Conformational Studies by Dynamic NMR. 32. Enantiomerization of Chiral Conformers in Hindered Naphthylamines and Naphthyl Nitroxides

Article abstract of DOI:10.1021/jo00230a013

Observation of anisochronous NMR signals in N-alkyl-N-methyl-1-naphthylamines at low temperature indicates that these molecules adopt a twisted conformation (yielding enantiomeric conformers at the equilibrium) as opposed to the corresponding N-alkyl-N-methyl-2-naphthylamines which give two planar (thus achiral) conformers.The barriers to enantiomerization in 1-naphthylamines have been measured by line-shape analysis for those amines containing prochiral substituents.The use at low temperature of one of the Pirkle's chiral alcohols as a discriminating agent allowed these barriers to be measured even in absence of prochiral grpups.Related alkyl 1-naphthyl nitroxides were also shown to prefer a twisted conformation, but the enantiomerization barriers are too low to be measured by ESR.Examination of a much more hindered nitroxide (2-tert-butyl-1-naphthyl ethyl nitroxide) showed that the methylenic hydrogens were anisochronous even at room temperature, indicating that this radical exist as a racemic mixture.The existence of an exponential relationship between the free energies of enantiomerization in the 1-naphthylamines and the nitrogen hyperfine splittings in the analogous nitroxides allowed the barrier for N,N-dimethyl-1-naphthylamine to be estimated.This barrier could not be measured directly because of the symmetry of the amine.