Institut für Organische und Biomolekulare Chemie, Universität Göttingen, Tammannstraße 2, 37077 Göttingen, Germany.
J Org Chem. 2021 Mar 5;86(5):3750-3757. doi: 10.1021/acs.joc.0c02504. Epub 2021 Feb 18.
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.
我们提出电喷雾电离（ESI）质谱法作为一种强大而有效的原位分析碳负离子的方法。ESI 质谱法选择性地探测取样溶液中的带电组分，因此非常适合检测游离碳负离子。我们通过分析 15 种不同的羧酸 AH 的乙腈溶液来证明这种方法的潜力，这些羧酸的酸度范围为 11.1 ≤ p(DMSO) ≤ 29.5。用强碱 KOBu 处理后，除了两个最酸性的化合物外，所有化合物都成功地被检测为游离碳负离子 A 和/或作为钾结合的聚集体 [KA]。通过添加冠醚 18-冠-6，可以将缔合平衡向更小的聚集体和游离碳负离子转移，从而便于评估质谱。当用其他碱（LiOH、LiNPr、NaH、NaOH、KOH、NBuOH）代替 KOBu 或用四氢呋喃或甲醇作为溶剂时，也可以成功地观察到碳负离子。为了进一步证明所提出方法的实用性，我们将其应用于去质子化二甲基酮与丁烯酮的迈克尔加成反应的分析。ESI 质谱法允许我们随时间跟踪反应物碳负离子的减少和产物碳负离子的积累。
