Effect of alkali cations on the methoxide ion addition to corands incorporating a thiopyrylium subunit

The incorporation of a thiopyrylium ring in a corand structure forms a system with a weak binding ability in methanol towards the alkali cations K+, Rb+ and Cs+. The binding substantially increases in basic solution because of the reversible addition of methoxide ion to the heteroaromatic ring to form a neutral 4H adduct. The effect of the length of the polyoxyethylene chain and of the presence of a bulky group (tert-butyl) in the ? position of the heteroaromatic ring has been quantitatively investigated. Results obtained from molecular mechanics calculations were used to rationalise the depressing effect of the ? tert-butyl group on the ligand ability of thiopyrans.