Unexpected Bisboronic Dicationic Acid Obtained from One‐Pot Condensation Reaction of 3‐Aminophenylboronic Acid and 2,6‐Pyridincarboxyaldehyde

Abstract

A one‐pot double condensation reaction between 3‐aminophenylboronic acid monohydrate and 2,6‐pyridincarboxyaldehyde forms the very interesting building block bisboronic dicationic acid (1), the reaction was carried out in methanol with the presence of sulfuric acid as catalyst. The product contains a central core consisting of five delocalized fused heterocycles being a dicationic moiety and including methylsulfate (CH3OSO3−) as anion (1a). Treatment with CaCl2 under heating produces the anion interchange leading the chloride as anion (1b). Both salts were crystallized by slow solvent evaporation and the X‐ray diffraction analysis reveals the presence of O−H⋅⋅⋅O hydrogen bonds favoring the 2D supramolecular arrangements. Three different motives were formed for 1a with the participation of water molecules, the methylsulfate anion and the boronic acid groups; by the way for 1b, the chloride anion interacts with a BOH group as well as a methanol molecule trough hydrogen bonds, additionally an electrostatic interactions were found with the cationic five membered rings. The possibility for syn‐anti isomerization at room temperature is confirmed by computations with a weak energy barrier of 17.7 kcal mol−1.