Selective synthesis of three conformational isomers of tetrakis[(ethoxycarbonyl)methoxy]thiacalix[4]arene and their complexation properties towards alkali metal ions†

Abstract

5,11,17,23-Tetra-tert-butyl-2,8,14,20-tetrathiacalix[4]arene-25,26,27,28-tetrol (TCA) underwent facile tetra-O-alkylation by treatment with ethyl bromoacetate in the presence of an alkali carbonate as base catalyst in DMF or acetone to provide a mixture of conformational isomers (cone, partial cone, and 1,3-alternate) of 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis[(ethoxycarbonyl)methoxy]-2,8,14,20-tetrathiacalix[4]arene (1), the stereochemistries of which were unambiguously assigned by ¹H NMR and X-ray analysis. The isomer distribution depended significantly on the base used, thus providing a facile route for the preparation of a particular conformer; Na₂CO₃, K₂CO₃, and Cs₂CO₃ gave cone- (77% yield), partial-cone- (58% yield), and 1,3-alternate-1 (78% yield) in acetone, respectively. Cone- and partial-cone-1, in turn, showed preference for Na⁺ and K⁺, respectively, in an ion-pair extraction study, while 1,3-alternate-1 preferred most Rb⁺ ion, followed by K⁺ and then Cs⁺. These results imply that the size of the cavities provided by the (ethoxycarbonyl)methoxy groups arranged on the periphery of the thiacalix[4]arene skeleton is in the order cone- < partial-cone- < 1,3-alternate-1. The ion selectivity of cone-1 was rather better than that of the methylene-bridged counterpart, 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis[(ethoxycarbonyl)methoxy]calix[4]arene (2). The stoichiometry of the complex of cone-1 with Na⁺ ion was determined to be 1∶1 with the stability constant of 10^2.85 mol^–1 dm³ in 50 (v/v)% CDCl₃–CD₃OD.