Acid-Catalyzed Borohydride Reduction of Vat Dyes

Vat dyes can be quickly and efficiently reduced to their leuco forms by sodium borohydride (NaBH₄) in acidic dimethyl sulfoxide (DMSO) at room temperature. Trifluoromethanesulfonic acid, methanesulfonic acid, or sulfuric acid were used to keep the pH ∼5 during the reaction. The acid is thought to catalyze the reaction by protonating the dye carbonyl groups, increasing their susceptibility to attack by hydride donors. DMSO protects the BH₄^-, which is rapidly decom posed by acid in aqueous or alcoholicsolvents.