Impact of Carbene Ligands on the Properties and Reactivity of Diazoboranes

While diazo compounds and organic azides have been extensively studied over the past two centuries, their boron analogs, diazoboranes, were only successfully isolated last year, and their properties remain largely unexplored. This study presents a systematic investigation of the synthesis, properties, and reactivity of diazoboranes. In this work, the diazoborane library is expanded to eight species by varying the carbene ligands coordinated to boron, allowing the identification of key correlations between ligand electronic properties and diazoborane behavior. The results demonstrate that increasing carbene π-acidity weakens the B–N bond while strengthening the N–N bond, significantly decreasing thermal stability. Kinetic studies reveal that less π-acidic carbenes promote a second-order decay mechanism, leading to dibora-azine formation, whereas more π-acidic carbenes favor first-order dinitrogen dissociation, facilitating borylene generation. Electrochemical studies indicate that redox potentials also correlate with carbene π-acidity. Three representative reactivity studies are presented in which the same substrate reacts with different diazoboranes to yield either different products or the same product via different reaction mechanisms. These insights provide a generalizable framework for tuning diazoborane properties, facilitating their use in cycloaddition chemistry and small-molecule activation.