Alkali metal salts of 1,2,3-benzodiazaborines: Platforms for late-stage N-functionalization and metal complexation

The standard procedure for the preparation of benzoid 1,2,3-diazaborines (DABs) is the condensation of 2-formylphenyl boronic acid with a hydrazine. The choice of hydrazine derivative irreversibly predetermines the N-substituent in most cases and is additionally limited by the availability and hazardous nature of the respective hydrazines. Options to subsequently modify the N-substituent are scarce. Herein, we explore an approach to postsynthetic N-functionalization via isolable, nucleophilic DAB alkali metal amides. The structures of these metalated DABs were extensively studied, utilizing ¹H DOSY NMR spectroscopy and XRD analysis. Subsequent reactivity studies of these unusual amides revealed an intricate, dualistic reactivity pattern. Upon treatment with mild electrophiles, the DAB amides react as N-nucleophiles, facilitating the straightforward introduction of functional groups at the N_α position. Due to the incorporation of the second N_β atom, they can moreover serve as anionic diazo ligands for the formation of µ-DAB-bridging coinage metal complexes, which bear a striking resemblance to well-studied complexes with pyrazolato (pz⁻) ligands. Overall, this work demonstrates how BN incorporation opens new avenues in ligand design and provides a valuable tool for post-synthetic modification of aryl DABs with organic and inorganic substrates.