Quaternary ammonium hydroxide‐functionalized g‐C 3 N 4 catalyst for aerobic hydroxylation of arylboronic acids to phenols

A 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‐C 3 N 4 catalyst has been described. The functionalized quaternary ammonium hydroxide (g‐C 3 N 4 ‐OH) has been prepared from graphitic carbon nitride (g‐C 3 N 4 ) scaffold by converting the residual –NH 2 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‐C 3 N 4 ‐OH was characterized by XRD, FTIR, field‐emission scanning electron microscope (FESEM), high‐resolution transmission electron microscope (HRTEM), N 2 adsorption/desorption isotherms, and acid–base titration. Tested as solid‐base catalysts, the g‐C 3 N 4 ‐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‐C 3 N 4 ‐OH solid‐base has been successfully reused four times with the minor loss of initial catalytic activity (10.5%).