Visible Light-Induced Borylation of C–O, C–N, and C–X Bonds

Boronic acids are centrally important
functional motifs and synthetic
precursors. Visible light-induced borylation may provide access to
structurally diverse boronates, but a broadly efficient photocatalytic
borylation method that can effect borylation of a wide range of substrates,
including strong C–O bonds, remains elusive. Herein, we report
a general, metal-free visible light-induced photocatalytic borylation
platform that enables borylation of electron-rich derivatives of phenols
and anilines, chloroarenes, as well as other haloarenes. The reaction
exhibits excellent functional group tolerance, as demonstrated by
the borylation of a range of structurally complex substrates. Remarkably,
the reaction is catalyzed by phenothiazine, a simple organic photocatalyst
with MW < 200 that mediates the previously unachievable visible
light-induced single electron reduction of phenol derivatives with
reduction potentials as negative as approximately – 3 V versus
SCE by a proton-coupled electron transfer mechanism. Mechanistic studies
point to the crucial role of the photocatalyst–base interaction.