Proton exchange equilibrium between bases and BMImBF4. An electrochemical procedure to evaluate the presence of carbene for synthetic applications

The actual ability of a base to yield the N-heterocyclic carbene (NHC) from the ionic liquid 1- butyl-3-methylimidazolium tetrafluoroborate (BMImBF4) was investigated by an electrochemical procedure. Accordingly, the proton exchange equilibrium between different bases (DBU, PIPG, Cs2CO3, 2,4,6-collidine, TEA) and the BMIm+ cation was studied in BMImBF4 as solvent. The concentration of NHC, generated by the addition of the bases to BMImBF4, is derivable from the anodic peak current (related to the NHC oxidation) of the voltammograms recorded. The electrochemical procedure allowed to evaluate the possible utilization of the solutions of bases in BMImBF4 as media able to induce known NHC-catalyzed and base-promoted syntheses. NHC-catalyzed oxidation of aldehydes to esters and base-promoted cyclization of bromoamides to β-lactams were taken as model reactions. Esters were achieved in good yields with DBU, PIPG and Cs2CO3, but no conversion was observed with 2,4,6-collidine or TEA. β-Lactams, instead, were isolated in good yields, irrespectively of the nature of the base. All the results from the syntheses are in line with the expectations from the electrochemical measurements.