Rapid and efficient diazotization and diazo coupling reactions on silica sulfuric acid under solvent-free conditions

A convenient, rapid, one-pot method for the synthesis of azo dyes has been developed by the sequential diazotization–diazo coupling of aromatic amines with NaNO2, silica sulfuric acid and coupling agents under solvent-free conditions at room temperature. Using this method, several types of aromatic amine, containing electron-withdrawing groups as well as electron-donating groups, were rapidly converted to the corresponding azo dyes in good yield. The ensuing aryldiazonium salts supported on silica sulfuric acid (aryl diazonium silica sulfates), ArN2+−OSO3–SiO2, were sufficiently stable to be kept at room temperature in the dry state. The use of mild reaction conditions and an inexpensive procedure are further advantages of this method.