A Cationic Catechol Derivative Binds Anions in Competitive Aqueous Media

A simple dihydroxy isoquinolinium molecule (3⁺) was prepared by a modification of a literature procedure. Interestingly, during optimisation of the synthesis a small amount of the natural product pseudopalmatine was isolated, and characterised for the first time by X-ray crystallography. Compound 3⁺ contains a catechol motif and positive charge on the same scaffold and was found to be a potent anion receptor, binding sulfate strongly in 8 : 2 d₆-acetone:D₂O and 7 : 3 d₆-acetone:D₂O (K_a>10⁴ and 2,100 M⁻¹, respectively). Unsurprisingly, chloride binding was much weaker, even in the less polar solvent mixture 9 : 1 d₆-acetone:D₂O. The sulfate binding is remarkably strong for such a simple molecule, however anion binding studies were complicated by the tendency of the molecule to react with BPh₄⁻ or BF₄⁻ species during anion metathesis reactions. This gave two unusual zwitterions containing tetrahedral boronate centres, which were both characterised by X-ray crystallography.