A novel structural interconversion unit based on intramolecular boroxine formation has been developed. A macrocyclic triboronic acid consisting of three phenylboronic acid units linked by covalent linkers preferentially underwent intramolecular rather than intermolecular boroxine formation, resulting in a quantitative formation of tricyclic boroxine. This structural transformation was accompanied by changes in the polarity, flexibility, and size of the molecule. Dynamic interconversion between the macrocyclic triboronic acid and the tricyclic boroxine was achieved by simple heating/cooling, whereas no boroxine formation occurred upon heating when three boronic acid units were not connected by linkers. Thermodynamic analysis revealed that the entropic advantage of the intramolecular boroxine formation process resulted in these unique features. The entropically stabilized tricyclic boroxine also shows high stability with respect to hydrolysis.
Keywords: boroxine; dynamic interconversion; entropic stabilization; intramolecular boroxine formation; macrocycles.
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