Ru-SYNPHOS^® and Ru-DIFLUORPHOS^®: Highly Efficient Catalysts for Practical Preparation of β-Hydroxy Amides

Abstract

Ru-SYNPHOS^® and Ru-DIFLUORPHOS^® catalysts were efficiently used for the synthesis of a wide variety of chiral β-hydroxy amides via asymmetric hydrogenation of the corresponding β-keto amides.

Key words

catalysts - hydrogenation - enantioselectivity - ruthenium

References

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TOP INDUSTRIE S. A., 80, rue Marinoni, BP 38, 77013 Vaulx le penil Cedex, France; www.top-industrie.com.

23

General Hydrogenation Procedure.
(S)-SYNPHOS (7.5 mg, 1.2 × 10^-2 mmol) and (COD)Ru(2-methylallyl)₂ (3.2 mg, 1.0 × 10^-2 mmol) were placed in a 5 mL flask and 2 mL of anhyd acetone were added. A methanolic solution of HBr (0.122 µL, 0.18 M) was added to the resulting suspension and the reaction mixture was stirred at r.t. for about 30 min. The solvent was removed under vacuum. The yellow solid residue was used as catalyst for the hydrogenation reaction. Then, MeOH (4 mL), acetoacetanilide (5, 177 mg, 1 mmol) and the catalyst were placed under argon in the TOP INDUSTRIE parallel hydrogenation system. The autoclave was pressurized to an initial pressure of 10 bar of hydrogen and the reaction was allowed to proceed at 50 °C for 2 h. The crude reaction mixture was purified by silica gel chromatography (cyclohexane-EtOAc, 50:50) to afford 163 mg of (R)-6 as a white solid (91%). Mp 125-126 °C; [α]_D +43 (c 1, CHCl₃); [α]_D +29 (c 1, CH₃OH). IR (KBr disk): 3331 (ν_OH-NH), 3111 (ν_CH3), 1645 (ν_CO) cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 2.50-2.59 (m, 2 H), 2.84 (d, J = 4.8 Hz, 3 H), 4.18 (br, 1 H), 5.09-5.16 (dd, J = 5.0, 7.6 Hz, 1 H), 5.83 (br, 1 H), 7.31-7.39 (m, 5 H). ¹³C NMR (CDCl₃): δ = 26.0, 44.4, 70.7, 125.4, 142.9, 152.4, 172.4. MS (70 eV): m/z (%) = 179 (21) [M⁺], 73 (100), 105 (26), 58 (24), 43 (25).