Imidazolium unit

The NHC class of nucleophilic carbenes are also bases. The proton-deuteron exchange of NHCs attached to macromolecules has been studied and the influence of counterion has been explored.99 Substitution, both directly on the imidazolium unit and on the pre-orientating calixarene backbone, was also studied. The results showed that substitution of the imidazolium salts has a large influence on the H-D exchange rates in wet methanol. These results were presented as having implications for Suzuki coupling. 

The homoleptic iron(II) phenoxy functionalised carbene complex was used in the ROP of e-caprolactone [64]. The polymerisation proceeds via a coordination-insertion mechanism [65-67] and the final product has the functionalised imidazolium unit as end group. Performance was riddled by poor control over number-average molecular weight and decreasing molecular weight with decreasing feedstock levels. This was attributed to inter-and/or intramolecular transesteriflcation processes [65,66],... 

Attachment of the catalyst to a polymeric support can also be facilitated using a bis-carbene as the ligand [253]. The bisimidazoUum salt featuring a hydroxyalkyl sidechain on each of the two imidazolium units was reacted with palladium (II) acetate to form the neutral catalyst complex (see Figure 4.82). Immobilisation was then achieved by reacting the functionalised catalyst with 4-(bromomethyl)phenoxy-methyl polystyrene, known as Wang resin, as the polymeric support. 

Counterions of ILs could be facilely exchanged in solution, thus based on a surfactant with an imidazolium unit, the ion-exchange reaction from bromine ion to hexafluorophosphate ion resulted in the amphiphilic-to-hydrophobic transition of the imidazolium... 

Scheme 2.32 Synthesis of monophosphine ligands with an imidazolium unit close to or remote from the phosphine group.

The reaction of the imidazolium salt with [Rh(OMe)(COD)]2 in dichloro-methane produced the supported NHC-rhodium precatalyst. Elemental analysis provided evidence that one rhodium atom is coordinated to each imidazolium unit. In a similar manner, also a binuclear rhodium complex has been synthesized. 

Figure 22.7 Pyrrolidine-derived organocatalysts bearing the imidazolium unit attached to a polymeric matrix or triazolyl fragment.

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