The acidity of calix[5]arenes and their linear analogues

Abstract

Five new calix[5]arenes containing a single p-nitrophenol unit as the most acidic phenolic unit have been synthesised by (3+2) fragment condensation of a trimer with a p-nitrophenol in the middle with various bishydroxymethylated alkanediyl diphenols. Their first acid constant (pK_al) has been determined in 2-methoxyethanol/water (9:1) at 22°C by optical titration. The pK_al values are distinctly lower (ΔpK_a > 2) than for the trimer, while no difference has been found in comparison to structurally analogous calix[4]arenes. Rigidification of the calix[5]arene skeleton by introducing a single alkanediyl bridge opposite to the p-nitrophenol unit has no effect on pK_al. All values are in agreement with a stabilisation of the monoanion via two chains of consecutive intramolecular hydrogen bonds OH…OH…O(−), a system which is favoured, if the acidity of the first donating OH group is increased. This effect is observed also in linear pentamers which show pK_al-values higher by only 0.8–1 units than those of the calix[5]arenes.