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Catalytic hydrodehalogenation is the replacement of a C-X (X= F, Cl, Br, I) bond with a new C-H bond[1]. Hydrodehalogenation occupies an important function in modern catalysis in terms of catalytic efficiency and environmental waste management. Hydrodehalogentation can contribute to undesirable side products and diminished yields in metal cross-coupling reactions as it offers a competing pathway for aryl halide systems[1].

Mechanisms

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Homogeneous Catalysis

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A radical chain catalyzed hydrodehalogenation mechanism.

Several homogeneous catalytic hydrodehalogenation mechanisms are known, including by radical chain process, halogenation of silanes, and by other transition metal mediated methods.

Radical chain reactions involving the hydrodehalogenation of alkyl halides to form alkanes are known to occur by tin[2][3], germanium[4], mercury[2], and other catalysts[5]. An example of a radical chain mechanism[6] using Bun3SnSnBun3 is shown to the left.

A catalytic cycle for the silane-mediated hydrodefluorination of RF.

The silane-mediated reduction of alkyl fluorides is known, where Et3Si+ abstracts fluoride from RF to form the carbocation R+, which subsequently gains a hydride from Et3SiH to give RH and regenerate Et3Si+[7]. A simple example of this mechanism is shown to the right.

The Pd catalyzed hydrodehalogenation of aryl iodide.

The catalytic hydrodehalogenation of aryl halides is possible using many transition metal catalysts, but is most commonly achieved using palladium catalysts. It is generally accepted that the key steps involve the oxidative addition of Ar-X (where X = halide) to the metal, ligand substitution of the halide with methoxide, β-elimination of formaldehyde, and finally the reductive elimination of Ar-H[8] to regenerate the catalyst. More recently[1], the ligand substitution of the halide with methoxide has been shown to perhaps be more complicated: when the halide is displaced it may not by the deprotonated methoxide, but rather by methanol, which is subsequently deprotonated with base before the β-elimination step.

Heterogeneous Catalysis

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The heterogeneous catalysis of hydrodehalogenation can be achieved using many different metal catalysts in the presence of hydrogen and alkyl or aryl halides, most commonly when the metal is palladium or nickel[5]. These gas-phase mechanisms have been less well understood than the homogeneous mechanisms, as the surface chemistry at the catalyst has been debated[9]. The hydrodechlorination of chlorofluorocarbons (CFCs) is thought to occur by a mechanism in which the CFC is adsorbed dissociatively onto the palladium surface, forming an adsorbed radical species, followed by a dechlorination step to make a :CF2 carbene. This can then be hydrodefluorinated twice to form :CH2, which reacts to form methane and a variety of other products[10].

A mechanism for the catalytic hydrodechlorination of chlorobenzene over palladium has also been proposed, where once again the reagents are adsorbed to the metal surface:

(1) Pd-H + C6H5Cl(g) → Pd-Cl + C6H6(g)

(2) Pd-H + HCl → Pd-Cl + H2

(3) Pd-Cl + H2 → Pd-H + HCl

However, whether the mechanism occurs via a radical pathway or by some ionic method (nucleophilic or electrophilic attack) is unknown[10].

Catalysts

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An expansive collection of compounds have been studied as catalysts for this hydrodehalogenation, both intentionally (e.g. through catalyst design) and incidentally (e.g. through competing catalytic processes). Remarkably, examples of catalysts based on elements from across the entirety of the periodic table have been reported for this process. Largely these catalysts are transition metal based, with representations from all groups and rows of the d-block and in a variety of compositional forms (e.g. zerovalent metals, metal salts, organometallic complexes, etc.)[5]. The most widely used hydrodehalogenation catalysts feature palladium, however ruthenium, rhodium, nickel, copper, and tin compounds are also common.

The typical order of reactivity in hydrodehalogenation is benzylic > allylic > vinylic > aromatic > aliphatic for the halogenated substrate[5] and I > Br > Cl >> F for the halide substituent [11], given that there is a thermodynamic dissociation energy related to the strength of the C-X bond. This can be advantageous for catalytic selectivity of systems that are poly- and/or inter-halogenated[5][8]. Catalyst design in this domain has notably focused on hydrodechlorination in light of concerns over environmentally persistent chlorinated compounds and their long-term management[12].

Palladium Catalysts

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Palladium has found the most frequent use for catalytic hydrodehalogenation in both heterogeneous and homogeneous catalytic systems[13]. Traditionally, Pd catalysts have employed the use of phosphine ligands[14], but have moved towards N-Heterocyclic carbene (NHC) ligands[8][13] in recent design. Numerous systems employ the use of alkoxide bases[8][14][15][16] which are important as they contain β-hydrogen moieties. Bases containing β-hydrogens are suggested to be crucial to hydrodehalogenation, and provides a two-fold function; 1) deprotonation of the imidazolium salt and 2) source of hydrogen for dehalogenation[8]. Inorganic bases such as K2CO3, or K3PO4 yield to be ineffective for the catalytic hydrodehalogenation process.[8]. Overall, palladium catalysts have been shown to be effective towards a plethora of I, Br, Cl substituted aromatic and polyaromatic systems[5][8][14][15][16][17].

Ruthenium Catalysts

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The ruthenium catalysts known for hydrodehalogenation are very similar to the rhodium and palladium examples, both mechanistically and compositionally. For example, the complex [RuCl2(COD)]x (COD = cyclooctadiene) when added to two equivalents of a phosphine (e.g. PCy3 or PiPr3) yields the catalyst precursor RuHCl(H2)2(PR3)2, which readily hydrodechlorinates singly or multi-chlorinated arenes in the presence of a secondary alcohol[18], as has been found for palladium[8], suggesting a transfer hydrogenation process. Similarly, RuCl2(PPh3)3 will react catalytically with chlorinated solvents, such as dichloromethane or carbon tetrachloride, when an alcohol is also present[19]. Reactivity is notably diminished when the ruthenium catalyst is substituted with nitroysl or chelating phosphine ligands (e.g. dppe = 1,2-bis(diphenylphosphino)ethane, etp = bis(diphenylphosphinoethyl)-phenylphosphine)[20] [19].

Rhodium Catalysts

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Classic organometallic and coordination compounds of rhodium have exhibited remarkable activity in a range of catalytic processes, one among them hydrodehalogenation. Complexes of the type L2RhCl3 (L = PPh3, Ph2P(CH2)2PPh2)[16] and L2Rh(H)Cl2 (L = PCy3, PiPr3)[21] have been reported as effective catalysts towards this process, as well as complexes featuring pi-donor ligands, such as the dimers (Cp*RhCl2)2 [22] and [Rh(μ-Cl)(COE)2]2 (COE = cyclooctene)[23]. These compounds are representative of the catalog of possible rhodium acceptor/phosphine donor complexes that may be similarly successful catalysts towards hydrodehalogenation. A minority of hydrodehalogenation catalysts are known to replace C-F bonds with C-H bonds given the C-F bond strength, however several rhodium catalysts have proven to be anomalous, for example (Me3P)3RhC6F5 and (Me3P)4RhH [24].

Nickel Catalysts

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Phosphine coordinated nickel complexes, analogous to those known for Ru, Rh, and Pd, are similarly effective towards catalytic hydrodehalogenation. In the presence of a hydrogen source, typically a borohydride reducing agent, the compounds (Ph3P)2NiCl2 [25] and (Et3P)2NiCl2 [26] selectively reduce aryl bromides and aryl iodides over aryl chlorides. Conversion of chlorinated, in addition to brominated and iodinated, substrates is observed for reaction mixtures involving NiCl2·2H2O lithium powder, and a catalytic amount of 4,4′-di-tert-butylbiphenyl (DTBB)[27]. Studies on the reactivity of Ni(0)/N-heterocyclic carbene catalysts reveal parallels with studies undertaken for palladium[8], indicating that the highest conversion of arylhalides is achieved for both Ni and Pd/NHC systems with 1,3-bis(2,4,6-trimethylphenylphenyl)imidazol-2-ylidene (IMes·HCl)[28].

Copper Catalysts

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Many forms of copper have been shown to catalyze hydrodehalogenation, though most simply copper metal and copper(II) oxide, which were studied in the context of environmental waste management by incineration[29]. These copper substances offer an accessible means to bulk hydrodehalogenation, given that they are readily available at low cost and may be handled with relative ease. Of further utility is the high degree of chemoselectivity that has been achieved with some copper-containing, heterogenous catalysts in hydrodehalogenation. CuSO4 on borohydride exchange resin (BER), for example, readily reduces alkyl and aryl iodides and bromides, but is ineffective for chloride analogues[30]. In contrast, the homogeneous catalyst copper(I) benzoate exhibits little chemoselectivity for aryl halides[31], but marked regioselectivity in halopyridines[32].

Tin Catalysts

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Studies of tin(IV) compounds as hydrogen transfer agents revealed facile, catalytic reduction of alkyl halides by trialkyltin hydrides in the presence of lithium aluminum hydride[33][34] and lead to their development as catalysts in the reduction of alkyl and aryl halides. Reactivity of tin(IV) hydrides has been found to increase with the number of hydride substituents at tin (e.g. R3SnH < R2SnH2 < RSnH3 for R = n-C4H9 and C6H5)[33]. Addition of the initiator azonitrile is known to further accelerates these processes, which supports the proposed radical stannyl mechanism[3][33]. Activity of the di- and tri-hydride alkyltin compounds is poorly understood and the involvement of mixed hydride tin halides in the reduction cannot be discounted. Consequently, R3SnH catalysts have been most widely used, tributyltin hydride especially (TBTH, R = n-C4H9)[2][6]. The reactivity of lithium aluminum hydride, however, limits the utility of this procedure and has led to the development of in situ generation of trialkyltin hydrides from trialkyltin halides in the presence of sodium borohydride[35].

Applications

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Synthesis

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Hydrodehalogenation can be an effective method to remove a C-Cl bond on a chloroarene ring in which the Cl moiety was employed as a protecting group for the regioselectivity of electrophilic aromatic substitution (see figure below)[21]. Furthermore, a high degree of selectivity can be observed when hydrodehalogenation catalysts are employed in the dehalogenation of molecules containing numerous functional groups that would otherwise not withstand traditional hydrogenation (Pd/C, H2) or metal hydride reduction without decomposition[36]. Additionally, catalytic hydrodehalogenation research has found potential for alternative approaches to deuterium labeling[37].

Hydrodehalogenation for Deprotection of Ar-Cl in Electrophilic Aromatic Substiution

Environment

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With the growing importance of environmental sustainability, catalytic hydrodehalogenation is finding its utility in the controlled degradation of the recalcitrant organic halides such as chlorofluorocarbons (CFCs), chlorinated-p-dibenzo-dioxins (dioxins), and polyhalogenated biphenyl's (PCBs, & PBBs)[5]. The catalytic process offers numerous advantages compared to traditional incineration methods in both cost and utility. This is because incineration, solvent extraction, microbial decomposition, and direct adsorption onto carbon does not provide sustainable management solutions[5]. Moreover, catalytic dehalogenation prevents toxic dibenzofurans and dibenzodioxins formation through incineration, and removes chlorofluorocarbons (CFC's) and polyhalogenated biphenyls (PCB's) compounds from entering the atmosphere experiencing the photodegredation that promotes free chlorine into the atmosphere and subsequently destroys the ozone layer[5].

References

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