Solvent-free mechanochemical synthesis of azo dyes†

An efficient diazotization of phenolic compounds with aryltriazenes is herein demonstrated by employing ball milling under catalyst-, promoter- and solvent-free conditions. The present protocol offers several advantages including mild conditions, good selectivity and high yields, simple operation and practical gram-scale synthesis. Overall, this novel strategy significantly improves the reaction efficiency, simplifies purification procedures of the diazotization reaction and provides potential for the industrial preparation of azo dyes.