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.

Anionic coordination polymerizations proceed via highly reactive intermediates, whose in situ analysis has remained difficult. Here, we show that electrosprayionization mass spectrometry is a promising method to obtain detailed information on the polymerization process. Focusing on polymerization reactions of 1,3dienes initiated by CoCl 2 /RLi (R = Me, nBu, tBu, Ph), we directly observe the growing polymer chains and characterize the active anionic cobalt centers by gasphase fragmentation experiments. On the basis of these results, we suggest a plausible mechanism for the polymerization reaction. Moreover, the ESI mass spectra permit the determination of molecular weight distributions, which are in good agreement with those derived from NMR-spectroscopic as well as MALDI mass-spectrometric measurements, and afford a wealth of kinetic data.

Anionische Koordinationspolymerisationen verlaufen über hochreaktive Zwischenstufen, deren in situ‐Analyse schwierig bleibt. Hier zeigen wir, dass Elektrosprayionisations‐Massenspektrometrie eine vielversprechende Methode ist, um detaillierte Informationen über den Polymerisationsprozess zu erhalten. Für die durch CoCl2/RLi (R=Me, nBu, tBu, Ph) initiierte Polymerisation von 1,3‐Dienen initiiert beobachten wir die wachsenden Ketten direkt und charakterisieren die aktiven Cobalt‐Zentren durch Fragmentierungsexperimente in der Gasphase. Darauf basierend schlagen wir einen möglichen Mechanismus für die Polymerisation vor. Die ESI‐Massenspektren erlauben zudem die Bestimmung der Molmassenverteilung, welche mit denen aus NMR‐Messungen und MALDI‐Massenspektrometrie gut übereinstimmen, und liefern eine Vielzahl an kinetischen Daten.

Anionic polymerizations are of exceptional practical importance, but difficult to analyze due to the high reactivity of the growing polymer chains. Here, we demonstrate that electrospray‐ionization mass spectrometry (ESI‐MS) permits direct observation of the active carbanionic intermediates formed in the anionic ring‐opening polymerization of 1‐cyanocyclopropanecarboxylate in tetrahydrofuran. This includes the identification of a side product, as well as real‐time analysis of the polymerization reaction. From the mass spectra obtained, we can derive not only the mean molar mass and the polydispersity, but also the rate constants for the initiation and the individual propagation steps. The initiation proceeds significantly faster than the propagation steps. Accordingly, the examined reaction corresponds to a living polymerization, as we also confirmed by additional control experiments. Besides giving detailed insight into the reaction system probed here, we also expect the presented methodology to make possible the in‐situ analysis of further anionic polymerizations.

Electrospray‐ionization mass spectrometry easily distinguishes the different living oligomers resulting from the anionic ring‐opening polymerization of the acceptor‐substituted cyclopropane Mcyclic. Kinetic modeling reproduces the measured time evolution of the growing oligomers and affords the rate constants of the individual propagation steps. More information can be found in the Research Article by N. F. Eisele, M. Peters, and K. Koszinowski (DOI: 10.1002/chem.202203762).