Direct Detection of Free and Counterion-Bound Carbanions by Electrospray-Ionization Mass Spectrometry.

We propose electrospray-ionization (ESI) mass spectrometry as a robust and powerful method for the in situ analysis of carbanions. ESI mass spectrometry selectively probes the charged components of the sampled solution and, thus, is ideally suited for the detection of free carbanions. We demonstrate the potential of this method by analyzing acetonitrile solutions of 15 different carbon acids AH, whose acidities cover a range of 11.1 ≤ p(DMSO) ≤ 29.5. After treatment with KOBu as a strong base, all but the two least acidic compounds were successfully detected as free carbanions A and/or as potassium-bound aggregates [KA]. The association equilibria can be shifted toward smaller aggregates and free carbanions by the addition of the crown ether 18-crown-6, which facilitates the evaluation of the mass spectra. When KOBu was replaced by other bases (LiOH, LiNPr, NaH, NaOH, KOH, NBuOH) or when tetrahydrofuran or methanol was used as a solvent, carbanions were also successfully observed. For further demonstrating the utility of the proposed method, we applied it to the analysis of the Michael addition of deprotonated dimedone to butenone. ESI mass spectrometry allowed us to follow the decrease of the reactant carbanion and the buildup of the product carbanion in time.