Investigation of Diazaborole Formation and Diazaborole-Linked Macrocycles with EthylHexyl Ester Substituents

Boron-containing polymers and macrocycles are currently an exciting field of study due to their wide variety of potential applications. The exploration of the dynamic covalent behavior of 2-phenyl-1,3,2-benzodiazaborole (DAB) under different solvent and temperature conditions may provide insight into the synthesis of larger diazaborole-based frameworks. The effects of solvent and temperature on the condensation of DAB were explored. The results showed that the DAB product formed faster in DMSO than in toluene. However, DAB formation showed higher percent conversion in toluene. In addition, Raman spectroscopy was used for in situ reaction monitoring of DAB formation. By using a calibration curve, we found that the formation of DAB in chloroform at 50 °C can achieve up to 68% conversion. However, the use of Raman spectroscopy for reaction monitoring of DAB formation was limited by solubility and by the baseline noise.
Furthermore, our research group is investigating the use of diazaboroles as the active linking unit in shape-persistent macrocycles. The present study describes the continuing efforts towards improving the solubility of monomers, oligomeric intermediates, and macrocyclic products. Ethylhexyl ester functional groups were installed on the monomer to improve solubility. The results illustrated that although the presence of branched 2-ethylhexyl ester had a positive effect on solubility of the monomer, the side chain interactions were not strong enough to overcome the pi-stacking forces of the macrocycles. The poor solubility of the co-reactant benzene-1,4-diboronic acid (BDBA) is a limiting factor for the formation of the macrocycle product. To overcome this issue, we investigated the use of BDBA-based esters instead of BDBA. The results revealed that the reactions between BDBA-based esters with various di- and tetraamines in chloroform at room temperature were slow and intramolecular reactions to form the macrocycle could not compete with the stability of the BDBA-based esters.