Electrogenerated NHCs in Organic Synthesis: Ionic Liquids vs Organic Solvents Effects

Abstract

In the last twenty years, N‐heterocyclic carbenes (NHCs) have been extensively studied for their application as organocatalysts in stereoselective synthesis as well as ligands for transition metals‐promoted synthetic methodologies. Derived mainly from azolium salts, NHCs have demonstrated exceptional versatility in their generation usually performed by deprotonation or reduction (chemical or electrochemical). In particular, the generation of NHC under electrochemical conditions, starting from azolium‐based ionic liquids, has proven to be a successful green approach and demonstrated wide applicability in organic synthesis. In this Personal Account, the application of electrogenerated NHCs in organic synthesis will be discussed, with a particular attention to the different reactivity in ionic liquids compared to classical organic solvents.

Electrochemical Generation of N‐Heterocyclic Carbenes for Use in Synthesis and Catalysis

“…Many azolium salts, depending on the N ‐substituents, are liquid at room temperature and can be classified as ionic liquids (IL). ILs have become a popular choice of electrochemical solvent due to the high thermal stability, non‐volatility, high polarity, high intrinsic conductivity, wide electrochemical windows and recyclability [9a,b,d,e,21] . In addition, intermediate radicals are often stabilized by the azolium [9d] .…”

“…Modification to include a reference electrode or to separate anodic and cathodic reactions with a membrane are possible. [1 ° ,2a,2h] Continuous electrochemical reactors have also been developed [1s–u,2i,m,20,2p] …”

Recent Advances in Imidazolium-Based Dicationic Ionic Liquids as Organocatalysts: A Mini-Review

“…ILs are considered, in most cases, green solvents, inert, easy to reuse, and with very low vapor pressure, reducing the possibility of air pollution, typical of common organic solvents. The wide choice of anions and cations gives the possibility to select the appropriate ionic liquid for a specific application [ 3 ]. In recent years, there has been an increased need to use organic synthesis methods that moderate the problem of environmental pollution.…”

“…DILs exhibit greater thermal and chemical stabilities, viscosity, flexibility, and lower volatility with respect to the corresponding monocationic ILs (see below). Moreover, the possibility to transform imidazolium ionic liquids (ILs and DILs) into the corresponding N-heterocyclic carbenes by electrochemical deprotonation, thus avoiding the use of any chemical base and the production of the corresponding byproducts [ 3 ], renders the use of these compounds even more attractive from the green chemistry point of view. For all these reasons, DILs show a growing interest in the field of catalysis.…”

Organocatalyst Design for the Stereoselective Annulation towards Bicyclic Diketones and Analogues

“…Organocatalytic procedures offer several advantages: the stability of these catalysts in the presence of air and moisture compared to metalcomplexes allows low-demanding reaction conditions; organocatalysts are usually cheaper than enzymes or metal-based catalysts and readily available; different activation modes, often simultaneous, are possible with respect to substrates, reagents, and reactions. Within this context, organocatalysis has been employed in many strategies to achieve enantioenriched chiral compounds with potential bioactivity and optically pure building blocks for the preparation of natural products [8][9][10][11][12][13][14].…”