Chemoselective Vicinal Dichlorination of Alkenes by Iron Ligand-to-Metal Charge-Transfer Catalysis

Jessica Stahl; Thilo Reiter; Burkhard König

doi:10.1055/a-2225-8858

Synlett, Table of Contents

Synlett 2024; 35(09): 1057-1061
DOI: 10.1055/a-2225-8858

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Chemical Synthesis and Catalysis in Germany

Chemoselective Vicinal Dichlorination of Alkenes by Iron Ligand-to-Metal Charge-Transfer Catalysis

Authors

Jessica Stahl‡

Faculty of Chemistry and Pharmacy, University of Regensburg, Universitätsstr. 31, 93053 Regensburg, Germany
Thilo Reiter‡

Faculty of Chemistry and Pharmacy, University of Regensburg, Universitätsstr. 31, 93053 Regensburg, Germany
Burkhard König∗

Abstract

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Abstract

We report the photocatalytic functionalization of terminal alkenes to vicinal dichlorides by using visible light and FeCl₃ as a catalyst, LiCl as a chloride source, and air as an oxidant. The transformation is proposed to be initiated by ligand-to-metal charge-transfer bond homolysis of a Fe–Cl bond, giving a highly reactive chloride radical able to initiate the functionalization of olefins. The process shows high chemoselectivity and broad functional-group tolerance with yields of up to 94% under mild conditions.

Key words

iron catalysis - photocatalysis - ligand-to-metal charge transfer - chlorination - alkenes - vicinal dichlorides

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References

References and Notes
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22 Preparative-Scale Iron-Catalyzed Dichlorination of Alkenes; General Procedure After successful screening at a 0.1 mmol scale, the preparative-scale reactions were performed in 6–12 parallel vials to achieve good reproducibility. Each vial was equipped with a magnetic stirrer bar and charged the appropriate alkene ester starting material (0.20 mmol, 1.00 equiv), FeCl₃ (0.04 mmol, 6.50 mg, 20.0 mol%), and LiCl (2.00 mmol, 85.0 mg, 10.0 equiv) in 9:1 AcOH–MeCN (0.5 mL). The glass vials were closed, and air was added by syringe through a semi-permeable septum in the cap of each vial. The vials were sealed with Parafilm and placed in the thermostatted (20 °C) metal block approximately 2 cm above the 390–395 nm light source, and the mixtures were irradiated for 3 h. The vials were then opened to air and their contents were combined, filtered through a thin layer of silica gel in a vacuum filtration device, washed with Et₂O, and collected in a 100 mL round-bottomed flask. The solvent volume was reduced under reduced pressure and the residue was extracted with H₂O (2 × 20 mL) and NaHCO₃ (20 mL). The combined organic layers were dried (Na₂SO₄), filtered, and then concentrated in vacuum. The oily residue was dried under reduced pressure and purified by flash column chromatography (silica gel, 20% acetone–PE). The purity of the product was confirmed by ¹H and ¹³C NMR analyses in CDCl₃ with triphenylmethane as a standard. 3,4-Dichlorobutyl Benzoate (1a) Colorless oil; yield: 0.023 g (47%, 0.093 mmol). ¹H NMR (300 MHz, CDCl₃): δ = 8.12–7.99 (m, 2 H), 7.62–7.49 (m, 1 H), 7.47–7.35 (m, 2 H), 4.62–4.41 (m, 2 H), 4.26 (dddd, J = 9.5, 7.3, 5.0, 3.5 Hz, 1 H), 3.83 (dd, J = 11.4, 5.0 Hz, 1 H), 3.72 (dd, J = 11.4, 7.3 Hz, 1 H), 2.50 (dddd, J = 14.5, 8.9, 5.8, 3.4 Hz, 1 H), 2.11 (ddt, J = 14.6, 9.7, 4.9 Hz, 1 H). ¹³C NMR (75 MHz, CDCl₃): δ = 166.3 (Cq), 133.2 (+), 129.9 (Cq), 129.6 (+), 128.5 (+), 61.4 (–), 57.6 (+), 48.3 (–), 34.3 (–). The spectroscopic results agreed well with those in the literature; see ref. 8.
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