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. (C) 2008 Elsevier Ltd. All rights reserved.