MSTFA及其含氟衍生物的特征质谱重排反应

N-甲基-N-（三甲基硅烷）三氟乙酰胺（MSTFA）是一种重要的衍生化试剂，其质谱峰的归属对于深入研究其反应机理具有深刻意义。本工作对含氟和不含氟的两类衍生化产物和含氟芳香酚化合物的质谱裂解进行研究，结果表明，通过MSTFA衍生化后的含氟化合物生成了具有显著特征的m/z 77碎片离子，在谱图上多为基峰或接近基峰。而不含氟化合物的衍生化产物未能生成该特征离子，经高分辨质谱分析确认该离子组成为C₂H₆FSi⁺，是骨架重排反应产物，表明了骨架重排在m/z 77产生过程的重要作用。推断该特征离子的生成途径为单分子内的亲核反应（SN1）：首先中性分子被电离得到分子离子，游离基中心定域在杂原子O(N)上，游离基中心诱导分子离子通过α裂解脱去甲基自由基形成π键，提供了稳定的电荷中心；电荷中心诱导迫使π键上的一对电子进行迁移，正电荷同时转移到硅原子上，形成了能够促进下一步反应进行的活化中心；最大电负性的氟原子携带组成C-F键的一对电子亲核进攻硅正离子，生成新的F-Si键，同时迫使O(N)-Si键的一对电子发生转移造成O(N)-Si键的裂解；通过分子内的亲核消除反应脱去中性分子，从而得到特征离子m/z 77。

Analysis of MSTFA and Its Fluorine Derivatives Using Mass Spectrometry

Chemical conversion of a compound to an appropriate derivative can some-times improve the resulting mass spectral data, either by increasing the compound’s vapor pressure or by making its spectrum more easily interpretable. As a kind of important derivative reagent, the attribution of mass spectrum peak in trifluoro -N-methyl-N -(trimethylsilyl) acetamide (MSTFA) has a great significance to further investigating its reaction mechanism. Through studying the fluorinated and non fluorinated derivatives as well as fluorinated aromatic phenolic compounds by gas chromatography mass spectrometry (GC/MS), it was found that fluorinated derivatives have characteristic m/z 77 even electron fragment ions, and non fluorinated derivatives can not form this characteristic ions. High resolution mass spectrometer confirms above characteristic ions is C₂H₆FSi⁺, indicating skeleton rearrangement has an important effect on the formation of m/z 77 fragment ions. Therefore, the formation way may be unimolecular nucleophilic reactions (S_N1): oxygen (nitrogen) is intermediate in its ability to influence either α or i reactions. In an aliphatic ether attraction of an electron pair initiated by the localized positive charge on the oxygen (nitrogen) can form the trimethylsilane ion and the alkoxyl radical. First molecular ion by α-cleavage loss of methyl radical at a reactive site, positive charge localized on the oxygen (nitrogen) atom. Because the positive charge attract a couple of electrons migrate, the charge site is moved localized on the silicon atom, then fluorine atom carries a couple of electrons nucleophilic attack on the silylium ion, and O(N)—Si bond’s a couple of electrons migrate, and nucleophilic elimination reaction in intramolecular is occur. The mechanism of these fragmentations has been studied with the GC/MS analysis and deuterium labeled analogs whose preparation is reported.