Perez Evan H, Schleif Tim, Messinger Joseph P, Rullán Buxó Anna G, Moss Olivia C, Greis Kim, Johnson Mark A
Sterling Chemistry Laboratory, Department of Chemistry, Yale University, New Haven, Connecticut 06520, United States.
Institut für Chemie und Biochemie, Freie Universität Berlin, Arnimallee 22, 14195 Berlin, Germany.
J Am Soc Mass Spectrom. 2022 Oct 5;33(10):1914-1920. doi: 10.1021/jasms.2c00188. Epub 2022 Sep 9.
Decarboxylation of carboxylate ions in the gas phase provides a useful window into the chemistry displayed by these reactive carbanion intermediates. Here, we explore the species generated by decarboxylation of two benzoate derivatives: 2-formylbenzoate (2FBA) and 2-benzoylbenzoate (2BBA). The nascent product anions are transferred to a cryogenic ion trap where they are cooled to ∼15 K and analyzed by their pattern of vibrational bands obtained with IR photodissociation spectroscopy of weakly bound H molecules. The structures of the quenched species are then determined by comparison of these spectra with those predicted by electronic structure calculations for local minima on the potential energy surface. The 2-phenide carbanion generated by decarboxylation of 2FBA occurs in two isomeric forms that differ in the orientation of the formyl group, both of which yield a very large (∼110 cm) redshift in the stretching frequency of the H molecule attached to the anionic carbon center. Although calculated to be a local minimum, the analogous 2-phenide species could not be isolated upon decarboxylation of 2BBA. Rather, the anionic product adopts a ring-closed structure, indicating efficient nucleophilic attack on the pendant phenyl group by the nascent phenide. The barrier for ring closing is evaluated with electronic structure calculations.
羧酸根离子在气相中的脱羧反应为研究这些活性碳负离子中间体所展现的化学性质提供了一个有用的窗口。在此，我们探究了两种苯甲酸衍生物：2-甲酰基苯甲酸（2FBA）和2-苯甲酰基苯甲酸（2BBA）脱羧反应所生成的物种。新生的产物阴离子被转移至一个低温离子阱中，在那里它们被冷却至约15 K，并通过对与弱结合的H分子进行红外光解离光谱所获得的振动带模式进行分析。然后，通过将这些光谱与势能面上局部极小值的电子结构计算所预测的光谱进行比较，来确定猝灭物种的结构。2FBA脱羧反应生成的2-苯基碳负离子以两种异构体形式存在，它们的甲酰基取向不同，这两种异构体都会使连接在阴离子碳中心上的H分子的伸缩频率产生非常大（约110 cm）的红移。尽管经计算它是一个局部极小值，但在2BBA脱羧反应中却无法分离出类似的2-苯基物种。相反，阴离子产物采用了一种闭环结构，这表明新生的苯基对侧链苯基进行了有效的亲核进攻。利用电子结构计算评估了闭环的势垒。
