Violations. How Nature Circumvents the Woodward-Hoffmann Rules and Promotes the Forbidden Conrotatory 4n + 2 Electron Electrocyclization of Prinzbach's Vinylogous Sesquifulvalene.
Violations. How Nature Circumvents the Woodward-Hoffmann Rules and Promotes the Forbidden Conrotatory 4n + 2 Electron Electrocyclization of Prinzbach's Vinylogous Sesquifulvalene.
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DOI：
10.1021/jacs.1c11058
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发表时间：
2021-12-29
影响因子：
15
通讯作者：
Houk KN
中科院分区：
文献类型：
--
作者：
Kukier GA;Turlik A;Xue XS;Houk KN
伍德沃德和霍夫曼在1969年发表的关于轨道对称性的论文中指出了“违规行为”。一个也没有！“1978年，普林斯巴赫报道，尽管有多种轨道对称性允许的过程，但乙烯类倍半富瓦烯的电环化反应仅通过伍德沃德-霍夫曼轨道对称性禁止的14π电子旋转途径进行。普林斯巴赫后来证明，18π电子同系物表现出同样的禁忌行为。然而，类似的乙烯基五富瓦烯和七富瓦烯都遵循轨道对称性规则，分别通过其允许的12π和16π的旋转过程。我们报道了用ωB97X-D密度泛函对这些反应的研究。通过这些计算，结合与遵循伍德沃德-霍夫曼规则的电环化反应的广泛分析和比较，现在已经阐明了公然违反伍德沃德-霍夫曼轨道对称性选择规则的普林斯巴赫破坏的物理根源。这种规则的显著逆转(发现14π电子禁止过程比允许的过程在能量上容易11千卡/摩尔)是由于这种烃的高度极化而发生的，使得乙烯基倍半富瓦烯的旋转电环化行为就像环戊二烯与硫代吡喃的结合一样。这些结果与由空间约束和应变释放或由反应物的双自由基特征驱动的其他禁忌周环过程进行了比较，这些特征促进了对称禁止反应的双自由基过渡态的形成。我们预测了很强的施主-受体取代可以怎样改变节点的性质来平衡允许的和禁止的电环反应势垒之间的差异，并给出了两种情况的计算预测。
Woodward and Hoffmann, in their treatise on orbital symmetry in 1969, stated “Violations. There are none!” Prinzbach reported in 1978 that the electrocyclization of vinylogous sesquifulvalene occurs exclusively through the Woodward–Hoffmann orbital-symmetry-forbidden 14π-electron conrotatory pathway, despite the availability of a variety of orbital-symmetry-allowed processes. Prinzbach later demonstrated that an 18π-electron homologue exhibits the same forbidden behavior. And yet, the analogous vinylogous pentafulvalene and heptafulvalene both follow the orbital symmetry rules, each proceeding through its allowed conrotatory 12π and 16π process, respectively. We report the investigation of these reactions with ωB97X-D DFT. The physical origins of the flagrant Prinzbach violations of the Woodward–Hoffmann orbital symmetry selection rules have now been elucidated by these calculations in conjunction with extensive analyses and comparisons to electrocyclizations that obey the Woodward–Hoffmann rules. This remarkable reversal of the Rules (the 14π-electron-forbidden process is found to be 11 kcal/mol more energetically facile than the allowed process) occurs due to the high degree of polarization of this hydrocarbon, such that conrotatory electrocyclization of vinylogous sesquifulvalene behaves like a cyclopentadienide combining with a tropylium. These results are compared to other forbidden pericyclic processes driven by steric constraints and strain release or by diradical character of the reactants that facilitates the formation of diradical transition states for symmetry-forbidden reactions. We predict how strong donor–acceptor substitution can modify nodal properties to level the difference between allowed and forbidden electrocyclic reaction barriers, and we provide computational predictions of two such cases.
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影响因子：
15
作者：
EPIOTIS, ND
通讯作者：
EPIOTIS, ND
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作者：
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作者：
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