Visible-Light-Induced Cyclization of Electron-Enriched Phenyl Benzyl Sulfides: Synthesis of Tetrahydrofurans and Tetrahydropyrans

Abstract

A new approach to the preparation of tetrahydrofurans and tetrahydropyrans through a photoredox catalytic process is described. The introduction of a phenylsulfanyl auxiliary group permits the substrates to be readily oxidized to form cationic intermediates for sequential intramolecular cyclization. The method features mild reaction conditions and operational simplicity.

Key words

photocatalysis - sulfides - cyclization - tetrahydrofurans - tetrahydropyrans

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• 12 The crystalline material 2l was prepared by a layer-to-layer diffusion method. 2l was dissolved in THF and layered with hexane. After seven days, a crystal suitable for single-crystal X-ray diffraction was obtained. CCDC 1431990 contains the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/getstructures.
• 13 tert-Butyl[4-(4-methoxyphenyl)-4-(phenylsulfanyl)butoxy]dimethylsilane (1a); Typical Procedure¹⁵ TMEDA (0.70 g, 6.0 mmol) was added to a 1.6 M solution n-BuLi in hexane (3.8 mL, 6.0 mmol) in THF (10 mL) at –30 °C, and the mixture was stirred for 0.5 h. A solution of 1-methoxy-4-[(phenylsulfanyl)methyl]benzene (1.150 g, 5.0 mmol) in THF (5 mL) was then added dropwise, and the mixture was stirred for a further 2 h. TBSO(CH₂)₃Br (1.25 g, 5.0 mmol) was added and reaction mixture was allowed to warm to 0 °C and stirred for 5 h. The reaction was quenched with aq NH₄Cl (20 mL) and the mixture was extracted with Et₂O (3 × 30 mL). The combined organic layers were washed with brine (60 mL), dried (MgSO₄), and filtered. The solvent was removed in vacuo to afford a crude product that was purified by flash chromatography (silica gel) to give a colorless liquid; yield: 1450 mg (72%). ¹H NMR (400 MHz, CDCl₃): δ = 7.29–7.11 (m, 7 H), 6.79 (d, J = 7.8 Hz, 2 H), 4.11 (t, J = 6.2 Hz, 1 H), 3.77 (s, 3 H), 3.55 (t, J = 6.2 Hz, 2 H), 2.12–1.81 (m, 2 H), 1.58–1.37 (m, 2 H), 0.86 (s, 9 H), 0.00 (s, 6 H). ¹³C NMR (150 MHz, CDCl₃): δ = 158.51, 135.12, 133.89, 132.39, 128.82, 128.59, 126.90, 113.65, 62.60, 55.19, 52.67, 32.61, 30.68, 25.93, 18.28, –5.35. GC/MS (EI, QMS): m/z (%) = 402 (<1), 345, 293, 235, 161 (100), 73.
• 14 Photocatalytic Cycloetherification of Phenyl Benzyl Sulfides; General Procedure A 10 mL round-bottomed flask equipped with a magnetic stirrer bar was charged with the appropriate phenyl benzyl sulfide 1 (0.1 mmol), TFA (0.15 mmol), Ru(bpy)₃Cl₂ (0.005 mmol), and anhyd MeNO₂ (1 mL). The mixture was irradiated with a blue LED (6 W) at r.t. for 4 h. The mixture was the filtered through a short plug of silica gel that was rinsed with EtOAc. The filtrate was concentrated and the residue was purified by flash column chromatography. 2-(4-Methoxyphenyl)tetrahydrofuran (2a) Colorless liquid; yield: 15.1 mg (85%). ¹H NMR (400 MHz, CDCl₃) δ = 7.25 (d, J = 3.6 Hz, 2 H), 6.87 (d, J = 8.6 Hz, 2 H), 4.83 (t, J = 7.2 Hz, 1 H), 4.08 (dd, J = 14.7, 7.2 Hz, 1 H), 3.91 (dd, J = 14.3, 7.9 Hz, 1 H), 3.80 (s, 3 H), 2.36–2.20 (m, 1 H), 2.11–1.94 (m, 2 H), 1.85–1.71 (m, 1 H). ¹³C NMR (150 MHz, CDCl₃): δ = 158.76, 135.30, 126.94, 113.65, 80.42, 68.47, 55.25, 34.45, 26.05. GC/MS (EI, QMS): m/z (%) = 178 (46), 177, 147, 136, 135 (100). HRMS (ESI); m/z [M + H]⁺ calcd. for C₁₁H₁₅O₂: 179.1072; found: 179.1067. 2-(2-Methoxyphenyl)tetrahydrofuran(2b)^5f Colorless liquid; yield: 13.0 mg (73%). ¹H NMR (600 MHz, CDCl₃) δ = 7.42 (d, J = 7.5 Hz, 1 H), 7.22 (t, J = 7.7 Hz, 1 H), 6.95 (t, J = 7.4 Hz, 1 H), 6.85 (d, J = 8.1 Hz, 1 H), 5.17 (t, J = 6.9 Hz, 1 H), 4.10 (dd, J = 14.1, 7.1 Hz, 1 H), 3.92 (dd, J = 14.2, 7.4 Hz, 1 H), 2.45–2.25 (m, 1 H), 2.02–1.86 (m, 2 H), 1.68–1.73 (m, 1 H). ¹³C NMR (150 MHz, CDCl₃): δ = 156.09, 132.19, 127.73, 125.55, 120.38, 109.99, 75.86, 68.50, 55.22, 33.05, 25.87. GC/MS (EI, QMS): m/z (%) = 178 (83), 177, 147, 136, 135 (100), 119. HRMS (ESI); m/z [M + Na]⁺ calcd. for C₁₁H₁₄NaO₂: 201.0891; found: 201.0881.
• 15 Smith III AB, Tong R. Org. Lett. 2010; 12: 1260