Synthesis, Structure, and Reactivity of Anionic sp2-sp3 Diboron Compounds: Readily Accessible Boryl Nucleophiles

Lewis base adducts of tetra-alkoxy diboron compounds, in particular bis(pinacolato)diboron (B(2)pin(2)), have been proposed as the active source of nucleophilic boryl species in metal-free borylation reactions. We report the isolation and detailed structural characterization (by solid-state and solution NMR spectroscopy and X-ray crystallography) of a series of anionic adducts of B(2)pin(2) with hard Lewis bases, such as alkoxides and fluoride. The study was extended to alternative Lewis bases, such as acetate, and other diboron reagents. The B(sp(2))-B(sp(3)) adducts exhibit two distinct boron environments in the solid-state and solution NMR spectra, except for [(4-tBuC(6)H(4)O)B(2)pin(2)](-), which shows rapid site exchange in solution. DFT calculations were performed to analyze the stability of the adducts with respect to dissociation. Stoichiometric reaction of the isolated adducts with two representative series of organic electrophilesnamely, aryl halides and diazonium saltsdemonstrate the relative reactivities of the anionic diboron compounds as nucleophilic boryl anion sources.