Dual Photoredox/Palladium-Catalyzed C–H Acylation of 2-Arylpyridines with Oxime Esters

Published as part of the Cluster Radicals – by Young Chinese Organic Chemists

Abstract

An unprecedented dual photoredox/palladium-catalyzed iminyl-radical-mediated C–C bond cleavage and directed ortho C–H acylation of 2-arylpyridines by using oxime esters is described. Oxime esters can serve as efficient acyl sources through formation of the corresponding acyl radicals by photoredox-catalyzed iminyl-radical-mediated C–C bond cleavage. This redox-neutral protocol features excellent regioselectivity, a broad substrate scope, and good functional-group tolerance with respect to both components, giving a broad range of aryl ketones with generally good yields.

Key words

photocatalysis - palladium catalysis - C–H activation - acylation - iminyl radical - oxime esters

• References and Notes

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• 26 1-(2-Pyridin-2-ylphenyl)ethanone (3aa); Typical Procedure A 10 mL, flame-dried, round-bottomed Schlenk flask equipped with a magnetic stirrer bar was charged with 1a (0.2 mmol, 31.04 mg), 2a (0.4 mmol, 57.26 mg), AgOTf (0.5 mmol, 128.47 mg), fac-Ir(ppy)₃ (0.002 mmol, 1.31 mg), and Pd(TFA)₂ (0.02 mmol, 6.65 mg). The flask was evacuated and backfilled with Ar three times, and the mixture was irradiated with 7 W blue LED strips until the reaction was complete (24–48 h; TLC). The mixture was then poured into a separatory funnel containing 20 mL of sat. aq NaCl and washed with CH₂Cl₂ (20 mL). The organic layers were separated, dried (Na₂SO₄), filtered, and concentrated under reduced pressure. The crude product was purified by flash chromatography (silica gel) to give a colorless oil; yield: 29.6 mg (75%). ¹H NMR (400 MHz, CDCl₃): δ = 8.64 (d, J = 4.4 Hz, 1 H), 7.78 (t, J = 7.7 Hz, 1 H), 7.60 (dd, J = 12.9, 7.7 Hz, 2 H), 7.53 (t, J = 6.9 Hz, 2 H), 7.47 (d, J = 8.1 Hz, 1 H), 7.27 (t, J = 5.9 Hz, 1 H), 2.23 (s, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 204.1, 157.6, 149.2, 141.5, 138.8, 136.7, 130.3, 129.1, 128.6, 127.6, 122.5, 122.3, 30.5. HRMS (EI): m/z [M + H]⁺ calcd for C₁₃H₁₂NO: 198.0913; found: 198.0918.