Nitrogen-15 Nuclear Magnetic Resonance. Structure of Sulfaguanidine

Abstract

Sulfaguanidine (1) is unusual compared to the many derivatives of sulfanilamide (4-aminobenzenesulfonamide) that contain the grouping -SO_2HN- in being insoluble in aqueous alkali. Its 4-amino group is comparably basic to the other sulfanilamides (pK_B = 11.25) but its guanidino group is only very weakly basic (pK_B = 13.52). The nonacidic character of the compound caused Schwenker to investigate its infrared and ^lH NMR spectra and, by comparison of the infrared spectra in KBr pellets with those of several model compounds, it appeared that 1 had no infrared absorption which could be ascribed to the N-H bond of an -SO_2NH- group. The ^1H NMR spectrum in dimethyl sulfoxide solution was less decisive because of serious overlap of the downfield N-H resonances with the resonances of the aromatic ring, but nonetheless, there appeared to be no evidence for the three different kinds of -SO_2NHC(=NH)NH_2 proton resonances predicted for the conventional guanidine structure. It was concluded therefore that the correct structure for sulfaguanidine is not 1a, but instead the tautomer 1b. Schwenker's work seems to have been largely, if not totally, ignored, and as recently as 1975, a ^(13)C investigation formulates sulfaguanidine as 1a in accord with the standard reference works.

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