Unexpected Intrinsic Lability of Thiol-Functionalized Carboxylate Imidazolium Ionic Liquids

Abstract

New thiol-functionalized carboxylate ionic liquids (ILs), varying both for the cation and for the anion structures, have been prepared as new potential redox switching systems by reacting either 3-mercapto propionic acid (3-MPA) or N-acetyl-cysteine (NAC) with commercially available methyl carbonate ILs. Different ratios of thiol/disulfide ILs were obtained depending both on the acid employed in the neutralization reaction and on the reaction conditions used. Surprisingly, the imidazolium ILs displayed limited thermal stability which resulted in the formation of an imidazole 2-thione and a new sulfide ionic liquid. Conversely, the formation of the imidazole 2-thione was not observed when phosphonium disulfide ILs were heated, thus confirming the involvement of the imidazolium ring in an unexpected side reaction. An insight into the mechanism of the decomposition has been provided by means of DFT calculations.

Ionic liquid-promoted green synthesis of biologically relevant diaryl thioethers

“…Moreover, the concurrent deactivation of the metal catalyst due to the strong coordinative and adsorptive properties of sulfur, caused a decreased catalytic activity(23). As reported(24), this inconvenient is promoted by non-anhydrous condition and O 2 presence. In order to overcome this drawback the following tests were performed under inert Ar atmosphere.…”

The dialkylcarbonate route to ionic liquids: purer, safer, greener?

“…It was also discovered that, even at moderate temperature (90 °C), the imidazolium ionic liquids decomposed to sulfide-based ionic liquid and imidazole 2-thione. 106…”

“…68,105 The same group also studied 92,93 the synthesis and the stability of thiol-functionalised carboxylate ionic liquids, made through acid/base neutralisation of imidazolium and phosphonium methylcarbonates, with 3-mercapto propionic acid or N-acetyl-cysteine (Table 5, entry 6). 106 Depending on the conditions (bubbling air or inert atmosphere), they obtained the expected carboxylate ionic liquids, or a dicarboxylate disulfide anion. It was also discovered that, even at moderate temperature (90 °C), the imidazolium ionic liquids decomposed to sulfide-based ionic liquid and imidazole 2-thione.…”

“…It was also discovered that, even at moderate temperature (90 °C), the imidazolium ionic liquids decomposed to sulfide-based ionic liquid and imidazole 2-thione. 106 Medoš et al studied the preparation of surfactant ionic liquids and the possibility of scaling up their synthesis. They compared the DMC route to the classical synthesis using alkyl halides to reach hexanoate [hex], octanoate [oct], and decanoate [dec] ammonium carboxylates.…”