Comparative investigation of the inclusion preferences of optically pure versus racemic TADDOL hosts for pyridine and isomeric methylpyridine guests

Abstract Here we consider the effect of the optical purity of TADDOL (α,α,α′,α’-tetraphenyl-2,2-dimethyl-1,3-dioxolane-4,5-dimethanol) on its discriminatory behaviour. We synthesized optically pure and racemic TADDOL, which form complexes with pyridine and the methylpyridines, and recrystallized them from various equimolar mixtures of these guests, and showed that both TADDOLs, surprisingly, discriminate similarly between them. The selectivity of optically active TADDOL was in the order 3-methylpyridine > 4-methylpyridine > pyridine > 2-methylpyridine, while this order was 3-methylpyridine > 4-methylpyridine > 2-methylpyridine > pyridine for the racemate. For many of the experiments, the extent of selectivity displayed by the two hosts was comparable. Single crystal X-ray diffraction experiments showed that preferred guests experienced stronger π-π and, even more so, H-bonding interactions with the host. Thermal analyses confirmed reports that suggested that guests that reside in channels in the host structure form less thermally stable complexes than those where the guests are accommodated in discrete cavities.