Özen Cihan, Shoji Yoshiaki, Fukushima Takanori, Maeda Satoshi
Department of Chemistry , Hokkaido University , Sapporo 060-8510 , Japan.
Laboratory for Chemistry and Life Science, Institute of Innovative Research , Tokyo Institute of Technology , Yokohama 226-8503 , Japan.
J Org Chem. 2019 Feb 15;84(4):1941-1950. doi: 10.1021/acs.joc.8b02917. Epub 2019 Feb 1.
One-electron oxidation of borepin derivatives that consists of a boron-containing seven-membered ring has been reported to cause deborylation/C-C coupling, yielding aromatic compounds. The reaction can be achieved not only by transition metal compounds but also by oxidants without transition metal such as O and other organic compounds. Despite numerous experimental attempts, the mechanism of this peculiar reaction as well as the fate of the BCl part eliminated from borepin remain unclear to date. Based on theoretical approaches using the artificial force induced reaction method, here we address the mechanism of the unusual boron-mediated C-C coupling. For this purpose, two borepin derivatives (1 and 35), bearing ethyl and phenyl groups, respectively, were used as reactants, and FeCl/MeNO and O were chosen as oxidants. The calculations revealed reaction pathways that provided an overall picture of the mechanism of the target reaction, which features four key steps, namely, (i) quaternization of the boron atom by the coordination of oxidant, (ii) intersystem crossing, (iii) skeletal rearrangement to form a six-membered ring, and (iv) elimination of a boron moiety. The intrinsic nature of boron, i.e., a strong tendency to accept a coordination ligand even under oxidative conditions, is responsible for the oxidative deborylation/C-C coupling of borepin.
据报道，由含硼七元环组成的硼宾衍生物的单电子氧化会导致脱硼/C-C偶联，生成芳香族化合物。该反应不仅可以通过过渡金属化合物实现，还可以通过不含过渡金属的氧化剂如氧气和其他有机化合物来实现。尽管进行了大量实验尝试，但这种特殊反应的机理以及从硼宾中消除的BCl部分的去向至今仍不清楚。基于使用人工力诱导反应方法的理论方法，我们在此探讨这种不寻常的硼介导的C-C偶联的机理。为此，分别使用带有乙基和苯基的两种硼宾衍生物（1和35）作为反应物，并选择FeCl/MeNO和氧气作为氧化剂。计算结果揭示了反应途径，这些途径提供了目标反应机理的整体图景，该机理具有四个关键步骤，即：（i）通过氧化剂的配位使硼原子季铵化；（ii）系间窜越；（iii）骨架重排形成六元环；（iv）消除硼部分。硼的固有性质，即在氧化条件下即使接受配位配体的强烈倾向，是硼宾氧化脱硼/C-C偶联的原因。
