A Vicinal Electrophilic Diborylation Reaction Furnishes Doubly Boron-Doped Polycyclic Aromatic Hydrocarbons

Abstract

Ten examples of unsymmetrically benzannulated, boron-doped polycyclic aromatic hydrocarbons (B-PAHs) were prepared by a one-pot protocol using 4,5-dichloro-1,2-bis(trimethylsilyl)benzene (1), BBr₃ , and selected PAHs-among them anthracene, benzo[a]pyrene, biphenylene, and fluoranthene. After mesitylation at the boron centers, the resulting air- and water-stable products were investigated by ¹ H/¹¹ B{¹ H}/¹³ C{¹ H} NMR spectroscopy, X-ray crystallography, cyclic voltammetry, and UV/Vis absorption/emission spectroscopy. The experiments were augmented by DFT calculations. Most of the B-PAHs are brightly luminescent (Φ_PL up to 90 %) and undergo reversible reduction at moderate half-wave potentials. The two chloro substituents of 1 are not only mandatory for accomplishing efficient diborylation, but can subsequently be used for Stille-type coupling reactions to introduce 2-thienyl moieties. Alternatively, Cl/H exchange is achievable with HSiEt₃ in a quantitative, Pd-catalyzed transformation.

Keywords: arenes; aromatic substitution; boron; electrophilic substitution; fluorescence spectroscopy.