The 3-thioxo-1,2-dithiol-4-yl group : A versatile one endowed also with a -R electronic effect

The pKa values in an aqueous solution of some 4-aminodithiolethiones, 4-aminodithiolones, 5-phenyl-4-hydroxydithiolethione, and 5-phenyl-4-hydroxydithiolone were determined by UV-VIS spectrophotometry at 298 K. These compounds exhibited unexpectedly very low pKa values in the range of 0–2 for amino derivatives and in the range of 7 for hydroxy derivatives. Electronic structure calculations by ab initio and DFT methods together with an X-ray measurement indicated that these results may be attributed to a markedly negative π charge on the carbon 5 together with a positive π charge on the basic substituent of the carbon 4. As a result, the dithiole nucleus exerts in this case a strong ─R effect. Although they were very low, pK a values of 4-aminodithiolethiones remained still higher than those found with 5-aminodithiolethiones, which were also determined in this work for comparison. The latter were not unexpectedly in the range -3 < pH < −2.