“Thermometer” Ions Can Fragment Through an Unexpected Intramolecular Elimination: These Are Not the Fragments You Are Looking For

Benzylpyridinium analogs are effective thermometer ions since monitoring the formation of the benzylium fragment produced from heterolytic cleavage of the C–N bond can be linked to the ion’s internal energy. In this study, three para-substituted benzylpyridinium ions containing ethoxy (OEt), isopropoxy (OiPr) and tert-butoxy (OtBu) substitutents were synthesized and evaluated as chemical thermometers. Intriguingly, the product ion spectra of the three benzylpyridinium ions were dominated by m/z 107 instead of the anticipated benzylium species. Deuterium labeling suggested that the m/z 107 fragment resulted from an intramolecular elimination (Ei), which formed via a four-membered transition state (TS). The fragmentation pathway appears to be an anomaly within the mass spectrometry literature, as four-membered pericyclic TSs are usually accompanied by the formation of an exceptionally stable neutral molecule (e.g., CO2). Quantum-chemical calculations confirmed our hypothesis that stabilization of the strained TS is afforded by hyperconjugation (ΔG‡ tert-butoxy < isopropyoxy < ethoxy).苄基吡啶类似物（benzylpyridinium analogs）是一类有效的温度计离子，因为通过监测C-N键异裂生成的苄鎓碎片的形成情况，可与该离子的内能建立关联。本研究中，我们合成了三种带有乙氧基（ethoxy, OEt）、异丙氧基（isopropoxy, OiPr）和叔丁氧基（tert-butoxy, OtBu）取代基的对位取代苄基吡啶离子，并将其作为化学温度计离子开展评估。值得注意的是，这三种苄基吡啶离子的产物离子谱图中，丰度占主导的碎片为m/z 107，而非预期的苄鎓离子物种。氘标记实验表明，m/z 107的碎片源于分子内消除反应（intramolecular elimination, Ei），该反应经由四元过渡态（transition state, TS）形成。该碎裂路径在质谱学文献中属于反常现象，因为四元环周环过渡态通常会伴随生成一种极为稳定的中性分子（如CO₂）。量子化学计算验证了我们的假设：带环张力的过渡态的稳定性可通过超共轭效应（hyperconjugation）实现，且吉布斯自由能活化能（ΔG‡）对应的稳定性顺序为叔丁氧基 < 异丙氧基 < 乙氧基。