The Conformation of Free and Complexed Oligoethers

Abstract

Abstract Recent X‐ray and 13 C NMR results on macrocyclic compounds containing oxyethylene units (1,4 dioxa groupings) are reviewed. When such “saturated” crown ethers and related compounds form complexes with cations and uncharged protonacidic molecules, they adopt conformations which contain chiefly ag ± a units or g ± g ± a units (genuine corners) and are thus of the types encountered, or conceived for corresponding cycloalkanes. Uncomplexed or incompletely complexed crown ethers, on the other hand, adopt as a rule unusual conformations never encountered in cycloalkanes. These latter crown ethers contain g ± g ± a units (pseudo corners) characterized by close 1,5 CH … O interactions. Such conformational units are also found in uncomplexed cryptands. In open chains, there is no evidence of this type of chain folding, except in the gas phase for 1,2‐dimethoxyethane. It is concluded that pseudo‐corner formation is resorted to only when solvation of ether oxygen is excluded, such as in the gas phase or in the inaccessible interior of macrocyclic compounds.