Conformational behaviors of tetra-O-methylsulfinylcalix[4]arenes - An approach to control the conformation of thiacalix[4]arenes by oxidizing sulfur bridges

(Chemical Equation Presented) The conformational behaviors of all four stereoisomers [5(rctt), 5(rcct), 5(rtct), and 5(rccc)] of tetra-O- methylsulfinylcalix[4]arene were studied by the ¹H NMR spectroscopic method. Variable-temperature (VT) NMR experiments of 5(rctt), 5(rcct), and 5(rtct) revealed that each compound adopted the same conformation as that in the crystals at low temperatures and exhibited a self-exchange between the two equivalent species of this conformation at elevated temperatures. The values of the activation enthalpy ΔH^‡ for the self-exchange were similar (∼70 kJ mol^-1). Further, the activation entropy ΔS^‡ was more important for 5(rtct) (-40 J mol ^-1 K^-1) than for 5(rctt) (-5 J mol^-1 K ^-1) and 5(rcct) (-7 J mol^-1 K^-1); consequently, the exchange rate of 5(rtct) was 150-180 times less than that of the other isomers at 273 K. On the other hand, 5(rccc) was in an equilibrium state between cone and partial-cone conformers at 253 K with the molar ratio being 85:15, which was in reasonable agreement with the relative stability between the two conformers calculated by the ab initio molecular orbital method.