Z,E Isomerism and hindered rotations in malonamides: an NMR study of N,N′-dimethyl-N,N′-dibutyl-2-tetradecylpropane-1,3-diamide

Abstract

The title propane-1,3-diamide has three geometrical isomers, ZZ, ZE and EE, in slow exchange on the NMR timescale. The distribution of isomers involves the two amide moieties in a statistical manner. Four types of conformational changes ZZ ⇄ ZE ⇄ EE are shown by exchange correlation spectroscopy and lineshape calculations. Isomers are identified on the basis of aromatic solvent induced shift (ASIS) and lanthanide induced shift (LIS) experiments. Barriers to rotation in each amide moiety are relatively low due to the bulkiness of N- and C-substituents, typically 71 kJ mol^–1 at 25 °C, and are weakly sensitive to the conformational state (Z or E) of the other amide moiety.