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

Abstract

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 NCH2−But bond, which has a high 1-fold rotational barrier. The N-inversion/rotation process for Me2NCH2But was rendered detectable by desymmetrizing the molecule as Me2NCHDBut. Here the decoalescence of separate 13C signals for the diastereotopic NMe groups allowed the measurement of the corresponding free energy of activation (ΔG⧧ = 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° 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.