Visible light induced generation of aryl radicals from haloarenes using a TiO2 as a heterogeneous photocatalyst and synthesis of boron compounds

A new visible light (390nm) induced photocatalytic borylation method in presence of TiO2 is reported. Substituted boronic acids are starting materials for C-C bond forming Suzuki coupling and C-N bond forming Chan-Lam 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 heterogenous photocatalyst, for generation of aryl radicals from haloarenes and followed by reaction with bis(pinacoloto)diboron to get the desired boronate compounds with moderate to very good yields (upto 84%). The catalyst was recycled twice, a new reaction mechanism was proposed and the radical intermediated was trapped by 2,2,6,6-tetramethyl-1-piperidinyloxy. This method was successfully applied for the synthesis of antifungal drug tavaborole and a key starting material used for synthesis of crisaborole.