Silver-Promoted Versatile Cross-Dehydrogenative Coupling of Quinaldine with Usual Ethers

Abstract

The application of small molecules as synthetic blocks is a field of great meaning and full of challenges, especially the use of inert ether molecules, such as THF as starting materials. A versatile and easily handled cross-dehydrogenative coupling between ethers and quinaldine is reported here. Compared to the developed reaction, Selectfluor is used as a mild oxidant, and a variety of functional groups are tolerated. Nitrogen protection, anhydrous systems, and external base are not needed, making this method a convenient route to functional heterocycles.

Key words

cross-dehydrogenative coupling - Selectfluor - quinaldine - ethers - silver

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• 17 6-Fluoro-2-methyl-4-(tetrahydrofuran-2-yl)quinoline (3a)To a 35 mL Schlenk tube equipped with a magnetic stir bar was charged 6-fluoro-2-methylquinoline (161 mg, 1.0 mmol, 1.0 equiv), Selectfluor (1.40 g, 4.0 mmol, 4.0 equiv), AgNO₃ (42.5 mg, 0.25 mmol, 0.25 equiv), H₂O (2.5 mL), and tetrahydrofuran (7.5 mL, THF/H₂O = 3:1). After stirring at 50 °C for the indicated time, the reaction mixture was diluted with 5 mL of saturated solution NaHCO₃ (5 mL) and extracted with EtOAc (3 × 20 mL). The combined organic extracts were washed with brine (2 × 20 mL), dried over Na₂SO₄, and concentrated in vacuo. Purification of the crude product by flash chromatography on silica gel using the indicated solvent system afforded the desired product. White solid, 201 mg, 87% yield. ¹H NMR (600 MHz, CDCl₃): δ = 8.01 (dd, J = 9.1, 5.6 Hz, 1 H), 7.45–7.39 (m, 3 H), 5.42 (t, J = 7.2 Hz, 1 H), 4.21 (td, J = 7.7, 5.7 Hz, 1 H), 4.02 (dd, J = 15.3, 7.2 Hz, 1 H), 2.70 (s, 3 H), 2.57 (dt, J = 14.1, 7.7 Hz, 1 H), 2.11–2.04 (m, 1 H), 2.03–1.96 (m, 1 H), 1.84–1.77 (m, 1 H). ¹³C NMR (151 MHz, CDCl₃): δ = 159.71 (d, J = 246.4 Hz), 158.31 (d, J = 2.6 Hz), 148.69 (d, J = 5.5 Hz), 144.96, 131.66 (d, J = 9.1 Hz), 124.40 (d, J = 9.3 Hz), 118.82 (d, J = 25.3 Hz), 117.93, 106.83 (d, J = 22.6 Hz), 76.73, 68.94, 33.57, 25.91, 25.31. ¹⁹F NMR (565 MHz, CDCl₃): δ = –113.79 (dd, J = 14.7, 8.6 Hz). HRMS (ESI): m/z calcd for C₁₄H₁₅FNO [M + H]⁺: 232.1132; found: 232.1132.