The thiocarbonyl ‘S’ is softer than thiolate ‘S’: A catalyst-free one-pot synthesis of isothiocyanates in water

Abstract

Treatment of the preformed or the in situ generated aryl/alkyl dithiocarbamates triethylammonium salt (ArNHCSS⁻.Et₃NH⁺) with methyl acrylate in an aqueous medium gave solely arylisothiocyanate (ArNCS), whereas the in situ generated aryl dithiocarbamic acid (ArNHCSS⁻.H⁺) yielded exclusively the thia-Michael adduct (ArNHCSSCH₂CH₂COOMe). This differential reactivity can be explained by two alternative mechanisms which is dependent both on the nature of the counter cation and on the pH of the reaction medium. Irrespective of the counter cations, the thiocarbonyl sulfur (=S) atom, having large orbital-coefficient, is softer compared to the thiol/thiolate sulfur (-SH/S⁻) in a dithiocarbamate salt and the former adds to the Michael acceptor by a 1,4-addition.