Abstract
Ionic liquids (ILs) are a convenient and inexpensive source of N-heterocyclic carbenes (NHCs). In this study, dialkyl imidazolium-based ILs are used as carbene precursors in a four-component synthesis of oxo triphenylhexanoates (OTHOs), where it was found that IL outperformed commonly used NHC precatalysts in terms of reaction efficiency. The reaction is highly stereoselective, delivering the anti-diastereomer (20:1 dr), and the OTHOs can be obtained in high-to-excellent yields. By virtue of the four-component reaction-setup, facile construction of the OTHO scaffold with a diverse set of functional groups (21 examples) can be achieved. In the context of sustainability, the IL can be recovered and reused several times without affecting selectivity or yield. Moreover, most compounds can be isolated by precipitation and filtration, mitigating the use of solvent-demanding chromatography.
Highlights
The separation, recovery, and reuse of a catalyst are major obstacles within homogeneous catalysis.In this respect, ionic liquids (ILs) have been shown to be an ideal medium for covalent and non-covalent catalyst immobilization and regeneration, due to their low vapor pressure [1,2,3,4,5,6]
Other activation strategies can be invoked by deprotonation of dialkyl imidazolium-based ILs with a base, generating an N-heterocyclic carbene (NHC) (Figure 1a)
The reaction between acetophenone, benzaldehyde, cinnamaldehyde, and methanol was chosen as a model reaction to investigate multicomponent oxo triphenylhexanoates (OTHOs) synthesis
Summary
The separation, recovery, and reuse of a catalyst are major obstacles within homogeneous catalysis. In this respect, ionic liquids (ILs) have been shown to be an ideal medium for covalent and non-covalent catalyst immobilization and regeneration, due to their low vapor pressure [1,2,3,4,5,6]. ILs themselves can be used as catalysts [7,8] This has been exemplified in several syntheses of heterocycles [9], cycloadditions [10], and acetylations of alcohols [11,12]. Other activation strategies can be invoked by deprotonation of dialkyl imidazolium-based ILs with a base, generating an N-heterocyclic carbene (NHC) (Figure 1a)
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