Boron Macrocycles Based on Multicomponent Assemblies using (3-Aminophenyl) boronic Acid and Pentaerythritol as Common Reagents; Molecular Receptors toward Lewis Bases

Four cyclic boronate esters were synthesized by using a multicomponent reaction from (3-aminophenyl)boronic acid, pentaerythritol, and aldehyde derivatives [isophthalaldehyde (1), dialdehyde A (2), (3-formylphenyl)boronic acid (3), and (2,4-difluoro-3-formylphenyl)boronic acid (4)]. All four reactions lead to the formation of macrocyclic compounds in good yields of more than 70%. The cavities of the compounds consist of 29-, 30- and 40-membered rings, and the macrocyclic structures contain two (2) and four (1 and 3-4) boron atoms as Lewis acids. Additionally, the molecules contain eight (1) and six (2-4) donor atoms (N,O) within the macrocyclic structure, thus constituting a ditopic cavity. X-ray analysis of 2 and 3 revealed the whole conformation within the trigonal planar geometry for the boron atoms and the inclusion of neutral guest molecules, chloroform for 2 and benzene for 3. Titration of the receptors with triethylamine, pyridine, and tetrabutylammonium fluoride show that the Lewis acids are available for interaction with Lewis bases. The interaction was follow by B-11 NMR and UV/Vis spectroscopy.