Potential-tuned selective electrosynthesis of azoxy-, azo- and amino-aromatics over a CoP nanosheet cathode

Abstract

Azoxy-, azo- and amino-aromatics are among the most widely used building blocks in materials science pharmaceuticals and synthetic chemistry, but their controllable and green synthesis has not yet been well established. Herein, a facile potential-tuned electrosynthesis of azoxy-, azo- and amino-aromatics via aqueous selective reduction of nitroarene feedstocks over a CoP nanosheet cathode is developed. A series of azoxy-, azo- and amino-compounds with excellent selectivity, good functional group tolerance and high yields are produced by applying different bias input. The synthetically significant and challenging asymmetric azoxy-aromatics can be controllably synthesized in moderate to good yields. The use of water as the hydrogen source makes this strategy remarkably fascinating and promising. In addition, deuterated aromatic amines with a high deuterium content can be readily obtained by using D₂O. By pairing with anodic oxidation of aliphatic amines to nitriles, synthetically useful building blocks can be simultaneously produced in a CoP||Ni₂P two-electrode electrolyzer. Only 1.25 V is required to achieve a current density of 20 mA cm^-2, which is much lower than that of overall water splitting (1.70 V). The paired oxidation and reduction reactions can also be driven using a 1.5 V battery to synthesize nitrile and azoxybenzene with good yields and selectivity, further emphasizing the flexibility and controllability of our method. This work paves the way for a promising approach to the green synthesis of valuable chemicals through potential-controlled electrosynthesis.

Keywords: azo- and amino-aromatics; azoxy; electrosynthesis; metal phosphides; nitroarenes; selectivity.

Figure 1.

Highly selective electrochemical reduction of nitroarenes to azoxy-, azo- and amino-aromatics over a CoP cathode.

Figure 2.

(a) SEM (inset: enlargement), (b) TEM, (c) HRTEM, and (d) EDS elemental mapping images of CoP nanosheets.

Figure 3.

(a) Cathodic products from the electrochemical reduction of 1a over CoP obtained at different potentials. (b) LSV curves of CoP at a scan rate of 5 mV s⁻¹ in 1.0 M KOH with and without 0.5 mmol of 1a. (c) Cycle-dependent selectivity and FEs of 2a over CoP at −0.8 V vs. Ag/AgCl. (d-f) Time-dependent conversion plots for the electrochemical reduction of 1a into 2a, 3a and 4a over a CoP cathode, respectively, in 1.0 M KOH at different potentials.

Figure 4.

(a) Illustration for coupling cathodic electro-reductive synthesis of 2a with anodic conversion of octylamine into octylnitrile in a CoP||Ni₂P electrolyzer. (b) LSV curves and (c) comparison of potential for achieving benchmark current densities in a CoP||Ni₂P electrolyzer at a scan rate of 5 mV s⁻¹ in 1.0 M KOH before and after adding 1.0 mmol of 1a to cathode chamber and 1.0 mmol of octylamine to anode chamber. (d-f) Time-dependent photos of a CoP||Ni₂P electrolyzer setup driven by 1.5 V battery for both anodic oxidation of octylamine and cathodic reduction of 1a to 2a.