Imidazolium Based Fluorous N‐Heterocyclic Carbenes as Effective and Recyclable Organocatalysts for Redox Esterification

Abstract

A series of new highly fluorophilic ionic liquids (f > 110) was synthetized from 3‐iodopropyltris(3,3,4,4,5,5,6,6,7,7,8,8,8‐tridecafluorooctyl)silane and N‐alkyl imidazoles, followed by anion exchange. N‐heterocyclic carbenes generated in situ from obtained imidazolium salts were employed to catalyze redox esterification (umpolung) of cinnamaldehyde with alcohols. The most effective N‐methyl derivative with iodide as a counter anion was studied in detail with respect to the optimization of reaction conditions, substrate scope and recyclability. Recovery of the precatalyst was achieved using either fluorous extraction or performing the reaction in suitable fluorous biphase system with direct recycling of the fluorinated precatalyst phase. For both tested options, the catalytic activity did not significantly decrease within 5 subsequent cycles. The redox esterification was shown to proceed also in supercritical carbon dioxide (scCO2) as an alternative solvent where the activity of the fluorinated catalyst was also superior to the nonfluorinated model, while retaining the benefit of easy recycling.

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

“…To our delight, the third model G 0 DippImI bearing a sterically demanding 2,6-diisopropylphenyl (Dipp) group manifested by far the highest activity, reaching the full conversion of the starting aldehyde in less than 20 min (entry 8) and confirming the previously observed positive influence of bulky aryl substituents on azolium on the catalyst performance [45]. All three models showed high selectivity for the formation of saturated ester and only small quantities of byproducts 4 and 5a reported previously to accompany this reaction [20] were detected by GC (Table 1). Exploiting the unprecedented high activity of G 0 DippImI, we examined the influence of the reaction temperature (Figure 1a) and decrease in catalyst loading.…”

“…Preliminary experiments were performed at 100 • C in toluene, with 10 mol% catalyst loading and 10 mol% of the base (DBU); dioctyl phthalate was added as an inert standard to enable accurate quantification of GC-FID data. As evident from entries 1 and 5, the catalysts generated from methyl-and iso-propyl-substituted model imidazolium salts G 0 MeImI and G 0 iPrImI by deprotonation showed similar performance to previously described fluorinated analogs, for which also a superior activity of the methyl-substituted compound was observed [20]. To our delight, the third model G 0 DippImI bearing a sterically demanding 2,6-diisopropylphenyl (Dipp) group manifested by far the highest activity, reaching the full conversion of the starting aldehyde in less than 20 min (entry 8) and confirming the previously observed positive influence of bulky aryl substituents on azolium on the catalyst performance [45].…”

“…Tert-butyl alcohol and phenol gave only traces of products. The lack of G 0 DippImI activity towards phenol contradicts our previous findings, as we have previously obtained fair yields of phenyl ester under the catalysis by fluorinated methylimidazolylidene [20]. A deeper insight into the complex reaction mechanism is needed to understand this unexpected behavior; a mechanistic study addressing the issue is currently underway.…”

“…In cases where the selectivity was fair but a large decrease in the reaction rate was observed when going from the model to dendritic catalysts, also the experiments with higher catalyst loading (10 mol% of active sites and 10% of DBU) were carried out. The reaction progress was monitored by CG-FID with sampling after 5, 24 and 100 h. In our recent study utilizing fluorous imidazolium as a precatalyst we have observed that the use of secondary alcohols led to a slight decrease in the reaction rate, yield and selectivity [20]. Compared to those fluorinated NHCs, Dipp substituted organocatalysts demonstrated much higher sensitivity to the structure of alcohol, even if the trends were similar.…”

“…Recently we developed a branched fluorous tag designed for easy fluorophilization of various substrates [18,19]. Highly fluorophilic imidazolium salts bearing this tag were used as NHC precursors and applied in redox esterification carried out in fluorous biphase system, which allowed for their effective recycling [20]. Such solution allows the reaction to proceed in fully homogeneous mode, but it is unsuitable for continuous processes.…”