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

Abstract

Electroreduction of N,N′-dioctyl-4,4′-bipyridinium bis(tri­flimide) [C₈V^²+][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, [C₈V^²+][Tf₂N^-]₂ was recovered and reused for generation of the organic reductant C₈V⁰.

Key words

coupling - electron transfer - halides - reductions - palladium

References and Notes

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14

Purity of [C₈V^²+][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. [C₈V^²+][Tf₂N^-]₂ was dried in vacuo at 100 ˚C overnight and used without further purification.

15

In a cathodic compartment of a divided cell was placed a THF (3 mL) solution of [C₈V^²+][Tf₂N^-]₂ (1 mmol), [Bu₄N⁺][Tf₂N^-] (2 mmol), and a Pt (1 × 1.5 cm^²) 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 [C₈V^²+][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%).

17

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 [C₈V^²+][Tf₂N^-]₂ (1 mmol) and [Bu₄N⁺][Tf₂N^-] (2 mmol) was electrolyzed under constant current conditions (30 mA) at r.t. until 1 F/mol [C₈V^²+][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), [C₈V^²+][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.