STEREOCHEMISTRIES OF AROMATIC N-METHYLAMIDES IN CRYSTAL AND SOLUTION - TEMPERATURE-DEPENDENT CONFORMATIONAL CONVERSION AND ATTRACTING AROMATIC-AROMATIC INTERACTIONS

Crystal structures and dynamic behavior in solution of some aromatic N-methylanilides were analyzed. Every N-methylamide examined was in cis form in the crystal, and exhibited cis-preference in solution. Among them, meta-substituted amides, such as N,N',N''-trimethyl-N,N',N''-triphenyl-1,3,5-benzenetricarboxamide (5), exist in the all-syn conformation, in which all the N-phenyl groups are on the same side of the plane of the central benzene ring in the crystal. H-1-NMR studies suggested that 5 existed mainly in all-syn conformation at low temperature as found in the crystal, while the anti conformation, in which one N-phenyl group is on the opposite side of the central benzene ring, becomes predominant at higher temperature, and a rapid equilibrium exists between the syn and the anti conformations. This is a result of the positive enthalpy (Delta H degrees = + 2.1 kcal/mol) and the identical-signed large entropy (Delta S degrees = + 8.4 cal/mol . K) for the conversion from syn to anti conformation. From NMR experiments, the enthalpy gained by assembly of the three phenyl groups in solution was deduced to be 2.8 kcal/mol.