CONFORMATIONAL STUDIES BY DYNAMIC NMR .39. CLEFTS IN SIMPLE ACYCLIC ORGANIC-MOLECULES - CORRELATED STEREODYNAMICS OF N-TERT-ALKYLBENZYLAMINES STUDIED BY DYNAMIC NMR-SPECTROSCOPY, X-RAY-DIFFRACTION, AND MOLECULAR MECHANICS CALCULATIONS

In N-tert-butyl-N-neopentylbenzylamine (1), the slowing of four processes can be observed in the NMR spectrum at low temperature. The rate constant for 120-degrees rotation of the N-tert-butyl group is three times that for 180-degrees rotation of the phenyl group at all temperatures studied, suggesting correlated rotation of these groups. Barriers for the two processes specified are 6.8 and 7.1 kcal/mol, respectively, at -120-degrees-C. The barrier to interconversion of enantiomeric configurations by nitrogen inversion plus rotation is 9.2 kcal/mol at -77-degrees-C. The barrier to rotation about the tert-butyl-CH2 bond is 5.95 kcal/mol at -143-degrees-C. The anti arrangement of the t-BuNCH2-t-Bu part of the molecule greatly limits the space available for the benzyl substituent, and only one conformation about the N-benzyl bond is populated. Molecular mechanics calculations suggest that the benzyl group occupies a pocket or cleft, defined by the tert-butyl groups, undergoing several kcal/mol of repulsion and attraction from opposite directions. The barrier to rotation about the phenyl-CH2 bond is unprecedentedly high for a simple molecule but is comparable to or smaller than some found in polypeptides of rigid tertiary conformation, where concerted rotation is also a feature. Similar results obtain when either tert-butyl group is replaced by an adamantyl group to give N-(1-adamantyl)-N-neopentylbenzylamine (2) and N-tert-butyl-N-(1-adamantylmethyl)benzylamine (3). A crystal structure determination of 2 shows a conformation very close to that predicted by molecular mechanics calculations.