Correlating Solution‐ and Solid‐State Structures of Conformationally Flexible Resorcinarenes : Significance of a Sulfonyl Group in Intramolecular Self‐Inclusion

Abstract

In this study, the synthesis of tetramethoxy resorcinarene podands bearing p‐toluene arms connected by ‐SO3‐ (1) and ‐CH2O‐ (2) linkers is presented. In the solid state, the resorcinarene podand 1 forms an intramolecular self‐inclusion complex of the p‐toluene group, whereas the resorcinarene podand 2 did not show self‐inclusion. The conformation of the flexible resorcinarene podands in solution was investigated by 1D and 2D NMR spectroscopic techniques using variable temperature experiments, as well as, with computational methods including conformational search and subsequent density functional theory (DFT) optimization for representative structures. The 1H NMR spectra of 1 and 2 at room temperature show a single set of proton signals that are in agreement with C4v‐symmetry. At low temperatures the molecules exist as a mixture of boat conformations featuring slow exchange on the chemical shift time scale. An energy barrier (ΔG‡298) of 55.5 kJ/mol and 52.0 kJ/mol was calculated for the boat‐to‐boat exchange of 1 and 2, respectively. The results of the ROESY experiments performed at 193 K and computational modelling suggest that in solution the resorcinarene podand1 adopts similar conformation to that present in its crystal structure, whereas podand 2 populates more versatile range of conformations in solution.