A detailed study defining the extent of preorganization of an aza macrocycle containing the phenyldinaphthomethane subunit (a three bladed propeller) using dynamic NMR and molecular dynamics

Abstract

The aza-propeller crown 4,7,10-tritosyl-4,7,10-triaza-1,13-dioxacyclooctadecane 1 has been examined by variable temperature 13C NMR studies, and the movement of each segment of the molecule (ArOCCN, NCCN, propeller) was determined kinetically. Several sets of coalescences were found to occur at the same activation energy, which was also that found for the flip of the propeller segment. This movement of the macrocycle as a whole is a relatively slow process (ms scale) compared to the torsional movements round the crown ether ring (ps scale). These torsional movements were explored by molecular dynamics (MD) modelling; the macrocyclic ring is mobile when simulated at 200 K. The tosyl groups are in permanent motion although none twists by more than a few degrees round the N–S torsion. The segments next to the propeller (which are both gauche in the crystal structure) converted from gauche to anti, while the TsNCCNTS segments remained anti throughout.