Chiarotto Isabella, Mattiello Leonardo, Pandolfi Fabiana, Rocco Daniele, Feroci Marta
Dipartimento di Scienze di Base e Applicate per l'Ingegneria, Sapienza University of Rome, Rome, Italy.
Front Chem. 2018 Aug 28;6:355. doi: 10.3389/fchem.2018.00355. eCollection 2018.
Ionic liquids (ILs) are considered in the majority of cases green solvents, due to their virtually null vapor pressure and to the easiness in recycling them. In particular, imidazolium ILs are widely used in many fields of Chemistry, as solvents or precursors of N-heterocyclic carbenes (NHCs). The latter are easily obtained by deprotonation of the C2-H, usually using strong bases or cathodic reduction. Nevertheless, it is known that weaker bases (e.g., triethylamine) are able to promote C2-H/D exchange. From this perspective, the possibility of deprotonating C2-H group of an imidazolium cation by means of a basic counter-ion was seriously considered and led to the synthesis of imidazolium ILs spontaneously containing NHCs. The most famous of this class of ILs are N,N'-disubstituted imidazolium acetates. Due to the particular reactivity of this kind of ILs, they were appointed as "organocatalytic ionic liquids" or "proto-carbenes." Many papers report the use of these imidazolium acetates in organocatalytic reactions (i. e., catalyzed by NHC) or in stoichiometric NHC reactions (e.g., with elemental sulfur to yield the corresponding imidazole-2-thiones). Nevertheless, the actual presence of NHC in N,N'-disubstituted imidazolium acetate is still controversial. Moreover, theoretical studies seem to rule out the presence of NHC in such a polar environment as an IL. Aim of this Mini Review is to give the reader an up-to-date overview on the actual or potential presence of NHC in such an "organocatalytic ionic liquid," both from the experimental and theoretical point of view, without the intent to be exhaustive on N,N'-disubstituted imidazolium acetate applications.
在大多数情况下，离子液体（ILs）被认为是绿色溶剂，这是由于它们几乎为零的蒸气压以及易于回收利用。特别是咪唑鎓离子液体在化学的许多领域中被广泛用作溶剂或N - 杂环卡宾（NHCs）的前体。后者通常通过使用强碱或阴极还原对C2 - H进行去质子化而容易获得。然而，已知较弱的碱（例如三乙胺）能够促进C2 - H/D交换。从这个角度来看，人们认真考虑了通过碱性抗衡离子使咪唑鎓阳离子的C2 - H基团去质子化的可能性，并导致了自发含有NHCs的咪唑鎓离子液体的合成。这类离子液体中最著名的是N，N'-二取代的咪唑鎓乙酸盐。由于这类离子液体的特殊反应性，它们被指定为“有机催化离子液体”或“原卡宾”。许多论文报道了这些咪唑鎓乙酸盐在有机催化反应（即由NHC催化）或化学计量的NHC反应（例如与元素硫反应生成相应的咪唑 - 2 - 硫酮）中的应用。然而，N，N'-二取代的咪唑鎓乙酸盐中NHC的实际存在仍然存在争议。此外，理论研究似乎排除了在离子液体这样的极性环境中存在NHC的可能性。本综述的目的是从实验和理论的角度，让读者对这种“有机催化离子液体中NHC的实际或潜在存在有一个最新的概述，而无意详尽阐述N，N'-二取代的咪唑鎓乙酸盐的应用。
