New tautomerism suggested by pKa determinations
Abstract
The UV/VIS spectrophotometric pKa determination of (E)- and (Z)-5-(1-hydroxyiminomethyl)-4-methyl-1,2-dithiole-3-thione is described; for each isomer, the pKa value was obtained by a non-linear least-squares methodology. This methodology was used to take into account some unusual experimental difficulties (very low solubility in water and E→Z isomerization). The pKaZ value (5.13) strongly suggests that the resonant basic Z form is chiefly a thiolate form of a heteropentalene. Spectrophotometric results were confirmed by 13C NMR spectroscopy. A new type of tautomerism is suggested to explain the results.