Amphiphilic Viologen: Electrochemical Generation of Organic Reductant and Pd-Catalyzed Reductive Coupling of Aryl Halides in Water

Manabu Kuroboshi; Takashi Yamamoto; Hideo Tanaka

doi:10.1055/s-0032-1317857

Synlett, Inhaltsverzeichnis

Synlett 2013; 24(2): 197-200
DOI: 10.1055/s-0032-1317857

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Amphiphilic Viologen: Electrochemical Generation of Organic Reductant and Pd-Catalyzed Reductive Coupling of Aryl Halides in Water

Manabu Kuroboshi*

The Graduate School of Natural Science and Technology, Okayama University, Tsushima-naka 3-1-1, Kita-Ku, Okayama 700-8530, Japan Fax: +86(255)3424 eMail: tanaka95@cc.okayama-u.ac.jp

,

Takashi Yamamoto

The Graduate School of Natural Science and Technology, Okayama University, Tsushima-naka 3-1-1, Kita-Ku, Okayama 700-8530, Japan Fax: +86(255)3424 eMail: tanaka95@cc.okayama-u.ac.jp

,

Hideo Tanaka*

The Graduate School of Natural Science and Technology, Okayama University, Tsushima-naka 3-1-1, Kita-Ku, Okayama 700-8530, Japan Fax: +86(255)3424 eMail: tanaka95@cc.okayama-u.ac.jp

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Abstract

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Abstract

Electroreduction of 1,1′-bis(methoxyethoxyethoxyethyl)-4,4′-bipyridinium tosylate generated amphiphilic organic reductants, which promoted the Pd-catalyzed reductive coupling of aryl bromides in water to give the corresponding biaryls. The yields and selectivity of biaryls depended on the length of ethyleneoxy groups and substituents of the aryl bromides.

Key words

electron transfer - palladium - reduction - coupling - arenes

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Referenzen

References and Notes

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5 Octyl viologen bis(triflimide) and the corresponding quinoid dissolve in organic solvent to promote the reductive homocoupling of aryl halide. See: Kuroboshi M, Kobayashi R, Nakagawa T, Tanaka H. Synlett 2009; 85
6 Viologen also promotes the reductive homocoupling of aryl halides in ionic liquid. See: Kuroboshi M, Kuwano A, Tanaka H. Electrochemistry 2008; 862
7 Preparation of 1a In a round-bottomed flask were placed 4,4′-bipyridyl (3.531 g, 23 mmol) and 3,6,9-trioxadecyl p-tosylate (14.39 g, 45 mmol). The mixture was stirred at 120 °C for 24 h under argon atmosphere to form a viscous layer. The supernatant was decanted off, and the viscous layer was washed with Et₂O and toluene (2 × 10 mL), successively. The viscous liquid layer was concentrated under reduced pressure to give 1,1′-bis(3,6,9-trioxadecyl)-4,4′-bipyridinium bis(p-toluenesulfonate) (1a, 17.14 g, 22 mmol, 96%) as a brown liquid. ¹H NMR (200 MHz, CDCl₃): δ = 2.28 (s, 6 H), 3.28 (s, 6 H), 3.42–3.53 (m, 16 H), 3.85 (br, 4 H), 4.93 (br, 4 H), 7.11 (d, J = 8.1 Hz, 4 H), 7.69 (d, J = 8.1 Hz, 4 H), 8.71 (d, J = 6.8 Hz, 4 H), 9.24 (d, J = 6.8 Hz, 4 H). ¹³C NMR (150 MHz, CDCl₃): δ = 21.04, 58.69, 61.18, 68.89, 69.91, 70.08, 70.14, 71.56, 125.66, 126.64, 128.71, 1139.42, 143.25, 146.60, 148.87; IR (neat): 3464, 3127, 3059, 2921, 2876, 1638, 1451, 1191, 1122, 822 cm^–1.
8 1,1′-Bis(2-methoxyethyl)-4,4′-bipyridinium Bis(p-toluenesulfonate) (1b) Colorless solids. ¹H NMR (200 MHz, D₂O): δ = 2.13 (s, 6 H), 3.19 (s, 6 H), 3.79 (t, J = 5.0 Hz, 4 H), 4.68 (t, J = 5.0 Hz, 4 H), 7.08 (d, J = 8.3 Hz, 4 H), 7.40 (d, J = 8.3 Hz, 4 H), 8.26 (d, J = 7.0 Hz, 4 H), 8.85 (d, J = 7.0 Hz, 4 H). ¹³C NMR (50 MHz, CD₃OD): δ = 21.3, 59.2, 62.7, 71.3, 126.8, 127.8, 129.8, 141.6, 143.5, 147.5, 151.3. IR (KBr): 3532, 3451, 3134, 3062, 2992, 2864, 1639, 1510, 1448, 1191, 1120, 1035, 568 cm^–1. 1,1′-Bis(3,6-dioxaheptyl)-4,4′-bipyridinium Bis(p-toluenesulfonate) (1c) Colorless solids. ¹H NMR (200 MHz, D₂O): δ = 2.15 (s, 6 H), 3.09 (s, 6 H), 3.32–3.36 (m, 4 H), 3.44–3.49 (m, 4 H), 3.86 (t, J = 4.4 Hz, 4 H), 4.63–4.76 (m, 4 H), 7.11 (d, J = 7.7 Hz, 4 H), 7.42 (d, J = 7.7 Hz, 4 H), 8.30 (d, J = 6.2 Hz, 4 H), 8.89 (d, J = 6.2 Hz, 4 H). ¹³C NMR (150 MHz, CD₃OD): δ = 21.3, 59.1, 62.7, 70.0, 71.2, 72.7, 126.9, 127.7, 129.9, 141.6, 143.7, 147.6, 151.2. IR (KBr): 3509, 3131, 3055, 2879, 1640, 1442, 1192, 1123, 1033, 685 cm^–1. 1,1′-Dimethyl-4,4′-bipyridinium Bis(p-toluenesulfonate) (1d) Yellow solids. ¹H NMR (200 MHz, D₂O): δ = 2.18 (s, 6 H), 4.31 (s, 6 H), 7.13 (d, J = 8.2 Hz, 4 H), 7.45 (d, J = 8.2 Hz, 4 H), 8.25 (d, J = 6.9 Hz, 4 H), 8.81 (d, J = 6.9 Hz, 4 H). ¹³C NMR (150 MHz, CD₃OD): δ = 21.3, 49.0, 126.8, 127.6, 130.0, 141.6, 143.7, 147.8, 150.4. IR (KBr): 3444, 3053, 3020, 2994, 1642, 1509, 1440, 1358, 1208, 1119, 818, 683 cm^–1. 1,1′-Dibutyl-4,4′-bipyridinium Bis(p-toluenesulfonate) (1e)Colorless solids. ¹H NMR (200 MHz, D₂O): δ = 0.71 (t, J = 7.4 Hz, 6 H), 1.13 (sext, J = 7.4 Hz, 4 H), 1.76 (quin, J = 7.4 Hz, 4 H), 2.07 (s, 6 H), 4.40 (t, J = 7.4 Hz, 4 H), 7.01 (d, J = 8.1 Hz, 4 H), 7.32 (d, J = 8.1 Hz, 4 H), 8.16 (d, J = 6.7 Hz, 4 H), 8.77 (d, J = 6.7 Hz, 4 H). ¹³C NMR (150 MHz, CD₃OD): δ = 13.8, 20.4, 21.3, 34.3, 62.9, 126.9, 128.2, 129.9, 141.6, 143.7, 147.0, 151.0. IR (KBr): 3434, 3127, 3054, 2957, 2870, 1643, 1450, 1169, 1120, 1035, 818 cm^–1.
9 Viologens V²⁺ were easily soluble in H₂O, whereas the reduced form V⁰ were soluble in organic solvents. Therefore, viologens were used in a mixed solution of H₂O–MeOH or H₂O–MeCN. The amphiphilic PEG group would increase the solubility of V²⁺/V⁰ as well as the substrates.
10 Recycle use of aqueous viologen/Pd solution was also examined. In the second run, the recovered aqueous solution was placed in the cathodic chamber. The electroreduction was carried out under constant current conditions to generate V⁰. Though the reaction conditions have not been optimized yet, 4a was obtained in 40% yield.