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C,C-环钯物种是一类含有两个C-Pd σ键的环金属络合物。近几十年来，这类物种作为关键反应中间体表现出独特的反应性，因而受到广泛关注。钯催化芳基或烯基卤的C-H官能团化是一类涉及C,C-环钯中间体的典型反应。自从Dyker和Catellani在20世纪90年代报道了几个开创性的例子以来，该领域取得了很大的进展。然而，大多数研究被限于C(芳基),C-环钯中间体的转化（图1a）。相比之下，由于更有利的竞争途径，涉及C(烯基),C(烷基)-、C(烷基),C(烷基)-环钯中间体的反应仍然难以实现（图1b）。事实上，该领域现有的报道主要集中在C(烯基),C(烷基)-、C(烷基),C(烷基)-环钯物种的分子内还原消除反应，而分子间的转化极为罕见。近年来，Dong和Liang在C(烯基),C(烷基)-环钯中间体的分子间转化方面取得了重要进展，他们发展了几个烯基Catellani反应（图1b，左）。但是到目前为止，利用偶联试剂捕获C(烷基),C(烷基)-环钯中间体的分子间反应仍未报道。
图1 湖南师范大学杨源团队致力于C,C-环钯中间体的转化反应研究。他们设想利用烯烃连接的烯基卤化物通过分子内碳钯化/C(sp³)-H过程，可以实现C(烷基),C(烷基)-环钯中间体的分子间转化（图1b，右）。然而，该设计存在几个挑战：（1）烯烃π键的环丙烷化反应（路径 A）；（2）σ-烷基钯(II) 物种的环丙烷化（路径 B）；（3）σ-烷基钯(II) 物种难以活化远端的惰性 C(sp3)-H键（路径 C）；（4）C(烷基),C(烷基)-环钯中间体的竞争性转化（路径E和F）。 近日，该课题组报道了首例基于C(烷基),C(烷基)-环钯中间体的级联C(sp3)-H环化反应（图2）。该方法通过烯烃连接的烯基溴化物经历分子内Heck/C(sp3)−H活化形成C(烷基),C(烷基)-环钯中间体，该物种进一步被α-溴代丙烯酸捕获，生成三环稠合的吡啶二酮。同时，该策略还可应用于吲哚连接的环烯基溴化物，从而构建五环稠合的吡啶二酮。此外，在没有α-溴代丙烯酸参与反应的情况下，可以合成一系列含有四元环的三环骨架。 图2 该反应具有广泛的底物普适性，产物衍生化进一步证明了其实用性（图3，左）。为了深入了解反应机理，作者设计了几个控制实验（图3，右），同时与北京化工大学雷鸣教授团队合作进行了DFT计算（图4），相关研究表明：C(烷基),C(烷基)-环钯物种是反应的关键中间体；该反应经历了Pd(0)→Pd(II)→Pd(IV)→Pd(II)→Pd(0)的催化循环，其中C(sp³)-H活化是决速步，反应能垒为26.6 kcal/mol。 图3 图4 总之，湖南师范大学杨源团队报道了首例基于C(烷基),C(烷基)-环钯中间体的级联C(sp3)-H环化反应，可以实现三类多环稠合骨架的构建。该方法的建立进一步推动了C,C-环钯化学的发展。 论文信息 A Cascade C(sp3)−H Annulation Involving C(alkyl),C(alkyl)-Palladacycle Intermediates Dr Liwei Zhou, Xiahong Chen, Qiong Peng, Zhiwei Li, Shujia Qiao, Dr. Guobo Deng, Prof. Dr. Yun Liang, Prof. Dr. Ming Lei, Prof. Dr. Yuan Yang Angewandte Chemie International Edition DOI: 10.1002/anie.202412336
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