Stability of electrogenerated 1-butyl-3-methylimidazol-2-ylidene in DMF. Part 2. Role of acid substrates. [1]

The stability of 1-butyl-3-methylimidazol-2-ylidene (NHC) in N,N-dimethylformammide (DMF) in the presence of acids was studied by cyclic voltammetry. The NHC was easily obtained by cathodic reduction of a solution of 1-butyl-3-methylimidazolium salt (BMImX) in DMF. The voltammetric measurements allowed to determine the NHC presence (and its concentration) in solution, due to the NHC oxidation peak. Moreover, carrying out the measurements as a function of time, it was possible to determine the half-life of NHC in DMF containing three different organic acids. The half-life was defined as the time to halve the NHC concentration (with respect to the initial one) t1/2. It was found that when acetic acid is present in solution t1/2 is considerably smaller than t1/2 in its absence (15 min vs 69), while in the presence of 2,6-dimethylphenol and diethyl malonate (weaker acids than the NHCH+) t1/2 is considerably higher (130 and 140 min vs 69). A hypothesis on the role of BMIm+ and X on the stability of NHC in DMF containing acids is reported. The reactivity of the NHC (studied in this paper) was tested in classical base-induced organic reactions, and the results are in accordance with the voltammetric analysis.