Lewis Base Catalyzed Asymmetric Hydrosilylation of α-Substituted β-Enamino Esters: Facile Access to Enantioenriched β²-Amino Esters via Dynamic Kinetic Resolution

Abstract

A chiral Lewis base organocatalyzed asymmetric hydrosilylation of α-substituted β-enamino esters is presented. The reactions proceeded through dynamic kinetic resolution to afford various enantioenriched β²-amino esters with high yields (up to 98%) in moderate enantioselectivities (up to 77% ee).

Key words

asymmetric catalysis - dynamic kinetic resolution - β²-amino acids - hydrosilylation - Lewis base

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• 13 General Experimental Procedure for the Enantioselective Hydrosilylation of α-Substituted β-Enamino Esters A solution of trichlorosilane (41 μL, 0.3 mmol, 2.0 equiv) in 160 μL of CH₂Cl₂ was added to a stirred solution of the corresponding α-substituted β-enamino ester (0.20 mmol) and the catalyst 3i (0.020 mmol) in CH₂Cl₂ (2.0 mL) at –10 °C. The mixture was stirred at the same temperature until the reaction reached completion. Then the reaction was quenched with a sat. aq solution of NaHCO₃ and was extracted with CH₂Cl₂. The combined organic layer was washed with brine, dried over anhydrous Na₂SO₄, and the solvents were evaporated. Purification by column chromatography (silica gel; hexane–EtOAc, 10:1) afforded the products. The ee values were determined using established HPLC techniques with chiral stationary phases.
• 14 Ethyl 2-(3,4-Dimethoxyphenyl)-3-(4-methoxyphenyl-amino)propanoate (2e) Yield 99%; 77% ee; light yellow oil. ¹H NMR (300 MHz, CDCl₃): δ = 6.76–6.84 (m, 5 H), 6.58 (d, J = 8.8 Hz, 2 H), 4.10–4.19 (m, 2 H), 3.86 (s, 6 H), 3.79–3.83 (m, 1 H), 3.72–3.74 (m, 4 H), 3.59 (br s, 1 H), 1.18–1.28 (m, 3 H) ppm. ¹³C NMR (75 MHz, CDCl₃): δ = 173.0, 152.3, 149.1, 148.4, 141.4, 129.2, 120.3, 114.9, 114.6, 114.3, 110.9, 60.9, 55.8, 55.7, 50.3, 47.9, 14.0 ppm. ESI-HRMS: m/z calcd for [C₂₀H₂₅NO₅ + H]⁺: 360.1811; found: 330.1804. [α]_D ²⁰ +112 (c 0.50, CHCl₃). AD-H column (n-hexane–2-PrOH, 80:20), flow rate = 1.0 mL/min, t _R = 16.7, 17.9 min.