PKA OF CALIXARENES AND ANALOGS IN NONAQUEOUS SOLVENTS

The acid dissociation constants (K(app)) of p-t-butylcalix[n]arenes (1n: n = 4, 6, and 8), O-methylated p-t-butylcalix[4]arene (2Me(n)), and their noncyclic analogs (3n) were estimated in THF at 25-degrees-C. As pH indicators, Et4N+ salts of p-nitrophenolate, 2,4-dinitrophenolate, and picrate were employed. The pK(app) values for 1n were lowered by at least four pK units from that of p-t-butylphenol because of strong intramolecular hydrogen-bonding interactions. Compounds 2n involved both the strong and weak hydrogen bonds: the proton in the strong hydrogen bond showed the acidity comparable with that of 1n whereas that in the weak hydrogen bond showed the relatively high pK(app). These properties were discussed in relation to delta-OH (chemical shift in H-1 NMR) and nu-OH (OH vibration band in IR). This is the first systematic discussion on ''pK(a)'' of p-t-butylcalix[n]arenes.