Preference for Eclipsed Conformations in Acyclic Neopentyldialkylamines and the Stereodynamical Consequences: An NMR and Molecular Mechanics Investigation1

A dynamic NMR study of a number of acyclic N,N -dialkylneopentylamines, supported by molecular mechanics calculations, is reported. With simple alkyl groups, eclipsed conformations are encountered for the NCH 2 −Bu t bond, which has a high 1-fold rotational barrier. The N-inversion/rotation process for Me 2 NCH 2 Bu t was rendered detectable by desymmetrizing the molecule as Me 2 NCHDBu t . Here the decoalescence of separate 13 C signals for the diastereotopic NMe groups allowed the measurement of the corresponding free energy of activation (Δ G ⧧ = 9.4 kcal mol - 1 ). With significantly more branched alkyl groups, the N−CH 2 Bu t bond adopts a conformation intermediate between staggered and eclipsed. Trineopentylamine and an analogue undergo a novel concerted back and forward 60° rotation through eclipsed conformations about the N−CH 2 Bu t bonds, with a barrier which dynamic NMR indicates must be at least 8.3 kcal mol - 1 .