopy and Molecular Mechanics Calculations

The dynamic stereochemistry of a range of N -neopentylpiperidines 1 with an eclipsed N−CH 2 - t -Bu bond is compared with that of the corresponding range of N -ethylpiperidines 2 with a gauche N−CH 2 Me bond. By using dynamic NMR spectroscopy, the relative importance of ring inversion, exocyclic bond rotation, and nitrogen inversion are elucidated and barriers are reported and discussed. Minor populations of the neopentyl-axial-eclipsed conformation are detected directly and identified with the help of molecular mechanics calculations. The corresponding N -alkylpyrrolidines are also reported.