School of Chemistry and Chemical Biology, University College Dublin, Ireland.
Dalton Trans. 2011 Oct 14;40(38):9911-20. doi: 10.1039/c1dt11116g. Epub 2011 Aug 30.
Rhodation of trimethylene-bridged diimidazolium salts induces the intramolecular activation of an alkane-type C-H bond and yields mono- and dimetallic complexes containing a formally monoanionic C,C,C-tridentate dicarbene ligand bound to each rhodium centre. Mechanistic investigation of the C(alkyl)-H bond activation revealed a significant rate enhancement when the carbene ligands are bound to the rhodium centre via C4 (instantaneous activation) as compared to C2-bound carbene homologues (activation incomplete after 2 days). The slow C-H activation in normal C2-bound carbene complexes allowed intermediates to be isolated and suggests a critical role of acetate in mediating the bond activation process. Computational modelling supported by spectroscopic analyses indicate that halide dissociation as well as formation of the agostic intermediate is substantially favoured with C4-bound carbenes. It is these processes that discriminate the C4- and C2-bound systems rather than the subsequent C-H bond activation, where the computed barriers are very similar in each case. The tridentate dicarbene ligand undergoes selective H/D exchange at the C5 position of the C4-bound carbene exclusively. A mechanism has been proposed for this process, which is based on the electronic separation of the abnormal carbene ligand into a cationic N-C-N amidinium unit and a metalla-allyl type M-C-C fragment.
亚甲基桥联二咪唑盐的铑化作用诱导烷烃型 C-H 键的分子内活化，并生成含有 formally 单阴离子的 C,C,C-三齿二碳烯配体的单核和双核配合物，每个铑中心都与之结合。对 C(烷基)-H 键活化的机理研究表明，当卡宾配体通过 C4（瞬时活化）与铑中心结合时，与 C2 结合的卡宾类似物相比（2 天后仍未完全活化），其 C-H 活化速率显著提高。在正常 C2 结合的卡宾配合物中缓慢的 C-H 活化允许中间体被分离出来，并表明醋酸盐在介导键活化过程中起着关键作用。光谱分析支持的计算模型表明，卤化物的解离以及形成加合物中间体在 C4 结合的卡宾中得到了极大的促进。正是这些过程区分了 C4 和 C2 结合的体系，而不是随后的 C-H 键活化，在这两种情况下，计算出的障碍非常相似。三齿二碳烯配体在 C4 结合的卡宾的 C5 位置上进行选择性的 H/D 交换。已经提出了该过程的一种机制，该机制基于异常卡宾配体的电子分离为阳离子 N-C-N 脒基单元和金属烯丙基型 M-C-C 片段。
