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Rationalization of Enantioselectivity Change in Rhodium-catalyzed Hydrogenation Reaction Assessed by DFT Calculations

In continuation of our efforts to optimize "backdoor induction" of chirality in rhodium catalyzed hydrogenation of methyl α-acetamidoacrylate (substrate), influence of rigidity of the used ligands was investigated. We have recently demonstrated enantioselectivity control simply by switching from two monodentate to one bidentate ligand without change in point chirality at the stereogenic centers nor helicity of the formed complexes. To find the origin of these results, we investigated mechanism of the reaction and differences in the reaction pathways that lead to either of two possible enantiomeric products. In this regard, we relied on the findings of Gridnev and Imamoto who identified a substrate double bond approach toward Rh atom as the process along which enantioselection takes place. Also, this process most likely occur after oxidative addition of hydrogen. Assuming similar mechanism, we modeled possible reaction pathways along which enantioselection takes place employing wB97XD DFT method. Four possible structures of each supramolecular and enediyne tethered semicoordinated complexes were used as the initial points. Experimentally observed enantioselectivity reversal was interpreted in terms of the reaction energetics and geometrical changes along the Rh-"ene" bond formation paths. Ligand change induces strong destabilization of otherwise the most probable S-path which becomes the least probable one. R-paths are also destabilized but to lesser extent.

enantioselectivity reversal, DFT method

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