The Long and the Short of It: Regiospecific Syntheses of Isomers of Dicarbomethoxydibenzo-27-crown‑9 and Binding Abilities of Their Pyridyl Cryptands

The two isomers 6 and 9 of cis(4,4′-)-dicarbomethoxydibenzo-27-crown-9
with tri- and tetra-(ethyleneoxy) linkages transposed were synthesized
regiospecifically in high yields (94 and 92%, respectively) by the
Wang–Pederson–Wessels (WPW) protocol and were converted
via the corresponding diols 7 and 10 to
the corresponding pyridyl cryptands 3 and 4 by reaction with pyridine-2,6-dicarbonyl
chloride. As expected from Corey–Pauling–Koltun (CPK)
models, the cryptand with the tri­(ethyleneoxy) arm para to the ester
linkages, “short-armed” cryptand 3, did
display a higher binding constant (Ka =
2.4 × 105 M–1) with paraquats than
the analogous dibenzo-30-crown-10-based cryptand previously studied;
however, the effect was only twofold. Its binding to diquat was reduced
by an order of magnitude (1.5 × 105 M–1), as expected on the basis of its narrower cavity. Also as expected,
the cryptand with the tetra­(ethyleneoxy) arm para to the ester linkages,
“long-armed” cryptand 4, possessed diminished
binding with both paraquats and diquat relative to the 30-crown-based
analogue; in these systems, 2:1 H:G complexes were also detected by
mass spectrometry. A crystal structure is reported for 3·DQ­(PF6)2.