Highly Selective Electrocatalytic Reduction of Substituted Nitrobenzenes to Their Aniline Derivatives Using a Polyoxometalate Redox Mediator

Abstract

Anilines and substituted anilines are used on the multi-ton scale for producing polymers, pharmaceuticals, dyes, and other important compounds. Typically, these anilines are produced from their corresponding nitrobenzene precursors by reaction with hydrogen at high temperatures. However, this route suffers from a number of drawbacks, including the requirement to handle hydrogen gas, rather harsh reaction conditions that lead to a lack of selectivity and/or toleration of certain functional groups, and questionable environmental sustainability. In light of this, routes to the reduction of nitrobenzenes to their aniline derivatives that operate at room temperature, in aqueous solvent, and without the requirement to use harsh process conditions, hydrogen gas, or sacrificial reagents could be of tremendous benefit. Herein, we report on a highly selective electrocatalytic route for the reduction of nitrobenzenes to their corresponding anilines that works in aqueous solution at room temperature and which does not require the use of hydrogen gas or sacrificial reagents. The method uses a polyoxometalate redox mediator, which reversibly accepts electrons from the cathode and reacts with the nitrobenzenes in solution to reduce them to the corresponding anilines. A variety of substituted nitroarenes are explored as substrates, including those with potentially competing reducible groups and substrates that are difficult to reduce selectively by other means. In all cases, the selectivity for the redox-mediated route is higher than that for the direct reduction of the nitroarene substrates at the electrode, suggesting that redox-mediated electrochemical nitroarene reduction is a promising avenue for the more sustainable synthesis of substituted anilines.

Figure 1

Illustration of the electrocatalytic hydrogenation of nitrobenzenes using a phosphotungstic acid redox mediator.

Figure 2

Schematic illustration of the reduction potentials of the examined substituted nitrobenzenes. The reduction potentials were extracted from the cyclic voltammograms and were recorded using a glassy carbon working electrode (surface area = 0.071 cm²), a Pt wire counter electrode, and an Ag/AgCl reference electrode. The electrolyte was 1 M aqueous H₃PO₄, and the scan rate was 10 mV/s in all cases. The scale bar is nonlinear to allow the relative ordering of the various redox processes to be more clearly seen.

Figure 3

Cyclic voltammogram of ethyl-3-nitrobenzoate in 10 mL of 1 M aqueous H₃PO₄, using 9.74 × 10^–5 mol of both the nitroarene and the redox mediator. A glassy carbon working electrode (surface area = 0.071 cm²), a Pt wire counter electrode, and an Ag/AgCl reference electrode were used. The scan rate was 10 mV/s.

Figure 4

Stacked ¹H NMR plot (MeOD, 300 K, 400 MHz) summarizing the outcome of the mediated electroreduction of methyl-2-nitrobenzoate. Shown in purple at the top is a spectrum of the extracted and concentrated reaction medium from a nonmediated reaction (direct substrate reduction at the cathode). Beneath this (turquoise) is a spectrum of the extracted and concentrated reaction medium from a mediated electrochemical reduction of methyl-2-nitrobenzoate. Beneath this (green) is a pure methyl-2-aminobenzoate standard sample. The bottom spectrum (red) is that of the starting material. The peak at 4.78 ppm originates from residual MeOH.

Figure 5

Voltage–time plot of the controlled current reduction of methyl-2-nitrobenzoate in 1 M aqueous H₃PO₄ at −20 mA using a 2-electrode setup. A platinum mesh was used as the counter electrode (anode), and carbon felt was used as the working electrode (cathode).

Figure 6

Cyclic voltammogram of methyl-2-nitrobenzoate in 10 mL 1 M aqueous H₃PO₄, using 9.74 × 10^–5 mol of both the nitroarene and the redox mediator. A glassy carbon working electrode (surface area = 0.071 cm²), a Pt wire counter electrode, and an Ag/AgCl reference electrode were used. The scan rate was 10 mV/s.