N-Functionalization of 1,2-Azaborines

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 Nα position. Due to the incorporation of the second Nβ 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.

Synthesis, Structure, and Properties of Two Planar BN-Benzofluorenes

Two planar BN doped benzofluorenes (BN-BkF and BN-BjF) were synthesized separately through palladium-catalyzed intramolecular C-C or C-N coupling reactions. The structures of both BN-BkF and BN-BjF are unambiguously confirmed by X-ray crystallographic analysis. Moreover, the BN unit doping leads to a much lower HOMO level and higher LUMO level as compared to their carbon analogues BkF and BjF. In comparison to BkF and BjF, both BN-BkF and BN-BjF exhibit blue shifts in their ultraviolet-visible (UV-vis) absorption and fluorescence emission spectra. Furthermore, halogenation of BN-BkF and BN-BjF afforded monohalogenated BN-benzofluorenes in good yields. These monohalogenated BN-benzofluorenes can serve as convenient intermediates for palladium-catalyzed coupling reactions to yield a series of functionalized BN-benzofluorene derivatives. The UV-vis absorption and emission spectroscopies in dichloromethane of these BN-benzofluorene derivatives were studied, and the photophysics of these compounds exhibited a high degree of substituent dependency.

Palladium-Catalyzed Atroposelective Kinetic C-H Olefination and Allylation for the Synthesis of C-B Axial Chirality

The direct C-H functionalization of 1,2-benzazaborines, especially asymmetric version, remains a great challenge. Here we report a palladium-catalyzed enantioselective C-H olefination and allylation reactions of 1,2-benzazaborines. This asymmetric approach is a kinetic resolution (KR), providing various C-B axially chiral 2-aryl-1,2-benzazaborines and 3-substituted 2-aryl-1,2-benzazaborines in generally high yields with excellent enantioselectivities (selectivity (S) factor up to 354). The synthetic potential of this reaction is showcased by late-stage modification of complex molecules, scale-up reaction, and applications.