Electrochemical Deoxygenation of N-Heteroaromatic N-Oxides

Published as part of the Cluster Electrochemical Synthesis and Catalysis

Abstract

An electrochemical method for the deoxygenation of N-heteroaromatic N-oxide to give the corresponding N-heteroaromatics has been developed. Several classes of N-heterocycles such as pyridine, quinoline, isoquinoline, and phenanthridine are tolerated. The electrochemical reactions proceed efficiently in aqueous solution without the need for transition-metal catalysts and waste-generating reducing reagents.

Key words

reduction - organic electrosynthesis - deoxygenation - electrolysis - heterocycles

• References and Notes

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• 5 General Procedure for the Electrochemical Deoxygenation Reactions A 10 mL three-necked round-bottomed flask was charged with the N-heteroaromatic N-oxide (0.30 mmol, 1.0 equiv) and Et₄NPF₆ (0.06 mmol, 0.2 equiv). The flask was then equipped with a condenser, a reticulated vitreous carbon (100 PPI, ca. 65 cm² cm⁻³, 1.2 cm × 1.0 cm × 0.8 cm) anode, and a Pb plate (1.0 cm × 1.0 cm) cathode and flushed with argon. MeCN and H₂O (4:1, 10.0 mL) were added. The electrolysis was carried out at 80 °C using a constant current of 10 mA until complete consumption of the substrate (monitored by TLC or ¹H NMR). The reaction mixture was concentrated under reduced pressure. The residue was chromatographed through silica gel eluting with ethyl acetate/hexane to give the desired product.
• 6 Spectral Data for 2b Colorless oil; yield 70%; 2.4 F mol^–1. ¹H NMR (400 MHz, CDCl₃): δ = 8.04–7.95 (m, 2 H), 7.73 (dt, J = 8.3, 1.6 Hz, 1 H), 7.65 (ddt, J = 8.5, 6.9, 1.6 Hz, 1 H), 7.45 (ddt, J = 8.0, 6.9, 1.2 Hz, 1 H), 7.26–7.21 (m, 1 H), 2.72 (s, 3 H). ¹³C NMR (101 MHz, CDCl₃): δ = 159.1, 148.0, 136.2, 129.5, 128.7, 127.6, 126.6, 125.7, 122.1, 25.5.
• 7 Spectral Data for 2c Colorless oil; yield 65%; 2.2 F mol^–1. ¹H NMR (400 MHz, CDCl₃): δ = 8.94–8.83 (m, 1 H), 8.10 (d, J = 8.2 Hz, 2 H), 7.82–7.63 (m, 2 H), 7.56–7.45 (m, 1 H), 7.35 (dq, J = 7.5, 3.8 Hz, 1 H). ¹³C NMR (101 MHz, CDCl₃): δ = 150.5, 148.4, 136.1, 129.6, 129.5, 128.4, 127.9, 126.6, 121.2.
• 8 Spectral Data for 2e Colorless oil; yield 77%; 4.0 F mol^–1. ¹H NMR (400 MHz, CDCl₃): δ = 8.45–8.37 (m, 2 H), 6.85–6.68 (m, 2 H), 3.82 (s, 3 H). ¹³C NMR (101 MHz, CDCl₃): δ = 165.7, 151.2, 110.0, 55.2.