N-Heterocyclic carbenes (NHCs) as organocatalysts and structural components in metal-free polymer synthesis

Abstract

The chemistry of N-heterocyclic carbenes (NHCs) has witnessed tremendous development in the past two decades: NHCs have not only become versatile ligands for transition metals, but have also emerged as powerful organic catalysts in molecular chemistry and, more recently, in metal-free polymer synthesis. To understand the success of NHCs, this review first presents the electronic properties of NHCs, their main synthetic methods, their handling, and their reactivity. Their ability to activate key functional groups (e.g. aldehydes, esters, heterocycles, silyl ketene acetals, alcohols) is then discussed in the context of molecular chemistry. Focus has been placed on the activation of substrates finding analogies with monomers (e.g. bis-aldehydes, multi-isocyanates, cyclic esters, epoxides, N-carboxyanhydrides, etc.) and/or initiators (e.g. hydroxy- or trimethylsilyl-containing reagents) employed in such "organopolymerisation" reactions utilizing NHCs. A variety of metal-free polymers, including aliphatic polyesters and polyethers, poly(α-peptoid)s, poly(meth)acrylates, polyurethanes, or polysiloxanes can be obtained in this way. The last section covers the use of NHCs as structural components of the polymer chain. Indeed, NHC-based photoinitiators, chain transfer agents or functionalizing agents, as well as bifunctional NHC monomer substrates, can also serve for metal-free polymer synthesis.

Advances in the Knowledge of N-Heterocyclic Carbenes Properties. The Backing of the Electrochemical Investigation

“…• imidazolium cations (as 2a-b, Figure 7) are the least acidic azolium cations (pK a = 22-24 in water and 22 in DMSO), while triazolium (as 2c) and thiazolium ones (as 2d-f) are the most acidic (pK a = 16-19 in water and 14 in DMSO) [4,49]; and • the nucleophilicity of imidazole NHC (as 1a-b, Figure 7) is three orders of magnitude greater than the thiazole based NHC (as 1d-f) [50].…”

“…The considerable role of NHCs as ligands for transition metals [3] and as catalysts in metal-free polymer synthesis [4] has been frequently emphasized. In addition, NHCs are often utilized as green and cheap organocatalysts in many C-C bond-forming reactions (benzoin condensation, Stetter reaction, etc.)…”

Metal‐Free Catalyst for the Chemoselective Methylation of Amines Using Carbon Dioxide as a Carbon Source

“…Notably, N-heterocyclic carbenes (NHCs) have attracted much interest, [15] as they can behave as nucleophiles and may activate CO 2 to form imidazolium carboxylates. [16] The propensity of NHCs to form carboxylates could be exploited to give metal-free catalysts for the transformation of CO 2 .…”

Enantiopure C1-symmetric N-Heterocyclic Carbene Ligands from Desymmetrized meso-1,2-Diphenylethylenediamine: Application in Ruthenium-Catalyzed Olefin Metathesis

“…The design of NHCs with tailored steric and electronic properties has enabled rapid development of highly efficient metal catalysts in a wide variety of academically and industrially important processes [1][2][3][4]. Among them, asymmetric transformations mediated by metal catalysts bearing NHCs with chiral architectures have attracted increasing interest, and intensive research efforts have permitted the obtainment of highly stereoselective catalysts [5,6].…”