Metal-Free Oxidative Coupling of Benzylamines to Imines under an Oxygen Atmosphere Promoted Using Salicylic Acid Derivatives as Organocatalysts

The oxidative coupling of benzylamines proceeds efficiently using salicylic acid derivatives as organocatalysts under an oxygen atmosphere, affording the corresponding N -benzylidenebenzylamines in high yields. Electron-rich salicylic acid derivatives such as 4,6-dimethoxysalicylic acid and 4,6-dihydroxysalicylic acid exhibit excellent catalytic activities for the oxidative coupling of benzylamines to give the corresponding imines. This amine oxidation can also be applied to the synthesis of nitrogen-containing heterocycles such as benzimidazole derivatives. Furthermore, to recycle the catalyst, silica gel supported with 4.7 wt % of 4,6-dihydroxysalicylic acid is prepared, which acts as a recyclable catalyst, oxidizing benzylamine to imine four times successfully.