On the Carbene‐Like Reactions of Imidazolium Acetate Ionic Liquids: Can Theory and Experiments Agree?. Issue 2 (7th October 2018)

• Record Type: Journal Article
• Authors: Gehrke, Sascha; Reckien, Werner; Palazzo, Ivan; Welton, Tom; Hollóczki, Oldamur
• Other Names: Schreiner Peter R. guestEditor.
• Is Part Of: European journal of organic chemistry. Issue 2/3(2019)
• Page Start: 504
• Page End: 511
• Publication Date: 2018-10-07
• Subjects: Ionic liquids -- N‐Heterocyclic carbenes -- Reaction mechanisms -- Solvent effects -- Interfaces; Chemistry, Organic -- Periodicals; Organic compounds -- Synthesis -- Periodicals; Bioorganic chemistry -- Periodicals; Chemistry, Physical organic -- Periodicals; 547
• DOI: 10.1002/ejoc.201801050
• Languages: English
• ISSNs: 1434-193X

Abstract

Abstract : The N‐heterocyclic carbene organocatalytic reactivity of the 1‐ethyl‐3‐methylimidazolium acetate ionic liquid was investigated on the model reaction between this solvent and anisaldehyde. The formation of carbenes by a proton transfer from the cation to the anion was compared to a direct reaction mechanism, in which the proton transfer and the C–C bond formation between catalyst and substrate occurs in a single elementary step. Interestingly, the two reaction mechanisms show a much smaller difference in activation energies than those observed for analogous catalytic systems with neutral bases, showing that the mechanism might switch from one to the other at different temperatures or with different substrates. In this particular case, however, the direct reaction mechanism, avoiding free carbenes in the solution, is apparently more feasible. Based on the detailed analysis of this reaction path, the earlier contradictions between theory and experiments can be resolved, resulting in a consistent mechanistic picture for the related processes. Additionally, we show on the example of a platinum surface that introducing metal probes into the liquid may induce carbene‐like reactions, as the formation of a strong coordinative bond between the carbene and a platinum atom at the surface is highly exothermic, shifting the acid‐base equilibrium considerably. Abstract : NHC‐like reactions of imidazolium acetate ionic liquids have been observed in the last decade. Does this presume the presence of free NHCs, or is there an alternative reaction mechanism? Through an inspection of the contradicting former results, and via further calculations and simulations the disagreements can be explained, and resolved.