Institute of Chemistry and The Lise Meitner-Minerva Center for Computational Quantum Chemistry, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel.
Chemphyschem. 2010 Jan 18;11(1):301-10. doi: 10.1002/cphc.200900848.
Herein we demonstrate that an external electric field (EEF) acts as an accessory catalyst/inhibitor for Diels-Alder (DA) reactions. When the EEF is oriented along the "reaction axis" (the coordinate of approach of the reactants in the reaction path), the barrier of the DA reactions is lowered by a significant amount, equivalent to rate enhancements by 4-6 orders of magnitude. Simply flipping the EEF direction has the opposite effect, and the EEF acts as an inhibitor. Additionally, an EEF oriented perpendicular to the "reaction axis" in the direction of the individual molecule dipoles can change the endo/exo selectivity, favouring one or the other depending on the positive/negative directions of the EEF vis-à-vis the individual molecular dipole. At some critical value of the EEF along the "reaction axis", there is a crossover to a stepwise mechanism that involves a zwitterionic intermediate. The valence bond diagram model is used to comprehend these trends and to derive a selection rule for EEF effects on chemical reactions: an EEF aligned in the direction of the electron flow between the reactants will lower the reaction barrier. It is shown that the exo/endo control by the EEF is not associated with changes in secondary orbital interactions.
在此，我们证明了外电场（EEF）可作为 Diels-Alder（DA）反应的辅助催化剂/抑制剂。当 EEF 沿“反应轴”（反应物在反应路径中接近的坐标）定向时，DA 反应的势垒会显著降低，相当于反应速率提高 4-6 个数量级。简单地翻转 EEF 方向会产生相反的效果，EEF 起到抑制剂的作用。此外，垂直于“反应轴”且沿单个分子偶极子方向的 EEF 可以改变内/外选择性，根据 EEF 相对于单个分子偶极子的正负方向，有利于一种或另一种选择性。在“反应轴”上 EEF 的某个临界值处，会发生涉及两性离子中间体的逐步反应机制的转变。价键图模型用于理解这些趋势，并推导出 EEF 对化学反应的影响的选择规则：与反应物之间的电子流方向一致的 EEF 将降低反应势垒。结果表明，EEF 对内/外控制与次级轨道相互作用的变化无关。
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