Chirality-memory molecule: Crystallographic and spectroscopic studies on dynamic molecular recognition events by fully substituted chiral porphyrins

X-ray crystallography of a mandelate complex of a D-2-symmetric saddle-shaped porphyrin such as 2,3,7,8,12,13,17,18-octamethyl-5,15-bis(2',6'-dimethoxyphenyl)-10,20-diphenylporphyrin (2) showed that two mandelate anions are hydrogen bonded to the pyrrole NH moieties in a monodentate fashion, where the absolute structure of the porphyrin macrocycle is determined in such a way that the least hindered section of the host molecule accommodates the phenyl group of the mandelate. IR and H-1 NOESY NMR spectroscopies in CH2Cl2 indicated that a similar binding mode is operative in solution. A series of fully substituted chiral porphyrins having different numbers of o-dimethoxyphenyl groups at the meso-positions (1-3) showed different chiral transfer efficiencies and ring inversion activities. Thermal racemization profiles of protonated 2 in a variety of achiral carboxylic acids indicated that the ring inversion rate is dependent on the steric factor as well as the acidity of the carboxylic acid solvent.