Preference for eclipsed conformations in acyclic neopentyldialkylamines and the stereodynamical consequences: An NMR and molecular mechanics investigation

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 NCH2But bond, which has a high I-fold rotational barrier. The N-inversion/rotation process for Me2NCH2But was rendered detectable by desymmetrizing the molecule as Me2NCHDBut. Here the decoalescence of separate C-13 signals for the diastereotopic NMe groups allowed the measurement of the corresponding free energy of activation (Delta G(double dagger) = 9.4 kcal mol(-1)). With significantly more branched alkyl groups, the N-CH2But bond adopts a conformation intermediate between staggered and eclipsed. Trineopentylamine and an analogue undergo a novel concerted back and forward 60 degrees rotation through eclipsed conformations about the N-CH2But bonds, with a barrier which dynamic NMR indicates must be at least 8.3 kcal mol(-1).