The ring-enlarged crown ethers, 16-and 17-crown-5 and 19-to 22-crown-6, were synthesized and their cation-binding abilities were evaluated by solvent extraction of aqueous alkali metal picrates. The cation-binding abilities of less symmetrical crown ethers, 3a-e and 4a,b, were generally lower than those of the common symmetrical crown ethers 15-crown-5 (5a) and 18-crown-6 (5b), for which the less symmetrical arrangement of the donor oxygen atoms must be responsible. Compared with 18-crown-6 (5b), the ring-extended crown ethers, 3d, 3e, and 4b, showed a significant shift in cation selectivity, probably due to the enlarged cavity size. The thermodynamic parameters for the extraction of sodium and potassium picrates with 3a, 3c, and 5a,b were calculated from the change of the extraction equilibrium constants (Kex) between 10-25 °C. The stability of the cation-crown ether complexes was shown to be governed in general by the enthalpy change. However, a significant contribution of the entropy factor was found in unfavorable size combinations of K+ with 3a and Na+ with 3c.
Oligoethylene glycols and some related alcohols were efficiently tosylated with p-toluenesulfonyl chloride in a tetrahydrofuran–water (1:1) mixture in the presence of excess sodium hydroxide. This method is advantageous over the conventional tosylation in pyridine both regarding the work-up procedure, the yield, and the purity of the product, and may be potentially useful for the tosylation of certain acid-labile alcohols.
As compared with symmetrical 3m-crown-m, the ring-contracted(3m–1)-crown-m showed drastic decrease in cation-binding ability, which is attributable not to the diminished cavity size but to the disordered conformation induced by ring contraction.
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