TiO₂-Catalyzed Borylation of Haloarenes under Visible Light and Synthesis of Benzoxaboroles

A new visible-light (390 nm)-induced photocatalytic generation of aryl radicals from haloarenes and subsequent borylation reactions in the presence of TiO₂ nanoparticles is reported. Substituted boronic acids are precursors for Suzuki coupling (C-C bond) and Chan-Lam (C-N bond) reactions. The familiar boron-bearing drug molecules are tavaborole, crisaborole, bortezomib, ganfeborole, and ixazomib. TiO₂ is a pharmaceutically accepted excipient, and it acted as a heterogeneous photocatalyst for the generation of aryl radicals from haloarenes, followed by reaction with bis(pinacolato)diboron to get the desired boronate compounds with moderate to very good yields (up to 84%). The catalyst was filtered and recycled (five cycles). A new reaction mechanism was proposed, and the radical intermediate was trapped by 2,2,6,6-tetramethyl-1-piperidinyloxy. This method was successfully applied for one-pot borylation and reduction of aldehydes, followed by cyclization under an acidic medium for the synthesis of benzoxaboroles.