通过DFT计算阐明了IBX对苯酚氧化的机理研究。,Organic & Biomolecular Chemistry

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通过DFT计算阐明了IBX对苯酚氧化的机理研究。
Organic & Biomolecular Chemistry ( IF 2.9 ) Pub Date : 2020-01-29 , DOI: 10.1039/c9ob02650a
Amritpal Kaur ₁ , Alireza Ariafard ₁

Affiliation

使用SMD / M06-2X / def2-TZVP // SMD / M06-2X / LANL2DZ（d），6-31G（d）水平的密度泛函理论（DFT）来研究高价酚的区域选择性双重氧化碘（v）试剂（IBX）生成邻醌。氧化脱芳香化作用始于IBX与苯酚之间的配体交换，生成酚盐配合物，然后进行第一个氧化还原过程，该过程将碘（v）还原为碘（iii）。发现这两个过程（配体交换和第一个氧化还原反应）均由不稳定的碘（v）物种异构体介导。我们发现，尽管第一个氧化还原过程优先通过缔合途径进行，但酚环上的吸电子取代基降低了其可及性。对氧化还原过渡态的电子结构的检查表明，参与碘（v）还原的酚盐具有一定的苯氧ium特性。计算出苯氧鎓离子的固有稳定性对苯酚环上的取代基高度敏感。由于吸电子取代基大大降低了苯氧ium的稳定性，因此它们使碘（v）变成碘（iii）的还原能量消耗。一旦第一个氧化还原步骤完成，就会形成邻苯二酚-碘（iii）络合物，第二个氧化还原过程会从该络合物通过羧酸盐辅助的过渡结构生成最终的邻醌产品。该过渡结构通过邻苯二酚OH和羧酸酯基团之间的氢键相互作用获得稳定性。

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Mechanistic investigation into phenol oxidation by IBX elucidated by DFT calculations.

Density functional theory (DFT) at the SMD/M06-2X/def2-TZVP//SMD/M06-2X/LANL2DZ(d),6-31G(d) level was used to explore the regioselective double oxidation of phenols by a hypervalent iodine(v) reagent (IBX) to give o-quinones. The oxidative dearomatization commences with the ligand exchange between IBX and phenol, yielding a phenolate complex, followed by the first redox process, which reduces iodine(v) to iodine(iii). Both the processes (the ligand exchange and the first redox reaction) were found to be mediated by a less stable isomer of iodine(v) species. We found that although the first redox process preferentially proceeds via an associative pathway, an electron withdrawing substituent on the phenol ring decreases its accessibility. The inspection of the electronic structure of the redox transition state indicates that the phenolate involved in the iodine(v) reduction has some phenoxenium character. The intrinsic stability of a phenoxenium ion is calculated to be highly sensitive to the substituent on the phenol ring. Since the electron withdrawing substituents considerably decrease the stability of the phenoxenium, they render the iodine(v) to iodine(iii) reduction energy consuming. Once the first redox step has completed, a catechol-iodine(iii) complex is formed, from which the second redox process produces the final o-quinone product via a carboxylate-assisted transition structure. This transition structure gains stability by hydrogen bond interaction between the catechol OH and carboxylate group. Such an interaction results in the phenolate not having any phenoxenium character in the transition structure, thus making the activation barrier to the second redox step independent from the substituent on the phenol ring.

更新日期：2020-02-13

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