A rapid and practical direct α-halogenation of carboxylic acids with N-bromosuccinimide (NBS) or trichloroisocyanuric acid (TCCA) using catalytic p-toluenesulfonic anhydride

A rapid and practical direct α-halogenation of carboxylic acids was developed using N-bromosuccinimide (NBS) or trichloroisocyanuric acid (TCCA) in the presence of catalytic amounts of p-toluenesulfonic anhydride (Ts2O) and H₂SO₄. The protocol proceeds without prior conversion to acyl chlorides and is applicable to bromination and chlorination. Compared with our previous system, the present method significantly shortens reaction times and facilitates product isolation. A broad range of carboxylic acids undergo smooth α-halogenation in good to excellent yields, and the reaction can also be performed under solvent-free conditions. The operational simplicity, scalability, and improved efficiency highlight the synthetic utility of this transformation.