Mechanochemical Synthesis and Properties of Boronic Ester Cage Compounds

Background: In the past decades, mechanochemical organic synthesis has become an attractive technique for solvent-free synthesis, which avoids the usage of harmful solvents and production of toxic wastes. This method has been utilized to promote the efficient formation of carbon–carbon and carbon–heteroatom bonds. The mechanochemical synthesis of macrocyclic molecules, rotaxanes and covalent organic frameworks has been reported recently. However, the mechanochemical construction of purely organic cage compounds is scanty and challenging. Objective: The primary objective of this study was to synthesize boronic ester cage compounds under high-speed vibration milling conditions and investigate their properties. Method: A mixture of triboronic acids 3 (0.04 mmol) and pentaerythritol (0.06 mmol) was vigorously shaken under high-speed vibration milling (HSVM) conditions at 58 Hz for 40 min. The resulting solid was dissolved in chloroform (5 mL) and then filtered. Cage compounds 4 were obtained by removal of the solvent, and dried under vacuum at 100 °C for 12 h. Cage compounds 4 were subjected to thermogravimetric analysis and chemical stability experiments. Result: Boronic ester cage compounds 4 were synthesized in nearly quantitative yields under high-speed vibration milling conditions. Furthermore, thermogravimetric analysis and chemical stability experiments showed that cage compounds 4 possessed high thermal stability (Tdec up to 320 °C) and good chemical stability in aqueous media. Conclusion: We have described an efficient solvent-free synthesis of boronic ester cage compounds 4 under the HSVM conditions. Furthermore, cage compounds 4 possessed high thermal stability and good chemical stability in aqueous media. Keywords: Mechanochemical synthesis, solvent-free reaction, boronic ester, cage compound, high-speed vibration milling (HSVM), hydrolysis.