University of Louisville, Department of Chemistry, Louisville, Kentucky 40292, USA.
J Mass Spectrom. 2012 Jun;47(6):676-86. doi: 10.1002/jms.2986.
The McLafferty rearrangement is an extensively studied fragmentation reaction for the odd-electron positive ions from a diverse range of functional groups and molecules. Here, we present experimental and theoretical results of 12 model compounds that were synthesized and investigated by GC-TOF MS and density functional theory calculations. These compounds consisted of three main groups: carbonyls, oximes and silyl oxime ethers. In all electron ionization mass spectra, the fragment ions that could be attributed to the occurrence of a McLafferty rearrangement were observed. For t-butyldimethylsilyl oxime ethers with oxygen in a β-position, the McLafferty rearrangement was accompanied by loss of the t-butyl radical. The various mass spectra showed that the McLafferty rearrangement is relatively enhanced compared with other primary fragmentation reactions by the following factors: oxime versus carbonyl, oxygen versus methylene at the β-position and ketone versus aldehyde. Calculations predict that the stepwise mechanism is favored over the concerted mechanism for all but one compound. For carbonyl compounds, C-C bond breaking was the rate-determining step. However, for both the oximes and t-butyldimethylsilyl oxime ethers with oxygen at the β-position, the hydrogen transfer step was rate limiting, whereas with a CH(2) group at the β-position, the C-C bond breaking was again rate determining. n-Propoxy-acetaldehyde, bearing an oxygen atom at the β-position, is the only case that was predicted to proceed through a concerted mechanism. The synthesized oximes exist as both the (E)- and (Z)-isomers, and these were separable by GC. In the mass spectra of the two isomers, fragment ions that were generated by the McLafferty rearrangement were observed. Finally, fragment ions corresponding to the McLafferty reverse charge rearrangement were observed for all compounds at varying relative ion intensities compared with the conventional McLafferty rearrangement.
麦拉弗蒂重排是一种广泛研究的碎裂反应，适用于来自各种官能团和分子的奇数电子正离子。在这里，我们展示了通过 GC-TOF MS 和密度泛函理论计算合成和研究的 12 种模型化合物的实验和理论结果。这些化合物由三个主要组组成：羰基、肟和硅基肟醚。在所有电子电离质谱中，观察到可以归因于麦拉弗蒂重排发生的碎片离子。对于β位含氧的叔丁基二甲基硅基肟醚，麦拉弗蒂重排伴随着叔丁基自由基的丢失。各种质谱表明，与其他初级碎裂反应相比，麦拉弗蒂重排通过以下因素得到相对增强：肟与羰基、β位的氧与亚甲基以及酮与醛。计算预测，对于除一种化合物以外的所有化合物，逐步机制比协同机制更有利。对于羰基化合物，C-C 键断裂是速率决定步骤。然而，对于β位含氧的肟和叔丁基二甲基硅基肟醚，氢转移步骤是限速步骤，而β位有 CH(2) 基团时，C-C 键断裂再次是速率决定步骤。带有β位氧原子的正丙氧基乙醛是唯一预测通过协同机制进行的化合物。合成的肟以（E）-和（Z）-异构体的形式存在，并且可以通过 GC 进行分离。在两种异构体的质谱中，观察到由麦拉弗蒂重排生成的碎片离子。最后，与传统的麦拉弗蒂重排相比，所有化合物在不同的相对离子强度下都观察到对应于麦拉弗蒂反向电荷重排的碎片离子。
