Quaternary ammonium hydroxide‐functionalized g‐C3N4 catalyst for aerobic hydroxylation of arylboronic acids to phenols

Abstract

AbstractA new and efficient metal‐free approach toward the synthesis of phenols via an aerobic hydroxylation of arylboronic acids by using a novel quaternary ammonium hydroxide g‐C3N4 catalyst has been described. The functionalized quaternary ammonium hydroxide (g‐C3N4‐OH) has been prepared from graphitic carbon nitride (g‐C3N4) scaffold by converting the residual –NH2 and –NH groups to quaternary methyl ammonium iodide by performing a methylation reaction with methyl iodide followed by ion‐exchange with 0.1 N KOH or anion exchange resin Amberlyst A26 (OH‐ form) by post‐synthetic modification. The resultant g‐C3N4‐OH was characterized by XRD, FTIR, field‐emission scanning electron microscope (FESEM), high‐resolution transmission electron microscope (HRTEM), N2 adsorption/desorption isotherms, and acid–base titration. Tested as solid‐base catalysts, the g‐C3N4‐OH showed excellent catalytic activity in the aerobic hydroxylation reaction by converting a variety of arylboronic acids to the corresponding phenols in high yields. More importantly, the g‐C3N4‐OH solid‐base has been successfully reused four times with the minor loss of initial catalytic activity (10.5%).