Detailed Reaction Mechanism of Anthracene-Bridged bis-(3-Pyridiniumboronic Acid) Chemosensor with D-Glucose

Detailed reaction mechanisms of 4-(N-methyl)pyridinium boronic acid (1) and anthracene-bridged bis(pyridinium boronic acid) compounds (diboronic acids 2 and 3) with D-glucose were kinetically investigated in aqueous methanolic solutions. The crystal structure of diboronic acid 3 was determined using X-ray crystallography. The pKa values of the two boronic acid moieties of diboronic acid (2 or 3) differed by one unit, indicating electronic interactions between the two boronic acid moieties. The kinetics of the reaction of boronic acid 1 with D-glucose showed that both trigonal boronic acid and tetrahedral boronate ions were reactive with D-glucose. UV-Vis spectrophotometry and 1H NMR spectrometry of the reactions of phenylboronic acid with some model compounds of D-glucose showed that the reactive moiety of alpha-D-glucofuranose towards phenylboronic acid was not the 1,2-diol site, but the 3,5,6-triol site. In the reactions of diboronic acids 2 and 3 with D-glucose, the 3,5,6-triol site of alpha-D-glucofuranose reacts first with one of the two equivalent boronic acid moieties of diboronic acid to form a 3,5-bicoordinate intermediate as a rate-determining step, followed by the reaction of the 1,2-diol site of alpha-D-glucofuranose with another boronic acid moiety of diboronic acid to form the final 1,2 : 3,5-tetracoordinate cyclic compound as a fast step. Spectrometry of the reactions of phenylboronic acid with some model compounds showed that the reactive moiety of alpha-D-glucofuranose towards phenylboronic acid was the 3,5,6-triol site. In the reactions with diboronic acids, the 3,5,6-triol site of alpha-D-glucofuranose reacts first as a rate-determining step, followed by the reaction of the 1,2-diol site to form the final 1,2 : 3,5-tetracoordinate cyclic compound as a fast step. image