New tautomerism suggested by pKa determinations

Abstract

The UV/VIS spectrophotometric pK_a determination of (E)- and (Z)-5-(1-hydroxyiminomethyl)-4-methyl-1,2-dithiole-3-thione is described; for each isomer, the pK_a 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 pK_a^Z 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.