Task-Specific Properties and Prospects of Ionic Liquids in Cross-Coupling Reactions

Ionic liquids (ILs) are considered as highly useful materials for potential diverse uses such as greener and more convenient alternatives to volatile organic solvents, reagents, additives, ligands and co-solvents. Thermal stability, negligible vapor pressure and high polarity with ionic environments have possibly conferred some unique physico-chemical properties and a wider electrochemical window on ILs. More importantly, these properties are tuneable, depending on variations in alkyl chains and counter-anions. On the other hand, various transition-metal-catalyzed cross-coupling reactions constitute an important backbone of contemporary organic synthesis. A vast number of C-C and C-heteroatom cross-coupling reactions are reported in the presence of ILs, often showing better performance. The influence of IL on the action of a given catalyst or on the course of a reaction can be relatively complex, and is not understood well enough to be able to draw succinct conclusions. However, there are a few reports in the literature that help understand the role of actual and active catalytic species stabilized in an IL environment. Stabilization, which can be either helpful or detrimental to catalysis depends on specific circumstances. This review article is aimed primarily at summarizing the various applications of ILs during the past decade, focusing as far as possible on the task-specific properties of ILs in transition-metal-catalyzed C-C and C-heteroatom cross-coupling reactions. Several successful achievements and noteworthy progress in this field of research leads to the sensible conclusion that future prospects in this field of research are not only bright but promise new horizons.

Efficient in situ N-heterocyclic carbene palladium(ii) generated from Pd(OAc)2 catalysts for carbonylative Suzuki coupling reactions of arylboronic acids with 2-bromopyridine under inert conditions leading to unsymmetrical arylpyridine ketones: synthesis, characterization and cytotoxic activities

N,N-Substituted benzimidazole salts were successfully synthesized and characterized by ¹H-NMR, ¹³C {¹H} NMR and IR techniques, which support the proposed structures. Catalysts generated in situ were efficiently used for the carbonylative cross-coupling reaction of 2 bromopyridine with various boronic acids. The reaction was carried out in THF at 110 °C in the presence of K₂CO₃ under inert conditions and yields unsymmetrical arylpyridine ketones. All N,N-substituted benzimidazole salts 2a–i and 4a–i studied in this work were screened for their cytotoxic activities against human cancer cell lines such us MDA-MB-231, MCF-7 and T47D. The N,N-substituted benzimidazoles 2e and 2f exhibited the most cytotoxic effect with promising cytotoxic activity with IC₅₀ values of 4.45 μg mL⁻¹ against MDA-MB-231 and 4.85 μg mL⁻¹ against MCF7 respectively.

The in situ prepared four component system Pd(OAc)₂, 1,3-dialkylbenzimidazolium halides 2a–i and 4a–i, K₂CO₃ under CO atmosphere catalyses carbonylative cross-coupling reaction of 2-bromopyridine with various boronic acids to yield unsymmetrical arylpyridine ketones.

Route to Benzimidazol-2-ones via Decarbonylative Ring Contraction of Quinoxalinediones: Application to the Synthesis of Flibanserin, A Drug for Treating Hypoactive Sexual Desire Disorder in Women and Marine Natural Product Hunanamycin Analogue

A simple and practical method to access a variety of benzimidazol-2-ones is reported here. A series of N-alkyl-substituted benzimidazol-2-ones were synthesized by decarbonylative ring contraction starting from corresponding quinoxalinediones for the first time. The utility of the method has been demonstrated by synthesizing recently approved controversial drug flibanserin (Addyi) and a urea analogue of marine antibiotic natural product hunanamycin-A.