3-Aminophenylboronic acid as building block for the construction of calix- and cage-shaped boron complexes

The reactivity of 3-aminophenylboronic acid toward 3,5-disubstituted salicylaldehyde derivatives (R = tBu, MeO, I, Br, NO2) was analyzed with the aim to synthesize macrocyclic boron compounds having a calix-like structure. The three-component condensation was carried out using different aliphatic alcohols (ROH, R = Me, Et, nPr, nBu, nPn, nHex) in order to replace the free B–OH groups by B–OR moieties and their effect on the structural and physicochemical properties of the resulting compounds was analyzed. When the reaction was carried out under reflux conditions, trimeric calix-like compounds are formed for the 3,5-tButyl salicylaldehyde derivatives. The absence of alcohol molecules during the reaction lead to the condensation of two calixarenes through two of the three B(OH) groups to form two B-O-B moieties, giving place to the formation of a hexanuclear cage. Using the salicylaldehyde derivatives having I, Br and NO2 groups, the Schiff bases resulting from the condensation reaction of the aldehyde and the boronic acid were isolated providing thus evidence for the previously proposed reaction sequence for the formation of the calix-shaped compounds.