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

摘要

An upper rim unsubstituted thiacalix[4]arene immobilised in the cone conformation was regioselectively oxidized to give all possible sulfoxide isomers using NaBO3 center dot 4H(2)O 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 H-1 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 Delta G barrier of the pinched cone-pinched cone equilibrium as determined by Delta G values 45.4 kJ/mol for monosulfoxide 5 and Delta G <32 kJ/mol for tetrasulfoxide 9. (C) 2016 Elsevier Ltd. All rights reserved.