Selective oxidation of thiacalix[4]arene (cone) to all corresponding sulfoxides

Abstract

An upper rim unsubstituted thiacalix[4]arene immobilised in the cone conformation was regioselectively oxidized to give all possible sulfoxide isomers using NaBO3·4H2O as the oxidizing reagent. Single crystal X-ray analysis undoubtedly assigned the stereochemistry of the sulfoxide group with the oxygen atom pointing toward the upper rim of the thiacalix[4]arene. As revealed by dynamic 1H NMR, the presence of the sulfoxide groups has dramatic consequences on the conformational behavior of the thiacalix[4]arenes. Namely, that the number of sulfoxide groups is inversely proportional to the ΔG barrier of the pinched cone–pinched cone equilibrium as determined by ΔG values 45.4kJ/mol for monosulfoxide 5 and ΔG<32kJ/mol for tetrasulfoxide 9.