The reaction of d-fructose with diphenylborinic acid (Ph2B(OH)), to which d-fructose cannot coordinate tridentately, was kinetically investigated in order to solve the reaction mechanism, and to find a clue to resolve the two-step reaction mechanism of boronic acid (RB(OH)2) with d-fructose. The title reaction was a single-step reaction under the pseudo first-order conditions with large excess d-fructose over Ph2B(OH), which suggests that the reaction of RB(OH)2 with d-fructose consists of the fast formation of a two-coordinate complex followed by the slow formation of a three-coordinate complex. The kinetic reactivity of Ph2B(OH) was considerably higher than that of its conjugate base, diphenylborinate ion (Ph2B(OH)2-). It was shown that Ph2B(OH)2- interacted with CHES buffer to form an outer-sphere complex, whereas no interaction was observed between Ph2B(OH) and CHES buffer. Both the free borinate ion and the restrained borinate ion into the outer-sphere complex react with d-fructose to form the chelate complex via stable binary and ternary outer-sphere complexes, respectively.
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