Trihaloisocyanuric Acids as Atom-Economic Reagents for Halogenation of Aromatics and Carbonyl Compounds in the Solid State by Ball Milling

It is shown that both tribromoisocyanuric acid (TBCA) and trichloroisocyanuric acid (TCCA) are excellent reagents for direct halogenation of electron-rich aromatics under solvent-free, ball-milling conditions. The results are remarkable in light of the fact that chlorination of aromatics without any acid catalyst is heretofore unknown. The procedure is operationally simple, and the reactions occur within a few hours, affording high yields of halogenated products. Similarly, crystalline -keto dicarbonyl compounds are shown to undergo direct ,-dihalogenation with TBCA and TCCA with ball milling. Whereas monoketones are found to be unreactive with TBCA/TCCA, addition of 25 mol-% of p-toluenesulfonic acid as a solid catalyst catalyzes the reaction, leading to the formation of -chloro/bromo ketones in very good isolated yields. The fact that each of the reagents, in principle, serves as a source of three halogen atoms should render solvent-free halogenation of aromatics and carbonyl compounds with TBCA/TCCA an invaluable synthetic protocol.