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.