Crown ethers azophenol

For the previously established azophenolic crown ether extractant a predictive single stage equilibrium model (Figure 1) was constructed and validated [14]. The extent of extraction is characterised by the distribution ratios Dr and Ds for each enantiomer ... 

The two enantiomers (RRRR and SSSS) of the chiral azophenol crown ether (87) have been designed to achieve enantiomeric selective coloration with chiral amines. Studies of complexation of the (i i )-norpseudoephedrine (88) by the two enantiomers revealed that the binding of the substrate was more efficient with (SSSS)- (87) than with (RRRR)- 97). In addition, over a certain range of concentration of (88), a color change from yellow to reddish violet was observed with (SSSS)-(87) whereas no such color change was detected under the same conditions with the RRRR)-(ST) enantiomer, i.e. the expected enantiomer selective coloration was achieved <93TCCI63>. 

Naemura, K. Ogasahara. K. Hirose, K. Tobe. Y. Preparation of homochiral azophenolic crown ethers containing l-phenylethane-1.2-diol and 2,4-dimethyl-3-oxapentane-l,5-diol as a chiral subunit Enantiomer recognition behaviour towards chiral 2-aminoetlianol derivatives. Tetrahedron Asymmetry 1997. 8 (1). 19-22. 

Key words. Azophenol crown ether, cation, lithium. 

Another type of complexation with crown ethers is concerned with a tripod arrangement by three ether oxygens of the crown ether. We recently observed such a perching type complexation of protonated amines by the use of the above-stated azophenol crowns. 

Figure 2 Molecular structures of (a) crown-ether-based dyes 1,2 and (b) azophenol-substituted spherand 3.

Before the study of complexation, pKa values of acerands 4 and of the reference azophenol 13 were determined by spectrophotometry in 10% dioxane-water (Table 1). The table shows the increase of pKa values with decreasing crown ring size. This fact is explained by destabilization of the phenolate anion due to both the electrostatic repulsion between the phenoxide anion and the lone pair electrons of ethereal oxygen atoms and the blocking of the crown ring against solvation of the anion. 

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