Six azocalix[4]arenes were synthesized for the first time by diazo-coupling reactions with calix[4]arene. The products showed unusual spectral properties including an azophenol–quinine–hydrazone tautomerism. Owing to the chromophoric nature, the four pKa’s for water-soluble p-(4-trimethylammoniopheylazo)calix[4]arene could be determined to be 0.5, 2.0, 10.0, and ca. 13. The marked pKa split was explained by the strong hydrogen-bonding interactions characteristic of calix[4]arene.
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Shinkai et al. (1989) studied this question.
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