CONFORMATIONAL STUDIES BY DYNAMIC NMR .40. CONFORMATIONAL ATROPOISOMERISM IN HIGHLY HINDERED NAPHTHYLAMINES

N,N-Dialkyl-1-naphthylamines substituted by alkyl groups R (R = Me, Et, i-Pr, t-Bu) in position 2 display anisochronous NMR signals owing to their twisted conformational arrangement. These conformers are enantiomerically related (conformational atropoisomers), and variable temperature NMR measurements allowed the enantiomerization barriers to be determined. The barriers increase with the increasing dimension of the substituents (covering the range 15.7-23.0 kcal mol-1), and the observed trend was reproduced by Molecular Mechanics calculations. The calculations also gave indications upon the structure of the conformers that correspond to energy minima. The final choice among the possible conformations could be achieved by comparing the computed interprotonic distances with the results of NOE experiments.