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Other Nonnitrogenous Organocatalysts
Published in Andrew M. Harned, Nonnitrogenous Organocatalysis, 2017
In Chapter 8, it was shown how imidazolium salts can serve as catalysts for the formation of homoenolates and related intermediates. This reactivity was predicated on the ability of imidazolium salts, and related compounds, to function as precursors for so-called N-heterocyclic carbenes (NHCs). This section will focus more on the use of NHC catalysts as a means of accessing the reactivity of acyl anion equivalents. The use of NHC catalysts has expanded rapidly over the last 10–15 years, making it difficult to include all this work in just two chapters. The reader is encouraged to seek out one or more reviews45 on this topic for more in-depth coverage. Throughout this chapter, the terms imidazolium salt (or relevant analogs thereof) and NHC may be used interchangeably. It should be remembered that for the most part, the salt is the precatalyst that is actually added to the reaction, but the NHC is what is actually responsible for the reactivity.
Synthesis of N-heterocyclic carbene-based silver complexes and their antimicrobial properties against bacteria and fungi
Published in Journal of Coordination Chemistry, 2022
Murat Kaloğlu, Nazan Kaloğlu, Selami Günal, İsmail Özdemir
N-Heterocyclic carbenes (NHCs) are nitrogen-based heterocyclic compounds containing a divalent carbon atom. Previously, many researchers tried numerous synthetic methods to isolate the stable NHCs, but they were not successful until the first stable free-carbene was isolated in 1991 as a crystal solid by Arduengo and co-workers [11]. Since then, the number of studies in carbene chemistry has increased considerably and has become a staple in research laboratories throughout the world. Today, NHCs are one of the important classes of ligands for coordination chemistry. NHCs have strong σ-donating but weak π-accepting properties, which show excellent support to stabilize various oxidation states of transition-metal. Also, they can provide steric and electronic properties for optimal design of transition-metal complexes [12–18]. The modification at the nitrogen atoms of the NHCs significantly influence the reactivity and binding affinity of the ligand, thus NHCs make strong metal-carbon bond with different metals. Transition metal complexes of NHCs are used as strong-, reactive- and selective-catalysts in many chemical reactions. Initially, the metal-NHC complexes were used extensively as a catalyst in organic transformations such as C-C, C-heteroatom cross-couplings, and C-H functionalization [19–30]. Also, in recent years, transition metal NHC complexes containing Au, Pd, Cu, Ru, Pt, Ag and Rh have been widely used in medicine and pharmacy as potential metallopharmaceutical agents against AMR [31–43].
The effect of short alkane bridges in stability of bisbenzimidazole-2-ylidene silver(I) complexes: synthesis, crystal structure and antibacterial activity
Published in Journal of Coordination Chemistry, 2019
Yen L. Loh, Umie F. M. Haziz, Rosenani A. Haque, A. A. Amirul, O. Noor Aidda, Mohd. R. Razali
Generally, NHC ligands possessed a dominant stability due to their high σ-donor and low π-acceptor ability, hence they can produce stable metal-NHC complexes with strong metal-carbon bonds [13]. This excellent structural advantage gives metal-NHC benefits in various fields such as biological activities and catalytic studies [14–16]. Furthermore, with this ability, Ag(I)-NHC complexes could also achieve a slower release rate of silver ions compared to other coordinated silver complexes or current antimicrobial agents. This thus enhanced the ability of the former complexes to kill the bacteria over a sustained period of time [17]. In addition, the effect of substituent moiety at the N atoms that related to the lipophilicity of the complexes can also affect the antimicrobial activities of the complexes [9, 18, 19]. Recently, structural-activity relationship of Ag(I)-NHC complexes in biological activities were also reported [20, 21].
N-Heterocyclic carbene-Pd(II)-PPh3 complexes as a new highly efficient catalyst system for the Sonogashira cross-coupling reaction: Synthesis, characterization and biological activities
Published in Journal of Coordination Chemistry, 2018
L. Boubakri, L. Mansour, A. H. Harrath, I. Özdemir, S. Yaşar, N. Hamdi
Since the discovery of the first N-heterocyclic metal-carbene complexes in 1968 by the Öfele [1] and Wanzlick [2] groups, these compounds have aroused great interest in the field of coordination chemistry, which has been redoubled after the isolation of the first free-NHC in 1991 by Arduengo [3]. Later studies from Herrmann showed that NHC and phosphine have important similarities in properties, which make them particularly useful ligands for applications in catalysis [4]. However, NHCs are in general more σ-donating and have lower electronic parameters than phosphines. This leads to the remarkable stability of the metal-carbene bond. Different transition metal-NHC complexes have been synthesized [5] and used as catalyst on various organic transformations [6]. The prominence of metal–NHC complexes is likely due to their catalytic efficiency and robustness against air, moisture and high temperature. The basis of this interest lies in the facile modulation of electronic and steric parameters reflected by a strong σ-donor and weak π-acceptor ability of NHC ligand. Complexes bearing sterically bulky and electron-rich ligands exhibit enhanced catalytic activity in oxidative addition and reductive elimination reactions that are key elemental steps of many catalytic reactions [7] (Scheme 1).