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

Abstract

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 ≤ pK a(DMSO) ≤ 29.5. After treatment with KO t Bu as a strong base, all but the two least acidic compounds were successfully detected as free carbanions A– and/or as potassium-bound aggregates [K n–1A n ]−. 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 KO t Bu was replaced by other bases (LiOH, LiN i Pr2, NaH, NaOH, KOH, NBu4OH) 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.

Live Monitoring of Anionic Living Polymerizations by Electrospray‐Ionization Mass Spectrometry

“…This effect is most pronounced for analytes with nonpolar residues sprayed from polar solvents, [12] but less so for the THF solutions of the quite polar polymers probed here. In summary, we do not expect the ESI process to result in significant discrimination effects, in line with numerous recent examples of successful quantitative analyses by ESI‐MS, [13–15] including the demonstration of approximately linear correlations between sample concentrations and ESI‐signal intensities for carbanions [8] …”

“…In summary, we do not expect the ESI process to result in significant Chemistry-A European Journal discrimination effects, in line with numerous recent examples of successful quantitative analyses by ESI-MS, [13][14][15] including the demonstration of approximately linear correlations between sample concentrations and ESI-signal intensities for carbanions. [8] Comparison with NMR spectroscopy and SEC…”

“…However, as we have recently demonstrated, ESI-MS is capable of detecting monomeric carbanions stabilized by electron-withdrawing groups, such as ester, nitrile, or nitro groups. [8] We reasoned that ESI-MS should be equally suitable for the detection of polymer carbanions bearing electron-withdrawing groups. Such carban-ions are expected to form the active species of various practically important polymerizations, including the anionic ring-opening polymerization of 1,1-disubstituted cyclopropanes.…”

“…This deficiency presumably results from the high reactivity of the (carb)anion intermediates involved in these reactions and the supposed inability of ESI‐MS to detect these species. However, as we have recently demonstrated, ESI‐MS is capable of detecting monomeric carbanions stabilized by electron‐withdrawing groups, such as ester, nitrile, or nitro groups [8] . We reasoned that ESI‐MS should be equally suitable for the detection of polymer carbanions bearing electron‐withdrawing groups.…”

Capturing Reactive Carbanions by Microdroplets

“…Kinetics and laser flash photolysis experiments revealed that the average lifetime of such carbanions could be as short as 10 –11 s (Supplementary Note 1). While the existence of persistent carbanions in organometallics or in the preparation of stable carbanion salts for nuclear magnetic resonance and mass spectrometric characterization is often described in the literature, − no general attempt was made so far to capture and isolate fleeting (metal-free) carbanion intermediate from a reaction medium for examining its time-dependent abundance. The lack of appropriate experimental probes may hide the actual reaction mechanism involving an elusive carbanion species in which some other plausible alternate mechanism is assumed to operate. , A recent report showed that the life expectancy of a carbanion intermediate generated by photolysis could be increased in dry zeolite by about 1 order of magnitude compared to that in the corresponding solution phase .…”