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.

Cu-containing polyoxometalate-based melamine in the environmental remediation of toxic organic pollutants

“…27 Furthermore, a POM redox mediator was applied for the reduction of nitro benzenes to anilines with high selective electrocatalytic activity at room temperature in aqueous solution without any sacrificial reagents. 28 In another study, a heterogeneous magnetic nanocatalyst, [Fe 3 O 4 @SiO 2 -NH 2 -Cu 20 P 8 W 48 ], with excellent yield in the reduction of NPs into the corresponding aminophenols was demonstrated. 29 We were interested in studying the catalytic performance of transition metalsubstituted POMs (MPOMs) with unique properties in these kinds of reactions.…”

Transition‐metal Substituted Polyoxometalates as Soluble RedOx Mediators in Electrocatalytic Biomass Conversion

“…[47] Recently, Stergiou and Symes published their studies on selective electrocatalytic reduction of substituted nitrobenzenes to their aniline derivates using H 3 [PW 12 O 40 ] as a redox mediator. [48][49][50] Although POMs represent an important catalyst class for biomass conversion, [4] electrochemical applications in this field are rare. [51,52] Electrochemical biomass conversion is considered an important 21 st century technique because it combines renewable carbon feedstocks and decarbonized electrical energy.…”

“…Regarding non‐aromatic substrates, the oxidation of light alcohols mediated by [{Ru 4 O 4 (OH) 2 (H 2 O) 4 }(γ‐SiW 10 O 36 ) 2 ] 10− (1[(γ‐SiW 10 O 36 ) 2 ] 10− ) was reported by Liu et al ., [46] as well as the oxidation of light hydrocarbons using{Fe 30 W 72 } by Neumann [47] . Recently, Stergiou and Symes published their studies on selective electrocatalytic reduction of substituted nitrobenzenes to their aniline derivates using H 3 [PW 12 O 40 ] as a redox mediator [48–50] . Although POMs represent an important catalyst class for biomass conversion, [4] electrochemical applications in this field are rare [51,52] .…”