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Synlett 2009(1): 85-88  
DOI: 10.1055/s-2008-1087386
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Electroreductive Generation of Recyclable Organic Reductant from N,N′-Dioctyl-4,4′-bipyridinium and Pd-Catalyzed Reductive Coupling of Aryl Halides

Manabu Kuroboshi, Ryoto Kobayashi, Takayuki Nakagawa, Hideo Tanaka*
Department of Applied Chemistry, Okayama University, Tsushima-naka 3-1-1, Okayama 700-8530, Japan
Fax: +81(86)2518079; e-Mail: tanaka95@cc.okayama-u.ac.jp;
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Publication History

Received 11 September 2008
Publication Date:
12 December 2008 (online)

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Abstract

Electroreduction of N,N′-dioctyl-4,4′-bipyridinium bis(tri­flimide) [C8V²+][Tf2N-]2 in THF gave a dark blue solution of the corresponding quinoid C8V0, which worked as an efficient organic reductant for Pd-catalyzed reductive coupling of aryl bromides to give the corresponding biphenyl derivatives in good yields. After usual workup, [C8V²+][Tf2N-]2 was recovered and reused for generation of the organic reductant C8V0.

Key words

coupling - electron transfer - halides - reductions - palladium

14

Purity of [C8V²+][Tf2N-]2 was confirmed by ¹H NMR, ¹³C NMR, IR, and elemental analysis. ¹H NMR (200 MHz, CD2Cl2): δ = 0.8-1.0 (m, 6 H), 1.20-1.50 (m, 20 H), 1.90-2.20 (m, 4 H), 4.64 (t, J = 7.6 Hz, 4 H), 8.46 (d, J = 6.9 Hz, 4 H), 8.93 (d, J = 6.9 Hz, 4 H). ¹³C NMR (50 MHz, CD2Cl2): δ = 13.41, 22.16, 25.59, 28.41, 28.48, 31.01, 31.21, 62.57, 119.28 (q, J = 319.1 Hz), 127.03, 144.96, 149.50. IR (KBr): 3136, 3104, 3074, 2952, 2929, 2861, 1644, 1347, 1203, 1133, 1058 cm-¹. Anal. Calcd for C30H42F12N4O8S4: C, 38.21; H, 4.49; N, 5.94. Found: C, 38.28; H, 4.57; N, 5.93. [C8V²+][Tf2N-]2 was dried in vacuo at 100 ˚C overnight and used without further purification.

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In a cathodic compartment of a divided cell was placed a THF (3 mL) solution of [C8V²+][Tf2N-]2 (1 mmol), [Bu4N+][Tf2N-] (2 mmol), and a Pt (1 × 1.5 cm²) cathode. In an anodic compartment was placed a THF (3 mL) solution of [Bu4N+][Tf2N-] (2 mmol) and a Mg anode. A constant current (25 mA) was applied at r.t. until 2 F/mol [C8V²+][Tf2N-]2 of electricity was passed. The resulting dark blue solution in cathodic cell was added to a mixture of 1a (0.5 mmol) and a catalytic amount of PdCl2(PhCN)2 (0.025 mmol) by means of cannulation. The whole mixture was stirred for 30 h at 60 ˚C. Usual workup and purification by column chromatography (SiO2) gave 2a (0.227 mmol, 91%).

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In the case of Pd/TDAE-promoted reductive coupling of aryl bromides, PdCl2, PdCl2(PhCN)2, Pd(OAc)2, and Pd2(dba)3 gave the coupling products, whereas Pd(PPh3)4 and PdCl2(PPh3)2 did not promote the coupling, and the starting aryl bromides were recovered quantitatively.

18

A THF solution of [C8V²+][Tf2N-]2 (1 mmol) and [Bu4N+][Tf2N-] (2 mmol) was electrolyzed under constant current conditions (30 mA) at r.t. until 1 F/mol [C8V²+][Tf2N-]2 of electricity was passed. The solution was used for the reductive coupling of 1a (0.5 mmol) in the presence of PdCl2(PhCN)2 (0.025 mmol) to give 2a and 1a in 59% and 32% yield, respectively.

20

In an undivided cell were placed a mixture of 1a (0.5 mmol), [C8V²+][Tf2N-]2 (0.025 mmol), [Bu4N+][Tf2N-] (2.0 mmol), a catalytic amount of PdCl2(PhCN)2 (0.025 mmol), and THF (3 mL). A Mg anode and a Pt cathode were immersed in the solution, and a constant current (5 mA) was supplied at 60 ˚C until 2 F/mol-1a of electricity was passed.