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.

Opportunely, we also found that VO(Hhpic)2 could also oxidize benzylamines to the corresponding imines with high selectivity using an ionic liquid solvent under an atmosphere of O2 (0.1 MPa), also with good recyclability (Scheme 2). 12(c)

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Amines are transformed into the corresponding imines by environmentally benign catalytic oxidation reactions. Gaseous oxygen or hydrogen peroxide is used as the oxidant, and water is the only byproduct. When a vanadium complex is used as the catalyst in an ionic liquid, the amine oxidation successfully proceeds with recycling of the catalyst. Amine oxidation with hydrogen peroxide as an oxidant in water is also attained by using copper(II) sulfate as catalyst. In addition, photoinduced oxidation of amines to imines is conducted by using oxygen as the oxidant in the presence of a zinc-chlorin complex as catalyst.

A catalytic oxidation reaction for Acid Blue 7 dye synthesis was evaluated in water. Without lead oxide or manganese oxide derivatives as oxidants, polyoxometalate catalysts were investigated to reduce the usage of harmful heavy metal. A catalyst was prepared by mixing silicotungstic acid with copper oxide, and aqueous hydrogen peroxide (30%) was used as an oxidizing agent. This reaction proceeded to produce Acid Blue 7 from the corresponding leuco acid after 45 min at 95 °C and was viable for a 10 g-scale synthesis.