Proton-Exchange Equilibrium between Bases and [BMIm][BF4]: An Electrochemical Procedure to Evaluate the Presence of Carbenes for Synthetic Applications

The ability of a base to yield an N-heterocyclic carbene (NHC) in a base-ionic-liquid solution is investigated by using an electrochemical procedure. Accordingly, the equilibrium of proton exchange between different bases (1,5-diazabicyclo[5.4.0]undec-5-ene, 1,1,2,3,3-pentaisopropylguanidine, Cs2CO3, 2,4,6-trimethylpyridine, and triethylamine) and the 1-butyl-3-methylimidazolium (BMIm(+)) cation was studied with the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4]) as a solvent. The concentration of NHC, in the different base-[BMIm][BF4] systems, is derivable from the anodic peak current (related to the oxidation of NHC) of the voltammo-grams recorded in base-[BMIm][BF4] solutions. The electro-chemical procedure allows utilisation of base-[BMIm][BF4] solutions as a media that is capable of inducing NHC-catalysed syntheses. The NHC-catalysed oxidation of aldehydes to esters is selected as a model reaction. The results of the syntheses are in accordance with electrochemical forecasts.