Neighboring Protonation Unveils Lewis Acidity in the BNO Heterocycle.

Boron serves a distinctive role in a broad range of chemistry disciplines. The utility of the element lies in its Lewis acidity, and thus, it is crucial to understand the properties of the boron atom in chemically different contexts. Herein, a combination of experiments and computations reveals the nuanced nature of boron in direct amidation reactions catalyzed by recently disclosed 1,3-dioxa-5-aza-2,4,6-triborinanes (DATBs). The most active DATB catalyst has been shown to bear an azaborine ring in its structure, thus having four boron atoms in a single molecule. Three chemically distinct boron atoms in the catalyst framework have been shown to serve different roles in the catalytic cycle, depending on their innate Lewis acidity. More specifically, the most Lewis acidic boron interacts with the amine, whereas the two boron atoms in the B-N-B substructure acquire Lewis acidity only upon protonation of the center nitrogen atom. Furthermore, although the least acidic boron atom in the azaborine ring did not act as a Lewis acid, it still plays an important role in the catalytic cycle by forming a hydrogen bond between carboxylic acid and the B-OH moiety. The mechanistic insights obtained from this study not only extend the knowledge on catalytic direct amidation but also provide a guiding principle for the further exploration of multi-boron compounds.