catalysts Review The Influence of Copper on Halogenation/Dehalogenation Reactions of Aromatic Compounds and Its Role in the Destruction of Polyhalogenated Aromatic Contaminants Tomáš Weidlich Chemical Technology Group, Institute of Environmental and Chemical Engineering, Faculty of Chemical Technology, University of Pardubice, Studentska 573, 53210 Pardubice, Czech Republic; [email protected]; Tel.: +420-46-603-8049 Abstract: The effect of copper and its compounds on halogenation and dehalogenation of aromatic compounds will be discussed in the proposed article. Cu oxidized to appropriate halides is an effective halogenation catalyst not only for the synthesis of halogenated benzenes or their derivatives as desired organic fine chemicals, but is also an effective catalyst for the undesirable formation of thermodynamically stable and very toxic polychlorinated and polybrominated aromatic compounds such as polychlorinated biphenyls, dibenzo-p-dioxins and dibenzofurans accompanied incineration of waste contaminated with halogenated compounds or even inorganic halides. With appropriate change in reaction conditions, copper and its alloys or oxides are also able to effectively catalyze dehalogenation reactions, as will be presented in this review. Keywords: halogenation; oxybromination; oxychlorination; copper-catalyzed; arylation; Ullmann Citation: Weidlich, T. The Influence reaction; coupling; hydrodehalogenation of Copper on Halogenation/Dehalogenation Reactions of Aromatic Compounds and Its Role in the Destruction of 1. Introduction Polyhalogenated Aromatic Aromatic carbon–halogen (Carom–X) bonds are common functional groups in organic Contaminants. Catalysts 2021, 11, 378. synthesis. They are frequently and widely applied in the preparation of numerous organic https://doi.org/10.3390/catal fine chemicals. In many cases, halogenated aromatic compounds (Ar-Xs) are valuable 11030378 intermediates for subsequent Carom–R bonds formation. The broad application of Ar-Xs is joined, however, with a high risk of environmental pollution by almost non-biodegradable Academic Editor: Sónia Carabineiro and toxic Ar–Xs which need subsequent chemical treatment of polluted matter with the aim of converting Ar–Xs to more-biodegradable non-halogenated products. Due to this Received: 27 February 2021 reason, even the facile cleavage of C –X with the aim of producing nonhalogenated and Accepted: 11 March 2021 arom Published: 14 March 2021 usually more biodegradable Carom–H or Carom–R has been intensively studied. In this article, the research advances on the copper-catalyzed and mediated Carom– Publisher’s Note: MDPI stays neutral X (X = F, Cl, Br, I) bond formation via direct Carom–H bond transformation and (hy- with regard to jurisdictional claims in dro)dehalogenation of Carom–X producing Carom–H or Carom–R, respectively. published maps and institutional affil- Copper and its salts exhibit broad catalytic activity mainly due to the easily accessible iations. and reasonable stability of Cu(0), Cu(I), Cu(II) and Cu(III) oxidation states. They are therefore effective catalysts not only for Carom–H bonds transformations by single electron transfer (SET) processes [1,2] but through using appropriate conditions even for Carom– halogen cleavage reactions by SET, two-electron transfer, or other mechanisms [3,4]. Copper also ranks among the cheap, earth-abundant first-row transition metals which are more Copyright: © 2021 by the author. Licensee MDPI, Basel, Switzerland. environmentally acceptable (relatively less toxic) metals in comparison with noble platinum This article is an open access article group metals, gold or silver. Copper-mediated reactions have also gained significant distributed under the terms and popularity in recent times. The excellent functional group tolerance of copper-mediated conditions of the Creative Commons reactions offers new opportunities for the synthesis of a large variety of organic compounds. Attribution (CC BY) license (https:// This review intends to cover most of the recent advances both on Cu-based halogena- creativecommons.org/licenses/by/ tion and dehalogenation reactions of aromatic compounds based on C–O and C–C bond 4.0/). formation. Catalysts 2021, 11, 378. https://doi.org/10.3390/catal11030378 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, x FOR PEER REVIEW 2 of 36 This review intends to cover most of the recent advances both on Cu-based halogen- Catalysts 2021, 11, 378 ation and dehalogenation reactions of aromatic compounds based on C–O and C–C 2bond of 34 formation. 2. Cu-Based Halogenations of Aromatic Compounds 2. Cu-Based Halogenations of Aromatic Compounds This section may be divided by subheadings. It should provide a concise and precise descriptionThis section of the may experimental be divided results, by subheadings. their interpretation It should provide as well a as concise the experimental and precise descriptionconclusions ofthat the can experimental be drawn. Aryl results, halides their ar interpretatione widely used asin wellorganic as thesynthesis experimental to form conclusionscarbon-carbon that and can carbon-heteroatom be drawn. Aryl halidesbonds un areder widely metal usedcatalysis in organicsuch as synthesisin Heck, So- to formnogashira, carbon-carbon Suzuki, and and Ullmann carbon-heteroatom coupling reactions bonds under [5]. Aryl metal halides catalysis are suchalso ashighly in Heck, ver- Sonogashira,satile synthetic Suzuki, intermediates and Ullmann for many coupling applications reactions in [5agrochemicals,]. Aryl halides pharmaceuticals are also highly versatileand materials synthetic [6,7]. intermediates Aryl iodides for are many generally applications more reactive in agrochemicals, in organic pharmaceuticalstransformations. andAlthough materials aryl [6chlorides,7]. Aryl or iodides aryl bromides are generally are relatively more reactive more in inert, organic they transformations. are much more Althoughcommonly aryl found chlorides in pharmaceuticals or aryl bromides and agrochem are relativelyicals, more in which inert, they they are are introduced much more to commonlymodify the foundphysical in pharmaceuticalsand biological properties and agrochemicals, of aromatic in rings which [8]. they are introduced to modifyTraditional the physical methods and biological for Ar–Xs properties syntheses ofinvolved aromatic two rings common [8]. preparatory routes: Traditional methods for Ar–Xs syntheses involved two common preparatory routes: direct halogenation via an electrophilic substitution reaction (SEAr) and a nucleophilic direct halogenation via an electrophilic substitution reaction (SEAr) and a nucleophilic aromatic substitution reaction (SNAr) especially of diazonium salts [9]. aromatic substitution reaction (SNAr) especially of diazonium salts [9]. Halogenation of aromatic compounds using highly toxic chlorine (Cl2) and bromine Halogenation of aromatic compounds using highly toxic chlorine (Cl2) and bromine (Br2) via electrophilic aromatic substitution (SEAr) reaction mechanism requires special (Br ) via electrophilic aromatic substitution (S Ar) reaction mechanism requires special caution2 with regards to handling and safety. InE addition, corresponding toxic and very caution with regards to handling and safety. In addition, corresponding toxic and very corrosive hydrogen halides (HXs) are produced as by-products during halogen-based hal- corrosive hydrogen halides (HXs) are produced as by-products during halogen-based ogenations in stoichiometric quantity. In addition, HX as an undesirable by-product (a) halogenations in stoichiometric quantity. In addition, HX as an undesirable by-product diminishes significantly X atom efficiency of halogenation processes based on X2 as a hal- (a) diminishes significantly X atom efficiency of halogenation processes based on X as ogenation agent (X atom economy is max. 50%) and (b) causes difficulties with subsequent2 a halogenation agent (X atom economy is max. 50%) and (b) causes difficulties with HX treatment. subsequent HX treatment. TheThe developmentdevelopment ofof convenientconvenient andand moremore efficientefficient methodsmethods usingusing aa sourcesource ofof halidehalide ionion combinedcombined withwith differentdifferent oxidantsoxidants forfor thethe synthesissynthesis ofof arylaryl andand heteroarylheteroaryl halideshalides hashas thereforetherefore attractedattracted increasingincreasing attentionattention [[10–20].10–20]. AA different wayway forfor increasingincreasing thethe ClCl atomatom economyeconomy inin thethe productionproduction ofof chlorinatedchlorinated aromatic compounds via SEAr exploiting gaseous Cl2 is based on the ex-situ oxidative aromatic compounds via SEAr exploiting gaseous Cl2 is based on the ex-situ oxidative treatmenttreatment ofof co-producedco-produced hydrogenhydrogen chloride.chloride. ElectrolyticElectrolytic andand catalyticcatalytic technologiestechnologies cancan bebe appliedapplied forfor thisthis purpose.purpose. HCl electrolysis isis basedbased onon thethe conversionconversion ofof aa 2222 wt.%wt.% HClHCl solution over graphite electrodes separated by a diaphragm to Cl2 and H2, respectively at solution over graphite electrodes separated by a diaphragm to Cl2 and H2, respectively at thethe anodeanode andand the the cathode. cathode. High High electricity electricity consumption consumption leads leads to to the the unfavorable unfavorable operating operat- costsing costs of this of this process process [21]. [21]. TheThe catalyticcatalytic optionoption isis basedbased