CONFORMATIONAL STUDIES BY DYNAMIC NMR .54. TRIGONAL NITROGEN INVERSION AND ENANTIOMERIZATION PROCESSES IN THE STEREOLABILE CHIRAL ISOMERS OF N-NAPHTHYLIMINES

摘要

Imines bearing the 1-naphthyl substituent bonded to the C=N nitrogen atom were found to have the planes containing the naphthyl and the imino moieties significantly twisted. Even for the less hindered derivative investigated (la, E-isomer), the corresponding dihedral angle in solution has been estimated (NOE experiment) to be approximately equal to 65 +/- 6 degrees. This feature allows for the existence of conformational enantiomers (atropisomers) due to restricted rotation about the nitrogen-naphthyl bond. This form of chirality was observed below -90 degrees C by means of dynamic NMR spectroscopy (H-1 or C-13) either by taking advantage of the presence of prochiral substituents (e.g., isopropyl groups) or by making use of chiral solvating agents (Pirkle's alcohols). Computer line shape simulation of the appropriate NMR signals yielded the free energies of activation for the R,S interconversion process (enantiomerization). The atropisomers were usually observed only in one of the two possible stereolabile E,Z isomers, except in one case (1e) where both of them displayed evidence of atropisomerism, with two different barriers for the related enantiomerization processes (8.4 and 9.9 kcal mol(-1) for the E- and Z-isomer, respectively). Also, the free energies of activation for the E to Z interconversion were measured and found to cover a range of values much higher (15.8-19 kcal mol(-1)) than the values for the enantiomerization processes (8.4-10.8 kcal mol(-1)), except in the crowded imine 2, which has two tert-butyl groups bonded to the C=N carbon atom. In this case, it was possible to demonstrate that both the exchange of the E,Z positions and the R,S interconversion occur via the same pathway, which must be consequently identified as an inversion of the trigonal nitrogen atom (lateral shift).