Organic Photoredox Carbonylation of Arenediazonium under Mild Conditions

Dedicated to the 60^th birthday of Burkhard König.

Abstract

The modular synthesis of diverse carbonyl compounds is at the heart of organic synthesis. An optimized protocol for photoredox carbonylation was developed that operates under milder conditions with mesitylacridinium as a photocatalyst. Arenediazonium salts were converted into benzoates (with alcohols), benzoic acids (with water), benzamides (with amines), and chlorides (with 1-butyl-3-methylimidazolium chloride) at 20 bar CO and 20 °C.

Key words

photoredox catalysis - carbonylation - carbon monoxide - visible light - arenediazonium compounds - radical reactions

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• 12 Arenediazonium Salt Formation; General Procedure The parent aniline (4.5 mmol) was dissolved in glacial HOAc (3 mL) and 48% aq HBF₄ (1.3 mL) at r.t. Isoamyl nitrite (1 mL) was dissolved in glacial HOAc (2 mL) and added to the aniline solution over 5 min. Then, Et₂O (15 mL) was added, and the mixture was cooled to –30 °C until a crystalline product precipitated. The precipitate was collected by filtration and redissolved in a minimal amount of acetone. Finally, the diazonium tetrafluoroborate was precipitated by addition of Et₂O (15 mL), collected by filtration, washed with Et₂O (2 × 10 mL), and dried in air. Photocarbonylation; General Procedure A glass vial (2 mL) containing a magnetic stirrer bar was charged with the appropriate arenediazonium salt (0.1 mmol), and a 1 mM solution of [MesAcr]ClO₄ in MeCN (1 mL). The rubber septum of the vial cap was punctured with a needle, which was left in place. The vial was then transferred to an autoclave equipped with a quartz-glass window at the bottom (Parr Instrument Co., Moline, IL). The reactor was sealed, placed on a magnetic stirrer, and slowly filled with CO (20 bar). The vial was irradiated with an external LED (λ_max = 450 nm) for 5 h at 20 °C. The gas pressure was then released and the vial was retrieved. H₂O (3 mL) was adsded, and the resulting mixture was extracted with EtOAc (2 × 5 mL). The organic phases were washed with brine (5 mL), dried (Na₂SO₄), and concentrated. The residue was purified by flash column chromatography (silica gel). Methyl 4-methoxybenzoate (2a) Purified by flash column chromatography [silica gel, pentane–EtOAc (20:1)] as a colorless solid; yield: 13.1 mg (79%). ¹H NMR (300 MHz, CDCl₃): δ = 8.01–7.98 (m, 2 H), 6.93–6.90 (m, 2 H), 3.88 (s, 3 H), 3.85 (s, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 167.0, 163.4, 131.7, 122.8, 113.7,s 55.6, 52.0. GC/MS (EI): m/z (%) = 166 (33) [M⁺], 135 (100), 107 (12), 92 (17), 77 (21).
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