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Is Enol Always the Culprit? The Curious Case of High Enantioselectivity in a Chiral Rh(II) Complex Catalyzed Carbene Insertion Reaction

Von Wiley-VCH zur Verfügung gestellt

The mechanism of Rh2(S-NTTL)4 catalyzed carbene insertion into C(3)–H of indole is investigated using DFT methods. Previously proposed mechanisms involved a metal-free enol intermediate, which naturally raises a conundrum regarding the mode of chiral induction. We now propose a new mechanistic pathway involving a metal-associated enol intermediate and stereochemical model to solve this puzzle. We show how the catalyst bowl-shaped structure along with substrate-catalyst binding is crucial for achieving high levels of enantioselectivity. The water molecules are held together by H-bonding interactions with the carboxylate ligands that is reminiscent of enzyme catalysis. Till date there is no transition state model that can explain the high enantioselectivity with such chiral Rh complexes. The metal-associated enol pathway is currently underrepresented in catalytic cycles and may play a crucial role in catalyst design. Since the enol pathway is commonly adopted in other metal-catalyzed X–H insertion reactions involving a diazoester, our results are not specific to the current reaction. Therefore, our study could provide the direction for achieving high levels of enantioselectivity which is otherwise difficult to achieve with a single metal catalyst.

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