Transient binding mode of phenolphthalein–β-cyclodextrin complex: the lactone dianion as an induced transition-state analogue trapped in β-cyclodextrin

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Abstract
1 H and 13C NMR spectroscopy of phenolphthalein disodium salt–β-cyclodextrin complex in Me2SO, and in mixtures of Me2SO–D2O, further supports the lactone dianion structure in the complex as a new and hitherto unforeseen form of phenolphthalein; the aryl residue bearing the lactone ring has penetrated the cavity, whereas both phenolate anions are held on the rim to modulate sensitively the binding mode of the complex, and the optimal space-filling molecular model shows that the lactone dianion has a remarkable close fitting contact to a β-CD, and also indicates that the distortion caused by the two phenolic groups on the shape of β-CD is a prerequisite and essential part of the subsequent structural transformation, destroying the quinone–phenolate system of the red-coloured phenolphthalein dianion to become flat on the rim, that is to adopt a conformation complementary to that of β-CD; in this way, the lactone-dianion is formed as an induced transition-state analogue trapped in β-CD for the enantiomerization of the helical three aromatic rings of a red coloured phenolphthalein in solution.