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.
References
1.
Gribble, G.W. , Lord, P.D., Skotnicki, J., Dietz, S.E., Eaton, J.T., and Johnson, J.L., Reactions of Sodium Borohydride in Acidic Media. I. Reduction of Indoles and Alkylation of Aromatic Amines with Carboxylic Acids, J. Amer. Chem. Soc.96, 7812 (1974).
2.
Kolthoff, I.M. , Chantooni, M.K., Jr., and Bhowmik , S., Dissociation Constants of Uncharged' and Monovalent Cation Acids in Dimethyl Sulfoxide, J. Amer. Chem. Soc.90, 23 (1968).
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Kreevoy, M.M. and Abts, L.M., "The Role of Water in the Hydrolysis of BH3-," Abstracts of Papers, 167th National Meeting, American Chemical Society, March 31-April 5, 1974, PHYS 088.
4.
Kreevoy, M.M. and Hutchins, J.E.C., H2BH3 as an Intermediate in Tetrahydridoborate Hydrolysis, J. Amer. Chem. Soc.94, 6371-6376 (1972).
5.
Kreevoy, M.M. and Taft, R.W., Jr., The Evaluation of Inductive and Resonance Effects in Reactivity. III. Extra Resonance Energies of Conjugation, J. Amer. Chem. Soc.79, 4016 (1957).
6.
Levine, L.A. and Kreevoy, M.M., SolventIsotope Effects on Tetrahydridoborate Hydrolysis, J. Amer. Chem. Soc.94, 3346-3349 (1972).
7.
Nair, G.P. and Shah, R.C., Sodium Borohydride in Vat Dyeing, Textile Res. J.40, 303-312 (1970).
8.
"Society of Dyers and Colourists Colour Index," 2nd ed., Vol. 2, The Society of Dyers and Colourists and the American Association of Textile Chemists and Colorists, Lowell Technological Institute, Lowell, Mass., 1956, pp. 2524-2554.
