Direct Synthesis of Enones by Visible-Light-Promoted Oxygenation of Trisubstituted Olefins Using Molecular Oxygen

Abstract

A one-step synthesis of enones from olefins is described. The reaction was performed under visible-light irradiation in the presence of molecular oxygen and a photocatalyst. The reaction proceeded with various types of trisubstituted olefins to give enones in good yields with high regioselectivity. In particular, oxygen- and nitrogen-containing functional groups, heteroaromatic rings, and cyclopropanes were tolerated. Mechanistic studies and previous reports indicated that the active oxygen species generated in the reaction system is singlet oxygen.

Key words

oxidation - oxygenation - photocatalysis - ruthenium catalysis - enones - alkenes

• References and Notes

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• 27 Enones 5a–m; General Procedure (0.3 mmol Scale) A test tube was charged with trisubstituted olefin 4 (0.3 mmol, 1.0 equiv.), Ru(bpy)₃(PF₆)₂ (3.0 μmol, 1 mol%), Cu(OTf)₂ (0.03 mmol, 10 mol%), and TEMPO (0.3 mmol, 1.0 equiv). The tube was then evacuated and backfilled with O₂. An O₂-filled balloon was attached to the tube and then DMA (3.0 mL, 0.1 M) was added from a syringe. The mixture was irradiated by a pair of 5 W blue LEDs (λ_max = 450 nm) at 3–5 cm distance, cooled by a fan to maintain room temperature. When the reaction was complete (TLC), H₂O was added to quench the reaction. The aqueous layer was extracted with Et₂O (×3), and the combined organic layers were washed with H₂O, dried (Na₂SO₄), filtered through a plug of cotton wool, and concentrated under reduced pressure. The residue was purified by flash column chromatography [silica gel, EtOAc–hexane (1:20 to 1:5)]. 4-Methyl-N-(3-oxo-4-phenylpent-4-en-1-yl)benzenesulfonamide (5a) Pale-yellow oil; yield: 73.2 mg (74%). IR (neat): 3296, 1682, 1598, 1162, 816 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 2.43 (s, 3 H), 3.01 (t, J = 5.6 Hz, 2 H), 3.24 (dt, J = 5.6, 7.2 Hz, 2 H), 5.13 (t, J = 7.2 Hz, 1H), 5.95 (s, 1 H), 6.14 (s, 1 H), 7.20–7.24 (m, 2 H), 7.31 (d, J = 8.4 Hz, 2 H), 7.34–7.36 (m, 3 H), 7.73 (d, J = 8.4 Hz, 2 H). ¹³C NMR (100 MHz, CDCl₃): δ = 21.5, 38.4, 39.0, 126.3, 127.0, 128.26, 128.34, 128.4, 129.8, 136.5, 137.0, 143.4, 148.6, 200.4. HRMS (ESI): m/z [M + Na]⁺ calcd for C₁₈H₁₉NNaO₃S: 352.0983; found: 352.0978.