Vat dyes can be quickly and efficiently reduced to their leuco forms by sodium borohydride (NaBH 4) 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 alcoholic solvents.
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