Diastereo- and Enantioselective Hydrogenation of α-Amino-β-Keto Ester Hydrochlorides Catalyzed by an Iridium Complex with MeO-BIPHEP and NaBArF: Catalytic Cycle and Five-Membered Chelation Mechanism of Asymmetric Hydrogenation

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Abstract

The development of Ir-catalyzed asymmetric hydrogenation of α-amino-β-keto ester hydrochlorides is described. This reaction proceeds through a dynamic kinetic resolution to produce anti-β-hydroxy-α-amino acid esters in a high diastereo- and enantioselective manner. Mechanistic studies have revealed that this unique asymmetric hydrogenation proceeds through reduction of the ketone moiety via the five-membered transition state involving the chelation between the oxygen of the ketone and the nitrogen of the amine function. The relationship studies between the hydrogen pressure and the stereoselectivity have disclosed two mechanisms dependent on hydrogen pressure. Under low hydrogen pressure (<15 atm), the reaction rate proportionally increased with the hydrogen pressure. However, under the high hydrogen conditions, the reaction rate exponentially accelerated along with the increasing hydrogen pressure, which suggests the participation of two or more of hydrogen atoms.

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