Room-Temperature Palladium(II)-Catalyzed Direct 2-Arylation of Indoles with Tetraarylstannanes

Abstract

A palladium(II)-catalyzed direct 2-arylation of indoles by tetraarylstannanes with oxygen (balloon) as the oxidant at room temperature has been developed. Various tetraarylstannanes can be employed as aryl sources for 2-arylation of indoles in up to 89% yield, providing a practical and efficient catalytic protocol for accessing 2-arylindoles.

Key words

indoles - arylation - palladium catalysis - tetraarylstannanes - C–H activation

• References and Notes

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• 30 1-Methyl-2-Phenyl-1H-indole (3a); Typical Procedure A Schlenk tube equipped with a magnetic stirrer bar was charged with N-methylindole (1a; 0.5 mmol, 1.0 equiv), Ph₄Sn (2a; 0.19 mmol), and (MeCN)₂Pd(OTs)₂ (10 mol%, 0.05 mmol). HOAc (5 mL) was then introduced by using a syringe. The tube was degassed (×3) then charged with O₂, and the O₂ atmosphere was maintained by using a balloon. The mixture was then stirred at 25 °C for 12 h. The HOAc was removed by rotary evaporation, and the residue was dissolved in CH₂Cl₂ (50 mL) and the solution was washed with aq NaHCO₃ (2 × 30 mL). The organic layers were combined, washed with brine, and dried (Na₂SO₄). The solvent was removed by rotary evaporation, and the crude product was purified by column chromatography (silica gel, PE–CH₂Cl₂) to give a white solid; yield: 89 mg (86%); mp 67.5–69.5 °C; R_f = 0.2 (PE–CH₂Cl₂, 10:1). ¹H NMR (300 MHz, acetone-d ₆): δ = 7.59–7.57 (m, 3 H), 7.50–7.48 (m, 2 H), 7.43 (d, J = 7.8 Hz, 2 H), 7.20 (t, J = 7.2 Hz, 1 H), 7.18 (t, J = 7.2 Hz, 1 H), 6.55 (s, 1 H), 3.75 (s, 3 H).¹³C NMR (75 MHz, acetone-d ₆): δ = 141.9, 139.2, 133.4, 129.7, 129.1, 128.7, 128.4, 122.0, 120.8, 120.1, 110.3, 101.9, 31.1. HRMS (ESI); m/z [M + Na]⁺ calculated for C₁₅H₁₃NNa: 230.0946; found: 230.0948. 1-Methyl-2-(4-tolyl)-1H-indole (3b) White solid; yield: 88 mg (80%); mp 97.5–100.5 ℃; R_f = 0.2 (PE–CH₂Cl₂, 10:1). ¹H NMR (300 MHz, acetone-d ₆): δ = (m, 1 H), 7.48–7.40 (m, 3 H), 7.33–7.31 (m, 2 H), 7.18 (t, J = 7.2 Hz, 1 H), 7.07 (t, J = 7.2 Hz, 1 H), 6.50 (s, 1 H), 3.75 (s, 3 H), 2.40 (s, 3 H). ¹³C NMR (75 MHz, acetone-d ₆): δ = 142.0, 139.1, 138.2, 130.5, 129.7, 129.3, 128.8, 120.6, 120.4, 120.1, 110.3, 101.5, 31.1, 20.9. HRMS (ESI); m/z [M + Na]⁺ calculated for C₁₆H₁₅NNa: 244.1102; found: 244.1104. Characterization data for other products are reported in the SI.