Mechanistic Study of Cp*CoIII/RhIII-Catalyzed Directed C–H Functionalization with Diazo Compounds
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- 作者：Shuanglin QuChristopher J. Cramer
- 刊名：Journal of Organic Chemistry
- 出版年：2017
- 出版时间：January 20, 2017
- 年：2017
- 卷：82
- 期：2
- 页码：1195-1204
- 全文大小：887K
- ISSN：1520-6904
文摘
Density functional theory calculations have been performed to provide mechanistic insight into a series of Cp*CoIII- and Cp*RhIII-catalyzed directed C–H bond functionalizations with diazo-compound substrates. Co-based catalysis proceeds through five steps: C–H bond activation; C–C coupling via a concerted 1,2-aryl transfer; proto-demetalation; nucleophilic addition; and solvent-assisted methanol elimination. C–H bond activation is predicted to be reversible, consistent with deuterium-scrambling experiments. The higher Lewis acidity of Co compared to Rh for two otherwise identical catalysts increases the susceptibility of a coordinated carbonyl group to nucleophilic addition in the former, facilitating the formation of cyclized products not observed for Rh. Methanol elimination is predicted to be the turnover-limiting step for one substrate, and this is facilitated by solvent 2,2,2-trifluoroethanol (TFE) acting as a proton shuttle. Theory suggests that further tuning of acidity may offer opportunities for improving catalysis. We also assess the role of a pyridine group that leads to a different series of final steps in one Rh-based catalytic cycle, thereby enabling access to the otherwise suppressed cyclization product. Our study of an alternative Rh-based system having acetate ligands replaced with MeCN indicates that C–H bond activation is sensitive to those ligands and variation can affect which step is turnover-limiting.