Impact of Excited-State Antiaromaticity Relief in a Fundamental Benzene Photoreaction Leading to Substituted Bicyclo[3.1.0]hexenes
Impact of Excited-State Antiaromaticity Relief in a Fundamental Benzene Photoreaction Leading to Substituted Bicyclo[3.1.0]hexenes
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DOI：
10.1021/jacs.9b13769
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发表时间：
2020-06-24
影响因子：
15
通讯作者：
Ottosson, Henrik
中科院分区：
文献类型：
--
作者：
Slanina, Tomas;Ayub, Rabia;Ottosson, Henrik
苯表现出丰富的光化学性质，可以提供在电子基态反应中难以获得的复杂分子支架。苯在基态是芳香族的，而在最低的π π *激发态是反芳香族的。本文阐明了激发态抗芳性（ESAA）的缓解在多大程度上触发了苯的基本光反应：在酸性介质中，溶剂对苯的光引发的亲核加成导致取代的双环[3.1.0]己二烯。实验探索了反应范围，发现硅基取代苯对双环[3.1.0]己烯衍生物的反应速度最快，形成具有三个立体中心的单异构体，一步产率可达75%。通过量子化学计算和实验，探讨了两个主要的机制假设，都涉及ESAA的缓解。第一种机制涉及激发态苯的质子化，随后重排为双环[3.1.0]己烯离子，被亲核试剂捕获，而第二种机制涉及苯的光重排为苯，然后是质子化和亲核加成。我们的研究表明，第二种机制是有效的。我们还澄清了类似的ESAA释放导致s -1态硅苯和吡啶离子的起皱，其中后者的光重排具有既定的合成用途。最后，我们确定了反应局限性的原因，这些信息对于探索类似的光反应应该是有价值的。综上所述，我们揭示了苯中的ESAA和6个pi-电子杂环如何引发光化学扭曲，从而从简单的反应物中获得复杂的三维分子支架。
Benzene exhibits a rich photochemistry which can provide access to complex molecular scaffolds that are difficult to access with reactions in the electronic ground state. While benzene is aromatic in its ground state, it is antiaromatic in its lowest pi pi* excited states. Herein, we clarify to what extent relief of excited-state antiaromaticity (ESAA) triggers a fundamental benzene photoreaction: the photoinitiated nucleophilic addition of solvent to benzene in acidic media leading to substituted bicyclo[3.1.0]hex-2-enes. The reaction scope was probed experimentally, and it was found that silyl-substituted benzenes provide the most rapid access to bicyclo[3.1.0]hexene derivatives, formed as single isomers with three stereogenic centers in yields up to 75% in one step. Two major mechanism hypotheses, both involving ESAA relief, were explored through quantum chemical calculations and experiments. The first mechanism involves protonation of excited-state benzene and subsequent rearrangement to bicyclo[3.1.0]hexenium cation, trapped by a nucleophile, while the second involves photorearrangement of benzene to benzvalene followed by protonation and nucleophilic addition. Our studies reveal that the second mechanism is operative. We also clarify that similar ESAA relief leads to puckering of S-1-state silabenzene and pyridinium ion, where the photorearrangement of the latter is of established synthetic utility. Finally, we identified causes for the limitations of the reaction, information that should be valuable in explorations of similar photoreactions. Taken together, we reveal how the ESAA in benzene and 6 pi-electron heterocycles trigger photochemical distortions that provide access to complex three-dimensional molecular scaffolds from simple reactants.