Asymmetric Tandem Reduction of 2-(Aroylmethyl)quinolines with Phosphine-Free Ru-TsDPEN Catalyst

Abstract

The phosphine-free ruthenium complex containing chi-ral N-(p-toluenesulfonyl)-1,2-diphenylethylenediamine (TsDPEN) showed excellent stereoselectivity in the tandem asymmetric reduction of 2-(aroylmethyl)quinolines. The reaction involves transfer hydrogenation of aromatic ketones and hydrogenation of quinolines, giving 1,2,3,4-tetrahydroquinoline derivatives with up to 99% ee and 95:5 dr.

asymmetric catalysis - tandem reaction - quinolines - ­ruthenium - hydrogenation

References and Notes

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12

Typical procedure for the Ru-catalyzed asymmetric ATH/AH reactions: Into a 50 mL glass-lined stainless steel reactor with a magnetic stirring bar was charged (R,R)-1b (0.6 mg, 0.001 mmol), substrate 2a (24.7 mg, 0.1 mmol) and degassed EtOH (1 mL) under a nitrogen atmosphere, and the mixture was stirred at r.t. for 24 h. Then, to the reaction mixture was added a solution of 1.0 M TfOH in EtOH (100 µL, 0.001 mmol, 1 mol% cf substrate) under a nitrogen atmosphere. The autoclave was closed, and H₂ was initially introduced into the autoclave at a pressure of 50 atm, before being reduced to 1 atm. After this procedure was repeated three times, the autoclave was pressurized with H₂ to 50 atm. Subsequently, the mixture was stirred under this H₂ pressure at r.t. for another 12 h. After carefully releasing the hydrogen, the mixture was concentrated to afford the crude product. The conversion and diastereoselectivity were determined by ^¹H NMR analysis of the crude product. Further purification was performed with a silica gel column (PE-CH₂Cl₂, 1:1) to give the pure product, (+)-1-phenyl-2-(1,2,3,4-tetrahydroquinolin-2-yl)ethanol (4a). Isolated yield: 94%; >95:5 dr; >99% ee; [α] _d ^²0 +67.9 (c 1.00, CHCl₃); ^¹H NMR (300 MHz, CDCl₃): δ = 7.38-7.29 (m, 5 H), 7.00-6.95 (m, 2 H), 6.67-6.62 (m, 1 H), 6.49 (d, J = 7.8 Hz, 1 H), 5.02 (t, J = 6.6 Hz, 1 H), 3.55-3.47 (m, 1 H), 2.88-2.69 (m, 2 H), 1.97-1.90 (m, 3 H), 1.88-1.81 (m, 1 H); ^¹³C NMR (75 MHz, CDCl₃): δ = 143.34, 143.14, 128.30, 127.56, 126.61, 125.70, 124.69, 120.69, 116.73, 113.99, 71.07, 47.79, 43.67, 26.95, 25.15; HRMS (ESI): m/z [M + H]⁺ calcd for C1₇H₂₀NO: 254.15394; found: 254.15385.