Self-Supported N-Heterocyclic Carbenes and Their Use as Organocatalysts

Abstract

Abstract:The study of N-heterocyclic carbenes (NHCs) as organocatalysts has proliferated in recent years, and they have been found to be useful in a variety of reactions. In an attempt to further expand their utility and to study their recyclability, we designed and synthesized a series of self-supported NHCs in which the catalytic carbene groups form part of a densely functionalized polymer backbone, and studied them as organocatalysts. Of the self-Supported NHCs examined, a benzimidazole derived polymer with flexible linkers connecting the catalytic groups was found to be the most efficient organocatalyst in a model benzoin condensation reaction, and thus it was used in a variety of such reactions, including some involving catalyst recycling. Furthermore, it was also used to catalyze a set of redox esterification reactions involving conjugated unsaturated aldehydes. In all of these reactions the catalyst afforded good yield of the desired product and its polymeric nature facilitated product purification.

N-Heterocyclic Carbenes in Materials Chemistry

“…This approach effectively reduces the number of steps, and thus the cost, required to access this class of materials, although it also introduces an element of uncertainty in terms of precise structure. Polyionic liquids are obvious precursors to polymeric NHCs (polyNHCs), which employ derivatives of, for instance, imidazolium, − benzimidazolium, and triazolium salts as their backbones.…”

From Au11 to Au13: Tailored Synthesis of Superatomic Di-NHC/PPh3-Stabilized Molecular Gold Nanoclusters

“…Benzimidazole, 1-benzyl imidazole, 1,3-dibromopropane, 1,3-dichloropropane, isopropyl bromide, benzyl chloride, potassium carbonate, lithium bromide, lithium chloride, (chloro)dimethylsulfide gold(I), tetraethylammonium chloride monohydrate, and ammonium hexafluorophosphate were purchased from Sigma-Aldrich and used as received. [Au 11 (PPh 3 ) 8 Cl 2 ]Cl, 33 1-benzyl benzimidazole, 56 1-isopropyl benzimidazole, 57 1,3-di(1 H -imidazol-yl)propane, 58 and 1,3-di(1- H -benzimidazol-yl)propane 59 were synthesized according to literature procedures. Tetrahydrofuran (THF), acetonitrile (CH 3 CN), dichloromethane (DCM), dichloroethane (DCE), diethyl ether (Et 2 O), methanol (MeOH), n-hexane, and deuterated solvents were purchased from Sigma-Aldrich.…”

Poly(imidazolium) Carbosilane Dendrimers: Synthesis, Catalytic Activity in Redox Esterification of α,β-Unsaturated Aldehydes and Recycling via Organic Solvent Nanofiltration

“…Over the last two decades, the practical application of so called umpolung principle provided access to a wide library of compounds [12,13]. Nevertheless, while many aspects of NHCcatalyzed umpolung reactions, such as activity and selectivity of the catalysts, were studied and optimized, only few studies dealing with the problem of catalyst separation and reuse have been reported so far, moreover, they were limited to benzoin condensation [14][15][16] and Stetter reaction [17].…”