Rhodium-Catalyzed Regioselective C7-Functionalization of Indole Derivatives with Acrylates by Using an N-Imino Directing Group

Dedicated to Professor Victor Snieckus on the occasion of his 80th birthday

Abstract

An efficient rhodium-catalyzed method for C–H olefination at the C7 position of indoles has been developed. The N-imino directing group was shown to be crucial for high regioselectivity and reactivity of the metal catalyst. The utility of this protocol was further demonstrated through a concise, four-step synthesis of pyroquilon from indole.

Key words

indoles - C7 regioselectivity - C–H activation - rhodium - olefination

• References and Notes


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• 18 The structure of 10ha was confirmed by X-ray crystal analysis; details are given in the Supporting Information.
• 19 General Experimental Procedure for the Synthesis of N-Imino Indole Substrates 8ha–hv, 14a and 14b: A sealed tube (50.0 mL) was charged with N-aminoindoles (10.0 mmol) and 2,6-dimethylbenzaldehyde (1.34 g, 10.0 mmol). The tube was evacuated and backfilled with argon before DCE (3.0 mL) was added. The reaction mixture was stirred at 120–130 °C for 4 h. After the reaction mixture was cooled to room temperature, the solvent was distilled off under ambient pressure by simple distillation. Purification of the residual oil by flash chromatography gave the desired products. (E)-1-(2,6-Dimethylphenyl)-N-(1H-indol-1-yl)methanimine (8ha): Yield: 2.3 g (92%); light-yellow solid; R _f = 0.72 (5% EtOAc/petroleum ether). ¹H NMR (500 MHz, CDCl₃): δ = 8.81 (s, 1 H), 7.81 (d, J = 8.3 Hz, 1 H), 7.76 (d, J = 3.6 Hz, 1 H), 7.66 (d, J = 7.9 Hz, 1 H), 7.36 (t, J = 7.6 Hz, 1 H), 7.25–7.20 (m, 2 H), 7.17 (d, J = 7.5 Hz, 2 H), 6.74 (d, J = 3.4 Hz, 1 H), 2.64 (s, 6 H); ¹³C NMR (125 MHz, CDCl₃): δ = 144.42, 138.11, 136.58, 130.93, 129.26, 129.17, 126.88, 123.44, 121.15, 121.02, 116.08, 110.81, 104.13, 21.84; HRMS (ESI): m/z [M + H]⁺ calcd for C₁₇H₁₇N₂: 249.1386; found: 249.1383. (E)-1-(2,6-Dimethylphenyl)-N-(5-methoxy-1H-indol-1-yl)methanimine (8hh): Yield: 2.4 g (88%); brown solid; R_f = 0.46 (5% EtOAc/petroleum ether). ¹H NMR (400 MHz, CDCl₃): δ = 8.76 (s, 1 H), 7.72 (d, J = 3.6 Hz, 1 H), 7.66 (d, J = 8.8 Hz, 1 H), 7.23–7.19 (m, 1 H), 7.15 (d, J = 7.6 Hz, 2 H), 7.09 (d, J = 2.4 Hz, 1 H), 6.99 (dd, J = 8.9, 2.4 Hz, 1 H), 6.64 (d, J = 3.5 Hz, 1 H), 3.88 (s, 3 H), 2.61 (s, 6 H); ¹³C NMR (100 MHz, CDCl₃): δ = 155.23, 144.11, 138.07, 131.82, 130.96, 129.21, 129.17, 127.21, 116.40, 113.51, 111.55, 103.71, 102.81, 56.01, 21.84; HRMS (ESI): m/z [M + H]⁺ calcd for C₁₈H₁₉ON₂: 279.1492; found: 279.1488.
• 20 General Experimental Procedure for the Synthesis of C7-Alkenylated Indole Derivatives 10ha–ht, 15a and 15b: A sealed tube (10.0 mL) was charged with indole substrates 8 (0.25 mmol), [RhCp*Cl₂]₂ (1.5 mg, 1 mol%), Cu(OAc)₂·H₂O (104 mg, 0.53 mmol), and AgSbF₆ (3.1 mg, 4 mol%). The tube was evacuated and backfilled with argon before acrylate (1.0 mmol) and dichloromethane (1.0 mL) were added. The reaction mixture was stirred at 120–140 °C for 48–72 h. After cooling to room temperature, the mixture was diluted with dichloromethane, filtered through a plug of diatomite, concentrated under vacuum and the residue was purified by chromatography on silica with a gradient eluent of petroleum ether and ethyl acetate to give the corresponding products. Methyl (E)-3-(1-{[(E)-2,6-Dimethylbenzylidene]amino}-1H-indol-7-yl)acrylate (10ha): Yield: 70 mg (85%); white solid; R_f = 0.25 (10% EtOAc/petroleum ether). ¹H NMR (400 MHz, CDCl₃): δ = 9.01 (d, J = 16.0 Hz, 1 H), 8.72 (s, 1 H), 7.68 (d, J = 3.7 Hz, 1 H), 7.58 (d, J = 7.5 Hz, 1 H), 7.48 (d, J = 7.5 Hz, 1 H), 7.16–7.05 (m, 4 H), 6.65 (d, J = 3.6 Hz, 1 H), 6.36 (d, J = 16.0 Hz, 1 H), 3.66 (s, 3 H), 2.52 (s, 6 H); ¹³C NMR (100 MHz, CDCl₃): δ = 167.56, 146.83, 143.25, 138.45, 133.06, 130.60, 129.68, 129.07, 128.61, 123.37, 122.76, 121.04, 121.03, 117.95, 117.67, 104.47, 51.52, 21.44; HRMS (ESI): m/z [M + H]⁺ calcd for C₂₁H₂₁O₂N₂: 333.1598; found: 333.1601. Methyl (E)-3-(1-{[(E)-2,6-Dimethylbenzylidene]amino}-3-[2-(1,3-dioxoisoindolin-2-yl)ethyl]-1H-indol-7-yl)acrylate (10hp): Yield: 88 mg (70%); white solid; R_f = 0.17 (25% EtOAc/petroleum ether). ¹H NMR (400 MHz, CDCl₃): δ = 9.06 (d, J = 16.0 Hz, 1 H), 8.75 (s, 1 H), 7.86–7.84 (m, 2 H), 7.77–7.71 (m, 4 H), 7.56 (d, J = 7.5 Hz, 1 H), 7.24–7.14 (m, 4 H), 6.42 (d, J = 16.0 Hz, 1 H), 4.10 (t, J = 7.4 Hz, 2 H), 3.75 (s, 3 H), 3.24 (t, J = 7.3 Hz, 2 H), 2.61 (s, 6 H); ¹³C NMR (100 MHz, CDCl₃): δ = 168.51, 167.55, 146.04, 143.22, 138.40, 134.17, 133.42, 132.23, 130.74, 129.52, 129.02, 128.23, 123.42, 122.99, 121.21, 121.03, 120.83, 117.98, 115.68, 114.45, 51.53, 37.96, 24.52, 21.46; HRMS (ESI): m/z [M + H]⁺ calcd for C₃₁H₂₈O₄N₃: 506.2074; found: 506.2068.
• 21 General Experimental Procedure for the Synthesis of 11a–d: A suspension of indole derivatives 10h (0.1 mmol) and activated Zn powder (5.0 mmol) in a 1:1 mixture of THF and saturated NH₄Cl solution (2.0 mL) was stirred at room temperature to 50 °C until consumption of the starting material as monitored by TLC. The mixture was diluted with EtOAc and filtered through a plug of diatomite. The filtrate was extracted with EtOAc and washed with brine. The organic layer was dried over anhydrous Na₂SO₄ and concentrated in vacuo. Residue was purified by chromatography on silica with a gradient eluent of petroleum ether and ethyl acetate to give the corresponding products. Methyl (E)-3-(1H-Indol-7-yl)acrylate (11a): Yield: 17 mg (85%); white solid; R_f = 0.38 (25% EtOAc/petroleum ether). ¹H NMR (500 MHz, CDCl₃): δ = 8.64 (br s, 1 H), 8.07 (d, J = 16.0 Hz, 1 H), 7.72 (d, J = 7.9 Hz, 1 H), 7.43 (d, J = 7.4 Hz, 1 H), 7.28 (t, J = 2.9 Hz, 1 H), 7.17 (t, J = 7.7 Hz, 1 H), 6.62 (dd, J = 3.2, 2.0 Hz, 1 H), 6.54 (d, J = 16.0 Hz, 1 H), 3.85 (s, 3 H); ¹³C NMR (125 MHz, CDCl₃): δ = 167.85, 141.47, 134.45, 129.06, 124.95, 123.59, 122.47, 120.30, 118.32, 117.71, 103.55, 51.95; HRMS (ESI): m/z [M + H]⁺ calcd for C₁₂H₁₂O₂N: 202.0863; found: 202.0862.
• 22 General Experimental Procedure for the Synthesis of 12a–d: C7-alkenylation of indole derivatives 10h (0.1 mmol) was dissolved in EtOH (3.0 mL). To the solution, an excess amount of Raney Ni slurry was added and the resulting mixture was stirred under H₂ atmosphere (1 atm) at 50 °C until consumption of the starting material as monitored by TLC. The mixture was filtered through a plug of diatomite and extracted with EtOAc, and washed with brine. The organic layer was dried over anhydrous Na₂SO₄ and concentrated in vacuo. Residue was purified by chromatography on silica with a gradient eluent of petroleum ether and ethyl acetate to give the corresponding products. Methyl 3-(1H-Indol-7-yl)propanoate (12a): Yield: 16 mg (78%); white solid; R_f = 0.20 (25% EtOAc/petroleum ether). ¹H NMR (400 MHz, CDCl₃): δ = 8.86 (br s, 1 H), 7.54 (d, J = 7.8 Hz, 1 H), 7.24 (t, J = 2.8 Hz, 1 H), 7.08 (t, J = 7.3 Hz, 1 H), 7.00 (d, J = 6.9 Hz, 1 H), 6.56 (dd, J = 3.1, 2 Hz, 1 H), 3.68 (s, 3 H), 3.22 (t, J = 7.0 Hz, 2 H), 2.79 (t, J = 7.0 Hz, 2 H); ¹³C NMR (100 MHz, CDCl₃): δ = 175.09, 135.01, 128.17, 124.42, 123.71, 121.77, 120.06, 119.25, 102.93, 52.07, 34.77, 26.43; HRMS (ESI): m/z [M + H]⁺ calcd for C₁₂H₁₄O₂N: 204.1019; found: 204.1017.
• 23 Procedure for the Synthesis of Pyroquilon: In a test tube C7-alkenylated indole derivative 10ha (0.1 mmol) was dissolved in EtOH (3 mL), and then an excess amount of Raney Ni slurry was added. The test tube was placed in a stainless steel autoclave, and autoclave was sealed under N₂ before purging with hydrogen for three times. The autoclave was charged with hydrogen (40 kg/cm²), and the reaction mixture was stirred at 70 °C for 6 h. After cooling to room temperature, the remaining hydrogen was released. The mixture was filtered through a plug of diatomite and extracted with EtOAc, and washed with brine. The organic layer was dried over anhydrous Na₂SO₄ and concentrated in vacuo. Residue was purified by chromatography on silica with a gradient eluent of petroleum ether and ethyl acetate to give pyroquilon. 1,2,5,6-Tetrahydro-4H-pyrrolo[3,2,1-ij]quinolin-4-one (13): Yield: 16 mg (95%); white solid; R_f = 0.14 (25% EtOAc/petroleum ether). ¹H NMR (400 MHz, CDCl₃): δ = 7.08 (d, J = 7.3 Hz, 1 H), 7.00 (d, J = 7.4 Hz, 1 H), 6.94 (t, J = 7.4 Hz, 1 H), 4.10 (t, J = 8.4 Hz, 2 H), 3.21 (t, J = 8.4 Hz, 2 H), 2.99 (t, J = 7.6 Hz, 2 H), 2.70 (t, J = 7.8 Hz, 2 H); ¹³C NMR (100 MHz, CDCl₃): δ = 167.82, 141.47, 129.07, 125.46, 123.42, 123.39, 120.35, 45.29, 31.80, 27.91, 24.53; HRMS (ESI): m/z [M + H]⁺ calcd for C₁₁H₁₂ON: 174.0913; found: 174.0913.
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