Bipyrrole-Strapped
Calix[4]pyrroles: Strong Anion
Receptors That Extract the Sulfate Anion
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Abstract
Cage-type
calix[4]pyrroles <b>2</b> and <b>3</b> bearing
two additional pyrrole groups on the strap have been synthesized.
Compared with the parent calix[4]pyrrole (<b>1</b>), they were
found to exhibit remarkably enhanced affinities for anions, including
the sulfate anion (TBA<sup>+</sup> salts), in organic media (CD<sub>2</sub>Cl<sub>2</sub>). This increase is ascribed to participation
of the bipyrrole units in anion binding. Receptors <b>2</b> and <b>3</b> extract the hydrophilic sulfate anion (as the methyltrialkyl(C<sub>8–10</sub>)ammonium (A336<sup>+</sup>) salt) from aqueous
media into a chloroform phase with significantly improved efficiency
(>10-fold relative to calix[4]pyrrole <b>1</b>). These two
receptors
also solubilize into chloroform the otherwise insoluble sulfate salt,
(TMA)<sub>2</sub>SO<sub>4</sub> (tetramethylammonium sulfate)