Horváth Ádám, Benkő Zoltán
Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics, Müegyetem rkp. 3, Budapest H-1111, Hungary.
Inorg Chem. 2022 May 23;61(20):7922-7934. doi: 10.1021/acs.inorgchem.2c00549. Epub 2022 May 9.
While Diels-Alder (DA) reactions involving neutral or cationic dienophiles are well-known, the characteristics of the analogous reactions with anionic dienophiles are practically unexplored. Herein we present the first comparative computational investigations on the characteristics of DA cycloadditions with anionic dienophiles on the basis of the reactions of [ECX] anions (E = P, As; X = O, S, Se) with 2-pyran-2-one. All of these reactions were found to be both kinetically and thermodynamically feasible, enabling synthetic access toward 2-phosphaphenolate and arsaphenolate derivatives in the future. This study also reveals that the [ECO] anions show clear regioselectivity, while for [ECS] and [ECSe] anions, the two possible reaction channels have very similar energetics. Additionally, the activation barriers for the [ECO] anions are lower than those of the heavier analogues. The observed differences can be traced back to the starkly differing nucleophilic character of the pnictogen center in the anions, leading to a barrier-lowering effect in the case of the [ECO] anions. Furthermore, analysis of the geometries and electron distributions of the corresponding transition states revealed structure-property relationships, and thus a direct comparison of the cycloaddition reactivity of these anions was achieved. Along one of the two pathways, a good correlation was found between the activation barriers and suitable nucleophilicity descriptors (nucleophilic Parr function and global nucleophilicity). Additionally, the tendency of the reaction energies can be explained by the changing aromaticity of the products.
虽然涉及中性或阳离子亲双烯体的狄尔斯-阿尔德（DA）反应广为人知，但与阴离子亲双烯体的类似反应的特征实际上尚未得到探索。在此，我们基于[ECX]阴离子（E = P，As；X = O，S，Se）与2-吡喃-2-酮的反应，首次对与阴离子亲双烯体的DA环加成反应的特征进行了比较计算研究。发现所有这些反应在动力学和热力学上都是可行的，这使得未来能够通过合成得到2-磷苯酚盐和砷苯酚盐衍生物。该研究还表明，[ECO]阴离子表现出明显的区域选择性，而对于[ECS]和[ECSe]阴离子，两条可能的反应通道具有非常相似的能量。此外，[ECO]阴离子的活化能垒低于较重类似物的活化能垒。观察到的差异可追溯到阴离子中pnictogen中心截然不同的亲核特性，这在[ECO]阴离子的情况下导致了能垒降低效应。此外，对相应过渡态的几何结构和电子分布的分析揭示了结构-性质关系，从而实现了对这些阴离子环加成反应活性的直接比较。沿着两条途径之一，发现活化能垒与合适的亲核性描述符（亲核帕尔函数和全局亲核性）之间存在良好的相关性。此外，反应能量的趋势可以用产物芳香性的变化来解释。
