Solvent Dependent Regioselective Hydrogenation of Substituted Quinolines

Abstract

Various substituted quinolines have been reduced under H₂ using Rh/Al₂O₃. Using methanol as solvent leads selectively to the 1,2,3,4-tetrahydroquinoline derivatives whereas in hexafluoro­isopropanol the decahydro compounds are obtained.

Key words

hydrogenation - substituted quinolines - rhodium - solvent effects - HFIP

References

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10

Rh/Al₂O₃ (5%) was purchased from Acros and used as received.

11

All the products have been fully characterized by ¹H NMR (300 MHz) and ¹³C NMR (75 MHz) and analyses are in agreement with already published data. Data not fond in the literature: Product 1b (mixture of diastereoisomers): ¹H NMR (300 MHz, CDCl₃): δ = 0.94-1.90 (m, 11 H), 2.18 (t, J = 10.3 Hz) and 2.37 (dd, J = 2.2 Hz, J = 10.7 Hz) for ¹H in the ratio 1:3, 2.69 (m, 2 H), 3.18 (m, 2 H), 3.50 (m)and 3.71 (dt, J = 4.5, J = 11.9 Hz) for ¹H in the ratio 1:3. ¹³C NMR (75 MHz, CDCl₃): δ = 19.3, 20.4, 23.4, 23.6, 23.7, 24.9, 28.2, 28.9, 30.9, 32.2, 32.4, 34.2 (C_t) and 34.6 (C_t), 45.6 (C_s) and 46.2 (C_s), 60.2 (C_t) and 64.0 (C_t) and 66.9 (C_t), 69.9 (C_t) and 70.4 (C_t) and 70.8 (C_t) and 70.9 (C_t). Product 3a: ¹H NMR (300 MHz, CDCl₃): δ = 1.83 (t, 2 H, J = 6.0 Hz), 2.63 (t, 2 H, J = 6.0 Hz), 3.19 (t, 2 H, J = 6.0 Hz), 6.34 (d, 1 H, J = 8.1 Hz), 6.97 (m, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 21.7, 26.9, 42.1, 109.6 (C_q), 116.6, 124.5 (C_q), 129.7, 132.2, 142.7 (C_q).

20

In a Typical Procedure: Quinoline derivative (1 mmol) and Rh/Al₂O₃ [10] (25 mg, 0.01 mmol) were stirred in 1.5 mL of the selected solvent under 50 bar H₂ at r.t. for the requisite time. The reaction mixture was filtered through celite, the solvent evaporated and the crude product analyzed by NMR without further purification.