Reduction of Nitroarenes to Anilines with a Benzothiazoline: Application to Enantioselective Synthesis of 2-Arylquinoline Derivatives

Published as part of the 30 Years SYNLETT – Pearl Anniversary Issue

Abstract

The metal-free reduction of nitroarenes to aniline derivatives was accomplished in a short time by using a benzothiazoline as the hydrogen donor in combination with a Brønsted acid. An enantioselective synthesis of 2-arylquinolines was achieved by using 1-aryl-3-(2-nitrophenyl)propan-1-ones as starting materials and a combination of a benzothiazoline and a chiral phosphoric acid.

Key words

benzothiazolines - phosphoric acids - isoquinolines - nitro­arenes - anilines - reduction

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• 13 2-Aryl-1,2,3,4-tetrahydroquinolines 10a–c; General ProcedureUnder a N₂ atmosphere, a mixture of the appropriate ketone 9 (0.10 mmol), benzothiazoline 2f (0.60 mmol), chiral phosphoric acid 8 (0.010 mmol), and MS 3Å (600 wt%, activated) in toluene (1.0 mL) was refluxed for 2 days. When the reaction was complete (TLC), it was quenched by adding sat. aq NaHCO₃. The crude mixture was filtered through a Celite pad and extracted with EtOAc (×3). The organic extracts were combined, washed with brine, dried (Na₂SO₄), and concentrated in vacuo. The residue was purified by preparative TLC.2-Phenyl-1,2,3,4-tetrahydroquinoline (10a)White solid; yield: 13 mg (60%, 92% ee); mp 52–54 °C; [α]_D ²⁴ –42 (c 0.75, CHCl₃). ¹H NMR (400 MHz, CDCl₃): δ = 1.94–2.05 (m, 1 H), 2.09–2.15 (m, 1 H), 2.74 (dt, J = 4.8, 16.4 Hz, 1 H), 2.93 (ddd, J = 5.6, 10.8, 16.4 Hz, 1 H), 4.04 (br s, 1 H), 4.43 (dd, J = 3.4, 9.2 Hz, 1 H), 6.53 (d, J = 8.4 Hz, 1 H), 6.65 (t, J = 7.6 Hz, 1 H), 6.99–7.02 (m, 2 H), 7.24–7.40 (m, 5 H). ¹³C NMR (100 MHz, CDCl₃): δ = 26.4, 31.0, 56.3, 114.0, 117.2, 120.9, 126.6, 126.9, 127.5, 128.6, 129.3, 144.7, 144.8.