Cobalt(III)-Catalyzed Allylation with Allyl Acetates by C–H/C–O Cleavage

Dedicated to the 69th birthday of Prof. Peter Vollhardt

Abstract

Versatile cobalt-catalyzed C–H allylations on arenes, indoles, and pyrroles were accomplished with allyl acetates. The C–H/C–O functionalization process was characterized by a broad substrate scope as well as an excellent functional-group tolerance and was shown to occur by initial C–H cobaltation.

Key words

allylation - C–H activation - C–O cleavage - cobalt - heteroarene

• References and Notes


Representative recent reviews on C–H activation:
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For examples of metal-catalyzed direct C–H bond arylations through challenging C–O bond cleavage with fluorine-free electrophiles, see:
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For recent progress in cobalt-catalyzed C–H functionalizations, see:
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Representative recent examples:
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For examples with ruthenium or rhodium complexes, see:
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• 19 General Procedure for the Cobalt-Catalyzed C–H Allylation To a solution of heteroarene 1 (0.50 mmol, 1.0 equiv), [Cp*Co(CO)I₂] (4, 0.025 mmol, 5.0 mol%), AgSbF₆ (0.05 mmol, 10 mol%), and AcOK (0.05 mmol, 10 mol%) in DCE (1.5 mL) allyl acetate (2a, 1.00 mmol, 2.0 equiv) was added. The mixture was stirred for 16 h at 80 °C. After completion of the reaction, sat. aq NH₄Cl solution (5 mL) was added at ambient temperature, and the mixture was extracted with MTBE (4 × 5 mL). Drying over Na₂SO₄, evaporation of the solvents, and purification by column chromatography on silica gel using n-hexane–EtOAc yielded the product. 2-Allyl-5-fluoro-1-(pyrimidin-2-yl)-1H-indole (3d) The general procedure was followed using indole 1d (107 mg, 0.50 mmol, 1.0 equiv), allyl acetate (2a, 107 mg, 1.00 mmol, 2.0 equiv), [Cp*Co(CO)I₂] (4, 11.9 mg, 0.025 mmol, 5 mol%), AgSbF₆ (17.3 mg, 0.05 mmol, 10 mol%), and AcOK (4.9 mg, 0.05 mmol, 10 mol%). Purification by column chromatography on silica gel (n-hexane–EtOAc = 10:1) yielded 3d (119 mg, 473 μmol, 94%) as a colorless solid; mp 65–67 °C. R_f  = 0.33 (n-hexane–EtOAc = 10:1). ¹H NMR (300 MHz, CDCl₃): δ = 8.77 (dd, J = 4.8, 0.5 Hz, 2 H), 8.23 (ddt, J = 9.1, 4.7, 0.6 Hz, 1 H), 7.18 (ddd, J = 9.0, 2.6, 0.5 Hz, 1 H), 7.14 (td, J = 4.8, 0.5 Hz, 1 H), 6.95 (td, J = 9.2, 2.6 Hz, 1 H), 6.45 (dt, J = 1.0 Hz, 1 H), 5.97 (ddt, J = 17.0, 10.1, 6.5 Hz, 1 H), 5.06 (ddt, J = 17.0, 1.6 Hz, 1 H), 5.03 (ddt, J = 10.1, 1.6 Hz, 1 H), 3.97 (dq, J = 6.6, 1.3 Hz, 2 H). ¹³C NMR (125 MHz, CDCl₃): δ = 159.1 (d, ¹ J _C–F = 237.0 Hz, C_q), 158.2 (CH), 158.2 (C_q), 141.7 (C_q), 135.4 (CH), 133.6 (C_q), 130.1 (d, ³ J _C–F = 10.1 Hz, C_q), 117.3 (CH), 116.8 (CH₂), 115.1 (d, ³ J _C–F = 9.0 Hz, CH), 110.4 (d, ² J _C–F = 25.0 Hz, CH), 106.4 (CH), 105.1 (d, ² J _C–F = 23.6 Hz, CH), 34.4 (CH₂). ¹⁹F NMR (282 MHz, CDCl₃): δ = –122.89 (td, J = 9.2, 4.7 Hz). IR (ATR): ν = 3080, 2922, 1558, 1442 1348, 1204, 927, 897, 809, 635 cm^–1. MS (EI): m/z(relative intensity): 276.1 (19) [M + Na]⁺, 254.1 (100) [M + H]⁺. ESI-HRMS: m/zcalcd for C₁₅H₁₃FN₃ [M + H]⁺: 254.1094; found: 254.1088. 2-Allyl-1-(pyrimidin-2-yl)-6,7-dihydro-1H-indol-4(5H)-one (7a) The general procedure was followed using indolone 6a (107 mg, 0.50 mmol, 1.0 equiv), allyl acetate (2a, 104 mg, 1.00 mmol, 2.0 equiv), [Cp*Co(CO)I₂] (4, 11.9 mg, 0.025 mmol, 5 mol%), AgSbF₆ (17.3 mg, 0.05 mmol, 10 mol%), and AcOK (5.0 mg, 0.05 mmol, 10 mol%). Purification by column chromatography on silica gel (n-hexane–EtOAc = 10:1) yielded 7a (116 mg, 462 μmol, 92%) as a colorless solid; mp 104–106 °C. R_f = 0.30 (n-hexane–EtOAc = 10:1). ¹H NMR (300 MHz, CDCl₃): δ = 8.78 (dd, J = 4.9, 0.7 Hz, 2 H), 7.28 (t, J = 4.9 Hz, 1 H), 6.43 (d, J = 1.0 Hz, 1 H), 5.76 (ddt, J = 16.8, 10.4, 6.5 Hz, 1 H), 4.87 (ddt, J = 10.3, 1.4 Hz, 1 H), 4.85 (ddt, J = 16.8, 1.6 Hz, 1 H), 3.59 (dq, J = 6.5, 1.2 Hz, 2 H), 2.95 (t, J = 6.2 Hz, 2 H), 2.48 (dd, J = 7.1, 5.7 Hz, 2 H), 2.09 (tt, J = 6.4 Hz, 2 H). ¹³C NMR (125 MHz, CDCl₃): δ = 194.9 (C_q), 158.9 (CH), 157.0 (C_q), 145.5 (C_q), 135.2 (CH), 134.2 (C_q), 121.6 (C_q), 119. 2 (CH), 116.4 (CH₂), 106.2 (CH), 37.9 (CH₂), 32.4 (CH₂), 24.2 (CH₂), 24.0 (CH₂). IR (ATR): ν = 2935, 1640, 1559, 1417, 1406, 1168, 994, 898, 823, 734 cm^–1. MS (EI): m/z (relative intensity): 292.1 (60) [M + K]⁺, 276.1 (51) [M + Na]⁺, 254.1 (100) [M + H]⁺. ESI-HRMS: m/z calcd for C₁₅H₁₆N₃O [M + H]⁺: 254.1293; found: 254.1288.
• 20 The removal of the N-pyrimidyl-substituted indoles was described in ref. 15.