Probing substituent effects in aryl-aryl interactions using stereoselective Diels-Alder cycloadditions.
Probing substituent effects in aryl-aryl interactions using stereoselective Diels-Alder cycloadditions.
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DOI：
10.1021/ja903653j
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发表时间：
2010-03-17
影响因子：
15
通讯作者：
Houk, K. N.
中科院分区：
文献类型：
--
作者：
Wheeler, Steven E.;McNeil, Anne J.;Mueller, Peter;Swager, Timothy M.;Houk, K. N.
对芳基-芳基夹层配合物中立体选择性Diels-Alder环加成探测取代基效应进行了实验和理论研究。模型系统的计算表明，这些反应的立体选择性是由竞争过渡态的微分π-堆叠相互作用介导的。这使得取代和未取代夹层配合物的相对堆叠自由能可以从测量的产物分布中得到。与气相计算相比，色散效应在取代基效应中似乎没有发挥重要作用，与先前的实验一致。实验π-堆积自由能与Hammett σm常数有良好的相关关系（r = 0.96）。这些取代基常数主要提供了取代基的感性给电子和吸电子性质的量度，而不是给入或给出苯π体系。目前的实验结果最容易用最近提出的苯夹层二聚体中取代基效应模型来解释，在该模型中，取代苯的π系相对不重要，取代基效应是直接通过空间相互作用产生的。具体来说，这些结果是第一次清楚地表明，尽管OMe是π电子供体，但它增强了π-堆叠相互作用。这与普遍认为芳基-芳基相互作用中的取代基效应是由芳基π体系的极化调节的模型相矛盾。
Stereoselective Diels-Alder cycloadditions that probe substituent effects in aryl-aryl sandwich complexes were studied experimentally and theoretically. Computations on model systems demonstrate that the stereoselectivity in these reactions is mediated by differential π-stacking interactions in competing transition states. This allows relative stacking free energies of substituted and unsubstituted sandwich complexes to be derived from measured product distributions. In contrast to gas-phase computations, dispersion effects do not appear to play a significant role in the substituent effects, in accord with previous experiments. The experimental π-stacking free energies are shown to correlate well with Hammett σm constants (r = 0.96). These substituent constants primarily provide a measure of the inductive electron-donating and withdrawing character of the substituents, not donation into or out of the benzene π-system. The present experimental results are most readily explained using a recently proposed model of substituent effects in the benzene sandwich dimer in which the π-system of the substituted benzene is relatively unimportant and substituent effects arise from direct through-space interactions. Specifically, these results are the first experiments to clearly show that OMe enhances these π-stacking interactions, despite being a π-electron donor. This is in conflict with popular models in which substituent effects in aryl-aryl interactions are modulated by polarization of the aryl π-system.
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影响因子：
3
作者：
Grimme, Stefan
通讯作者：
Grimme, Stefan
影响因子：
16.6
作者：
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通讯作者：
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影响因子：
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作者：
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通讯作者：
Iverson, BL
影响因子：
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作者：
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通讯作者：
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影响因子：
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作者：
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通讯作者：
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