Highly Enantioselective Hydrogenation of α-Keto Esters Catalyzed by Ru-Tunephos Complexes

Abstract

Various enantiomerically pure α-hydroxy esters were synthesized by asymmetric hydrogenation of α-keto esters catalyzed by Ru-C_n-Tunephos complex. Up to 97.1% ee has been achieved for both α-aryl and α-alkyl substituted α-keto esters.

Key words

asymmetric catalysis - enantioselectivity - ruthenium - hydrogenation - α-keto esters

References and Notes

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15

General Procedure for the Asymmetric Hydrogenation of α-Keto Esters.
[Ru(cymene)Cl₂]₂ (6.2 mg, 0.01 mmol) and (S)-C₃-Tunephos (12.5 mg, 0.021 mmol) were dissolved in degassed DMF (3 mL) in a Schlenk tube under N₂. The solution was heated at 100 °C for 3.5 h. After the mixture was cooled to 50 °C, the solvent was removed under vacuum to give the catalysts as an orange-red solid. The catalyst was dissolved in degassed MeOH (8 mL) in a glovebox and distributed equally between four vials. Substrate 6a (82 mg, 0.5 mmol) was then added to the catalyst solution. The resulting mixture was transferred into an autoclave and charged with 5 atm pressure of H₂. The autoclave was stirred at r.t. for 20 h. The autoclave was then cooled to r.t. and the H₂ was carefully released. The reaction solution was then evaporated and the residue was purified by column chromatography to give the corresponding hydrogenation product (76 mg, 92% yield). ¹H NMR (300 MHz, CDCl₃): δ = 7.34-7.44 (m, 5 H), 5.18 (d, J = 3.5 Hz, 1 H), 3.76 (s, 3 H), 3.59 (d, J = 3.5 Hz, 1 H). ¹³C NMR (75 MHz, CDCl₃): δ = 174.5, 138.7, 129.0, 128.9, 127.0, 73.3, 53.4.
[α]_D ²⁰ +180.5 (c 1.3, CHCl₃) for 97.1% ee; Gamma Dex 225, 30 m × 0.25 mm, column temperature: 130 °C, carrier gas: He, 1 mL/min, t ₁ = 19.000 min, t ₂ = 21.593 min.