Xanthopinacol Boronate: A Robust, Photochemically Assembled and Cleavable Boronic Ester for Orthogonal Chemistry

Organoboronic acids impact numerous fields of application from organic synthesis to materials science and drug discovery. However, their highly polar nature makes them challenging to handle, purify, and characterize. Boronic esters help overcome these issues, and pinacol esters (Bpin) have become the dominant boronic acid surrogate in organic synthesis. Despite its popularity, Bpin is not without drawbacks as it is intrinsically reversible in the presence of water or alcohols, which may cause issues of premature release leading to losses during reactions and purification. This reversibility complicates the hydrolytic regeneration of free boronic acids, which often requires additional steps to destroy the pinacol by-product. Although other boronyl protecting groups exist, their cleavage to afford the boronic acid tends to require harsh pH conditions. To address these issues, we developed xanthopinacol boronates (Bxpin), a robust protecting group for boronic acids with excellent orthogonality in various chemical reactions and mild irreversible removal. Xpin boronates can be obtained directly by irradiation of a mixture of free boronic acid and xanthone with UV light, causing an in-situ dimerization of xanthone to the required xanthopinacol. The unique attributes of xpin boronates were further studied by UV-Vis spectrophotometry, X-ray crystallography, cyclic voltammetry, and various stability tests.