Kumar M Kiran, Sateesh B, Prabhakar S, Sastry G Narahari, Vairamani M
National Center for Mass Spectrometry, Indian Institute of Chemical Technology, Hyderabad 500 007, India.
Rapid Commun Mass Spectrom. 2006;20(6):987-93. doi: 10.1002/rcm.2400.
Regiospecific formation of carbanions from a set of geometrical (cis and trans isomers) and five different sets of positional isomers (ortho, meta and para isomers) of aromatic carboxylic acids is reported under negative electrospray ionisation conditions by decarboxylation of the carboxylate anions. The structures of decarboxylated anions, (M-H)-CO(2), are studied by ion-molecule reactions with carbon dioxide in the collision cell of a triple quadrupole mass spectrometer. The (M-H)-CO(2) ions generated from the trans and meta/para isomers react with CO(2) to produce product ions corresponding to the addition of one CO(2), which confirms the survival of the (M-H)-CO(2) ions as carbanions. On the other hand, the (M-H)-CO(2) ions generated from cis and ortho isomers failed to react with CO(2) due to rapid isomerisation of the initially generated carbanion to a aromatic carboxylate/oxide anion, which is unreactive with CO(2), through a facile intramolecular proton transfer from the proton-containing substituent to the carbanion site. When the experiments were performed at high desolvation temperatures (300 degrees C), instead of 100 degrees C, the relative abundance of (M-H)-CO(2) ions and the corresponding CO(2) adduct in ion-molecule reaction experiments increased significantly due to minimisation of proton exchange. Quantum chemical calculations on some of the generated isomeric carbanions and their isomerised products due to proton transfer support the selective stability of carbanions.
报道了在负电喷雾电离条件下，通过羧酸根阴离子的脱羧反应，从一组几何异构体（顺式和反式异构体）以及五种不同的位置异构体（邻位、间位和对位异构体）的芳香族羧酸中区域特异性地形成碳负离子。在三重四极杆质谱仪的碰撞池中，通过与二氧化碳的离子 - 分子反应研究了脱羧阴离子[(M - H)-CO₂]⁻的结构。由反式异构体以及间位/对位异构体产生的[(M - H)-CO₂]⁻离子与CO₂反应生成对应于加成一个CO₂的产物离子，这证实了[(M - H)-CO₂]⁻离子作为碳负离子的存活。另一方面，由顺式异构体和邻位异构体产生的[(M - H)-CO₂]⁻离子未能与CO₂反应，这是由于最初生成的碳负离子通过从含质子取代基到碳负离子位点的 facile 分子内质子转移迅速异构化为与CO₂不反应的芳香族羧酸盐/氧化物阴离子。当实验在高脱溶剂化温度（300℃）而非100℃下进行时，由于质子交换的最小化，离子 - 分子反应实验中[(M - H)-CO₂]⁻离子及其相应的CO₂加合物的相对丰度显著增加。对一些生成的异构碳负离子及其因质子转移而异构化的产物进行的量子化学计算支持了碳负离子的选择性稳定性。
