Enzymatic Knoevenagel condensation in dual-functionalized water-mimicking ionic liquids and tertiary amide solvents

BACKGROUND: Knoevenagel condensation is an important tool for building carbon–carbon (C-C) bonds, especially when catalyzed by enzymes to enable a potentially high chemo-, regio- and/or stereoselectivity. Although many Knoevenagel condensation reactions are carried out in aqueous solutions, insoluble hydrophobic substrates often lead to poor catalytic efficiencies. The use of water-miscible organic solvents improves the substrate solubilization, but usually induces activity suppression or inactivation of enzymes. There is a great need to develop alternative solvents for both substrate dissolution and enzyme compatibility in C-C bond formation reactions. RESULTS: Our group previously developed dual-functionalized water-mimicking ionic liquids (ILs) for the activation and stabilization of hydrolases (e.g. lipase and protease). In the present study, we evaluated the Knoevenagel condensation of 4-chlorobenzaldehyde with acetylacetone, and found that porcine pancreas lipase in water-mimicking ILs carrying ammonium, imidazolium and benzimidazolium cations enabled higher reaction rates (up to 3.22 μmol min⁻¹ g⁻¹ lipase) and better yields than tert-butanol, glymes and [BMIM][Tf₂N]. Interestingly, tertiary amide solvents such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMAc) led to 8.2- to 11.1-fold increases in the initial rate (up to 35.66 μmol min⁻¹ g⁻¹ lipase) when compared with dual-functionalized ILs, which is likely due to some synergistic effect of these tertiary amides with the lipase. CONCLUSION: Dual-functionalized ILs based on ammonium, imidazolium and benzimidazolium cations improved Knoevenagel condensation reaction rates and yields when compared with tert-butanol and glymes. Tertiary amides (NMP, DMF and DMAc) significantly increased the reaction rate.