An approach to helical tubular self-aggregation using C2-symmetric self-complementary hydrogen-bonding cavity molecules

In an approach to helical self-aggregation, C2-symmetric cavity compounds based on the fusion of the bicyclo3.3.1]nonane and indole framework and incorporating two 2-pyridone hydrogen-bonding motifs, compounds (-)-4 (pyrrole N-butyl) and (-)-5 (pyrrole N-decyl), have been synthesized. The 2-pyridone AD-DA hydrogen-bonding motif failed to operate in the solid state as demonstrated by X-ray diffraction analysis of (-)-4. Instead, the hydrogen-bonded (D-A) chains ...O=C-N-H...O=C-N-H...O=C-N-H..., interconnecting columnar stacks, comprise helices of the right-handed (P) chirality motif. In solution, the aggregation of (-)-5 was studied by NMR, electronic, and CD spectroscopies, and VPO measurements. These investigations strongly suggest that (-)-5 associates to oligomers in CHCl3 and CH2Cl2 using the 2-pyridone motif, fitting the equal K model, and that pi-stacking can be ruled out as a mode of aggregation. We conclude that the so formed aggregates of (-)-5 have a helical structure, based on the fact that only helical tubular structures can result when enantiomerically pure 5 uses its 2-pyridone AD-DA hydrogen-bonding motifs for aggregation.