(371e) Breaking Aliphatic Carbon-Carbon Bonds Via Electrochemically Mediated Hydrogen Atom Transfer Reactions and Its Application to Polystyrene Deconstruction

The selective cleavage of carbon-carbon (C-C) bond is central to plastic deconstruction and biomass valorization.^1–4 However, the inertness of the σ C-C linkages hinders the selective and energy-efficient depolymerization and upcycling of polyolefinic plastic waste.⁵ The activation of σ C-C bonds is challenging due to several thermodynamic and kinetic constraints,⁶ including high bond dissociation energies (BDEs) of ~90 kcal mol^−1,⁶ steric inaccessibility caused by surrounding C-H bonds, and unfavorable orbital directionality towards cleavage requiring the rotation of two carbon sp³ orbitals.⁷Except for a limited number of cases where σ C-C bond scission is facile such as strained molecules, and structures that introduce aromaticity upon cleavage,^6,7 most σ C-C bonds do not fall into these special categories and necessitate more aggressive reaction conditions for activation. For example, thermochemical pathways used for plastic depolymerization, such as pyrolysis and thermal cracking, operate at temperatures > 400°C and suffer from low product selectivity.⁸ Alternatively, oxidative C-C bond cleavage produces oxygenates as valuable chemicals,² but have low environmental factors, urging the development of new oxidative schemes.

Electrocatalytic oxidation presents a promising approach to activating and cleaving inert σ C-C bonds under mild conditions.^9–11 During direct electrooxidation, the substrate is activated at the electrode to generate intermediates or products (Figure a).^1,12 While simple, oxidizing inert molecules in this setup often requires highly anodic potentials where side reactions, such as solvent oxidation, can occur. On the other hand, a redox-active species can be used to mediate the process indirectly by first undergoing oxidation at the electrode, then selectively oxidizing the substrate of interest in solution, and lastly regenerating at the electrode to start the cycle over (Figure b). Since the mediator is designed to undergo rapid electron transfer at the electrode surface, a significantly lower oxidation potential is necessary to drive the conversion. Furthermore, mediators can be engineered to react with the substrate selectively, thus eliminating many side reactions.

Results

Here, we employ a mediator, N-hydroxyphthalimide (NHPI), that is readily oxidized at a carbon electrode into the phthalimide-N-oxyl (PINO) radical.^13–15 PINO is highly adept at benzylic C-H hydrogen atom abstraction to return to its reduced form, NHPI, and complete one catalytic cycle. The resulting benzylic carbon radical is susceptible to further oxidation (e.g., in the presence of molecular oxygen) to form a peroxide species that decomposes into oxygenate products (Figure b). This indirect, mediated approach for σ C-C bond cleavage reduces the oxidation potential by 1.26 V compared to the direct oxidation of the substrate (Figure c), thereby eliminating deleterious side reactions, such as solvent oxidation, that may occur at high potentials.

Studies with a bibenzyl model compound revealed a branched reaction pathway following the initial HAT step, which leads to either C-C bond cleavage products such as benzaldehyde and benzoic acid, or C-H oxygenation products such as 1,2-diphenylethanone and benzil (Figure d). We have optimized the bulk electrolysis conditions and have studied the effects of NHPI quantity, NHPI substituents, basicity, electrolyte type, solvent, O₂ partial pressure, temperature, and oxidation current. The optimal reaction conditions suppress the self-decomposition of PINO and maximize the conversion of bibenzyl to 61.0% with a selectivity of 38.4% to C-C bond cleavage products and a selectivity of 39.2% to C-H bond oxygenation products.

To probe the structural factors that determine the C-C bond cleavage selectivity, we changed the length of the alkyl chains that separate the two phenyl rings and added substituents to the benzylic carbon. The substrate screening study suggests that 1) PINO is adept at HAT of benzylic C-H; 2) methyl groups, alcohol hydroxyl groups, or carbonyl groups do not react with PINO; 3) the competition between C-C bond cleavage and C-H bond oxygenation depends on their relative bond strength in that a weaker C-C bond results in higher C-C bond cleavage selectivity. For example, bibenzyl with a weaker C-C bond displays a higher C-C cleavage selectivity of 38.4% compared to 8.1% for 1,4-diphenylbutane.

Next, we investigated the C-C bond cleavage of polystyrene (PS) oligomers with an average molecular weight of 510 Da. With oxidation, the starting oligomers were completely converted into oxygenates, with a yield of 77.8% for monomers and dimers (Figure e). Similar to the model compounds, the oxidation is initiated by HAT and followed by C-C bond cleavage.

As a proof-of-concept, we employed NHPI to mediate the oxidative depolymerization of high molecular weight PS (average M_w = 9,500 Da). GPC results confirmed that PS was successfully broken down into shorter-chain products via C-C bond cleavage (Figure f). The yield of monomers and dimers reached 12% after 4.29 electrons were transferred to each styrene unit in average.

Implications

The NHPI-mediated oxidation strategy demonstrates the great potential of electrocatalysis in activating inert chemical bonds under mild conditions. Given the wide variety of redox mediators, this study opens the door to employing renewable electricity and targeted redox mediators to catalyze challenging chemical transformations. Future studies will focus on engineering the redox mediator to improve its stability in order to boost the conversion and yield of C-C bond cleavage products. Additionally, strategies to immobilize the redox mediators onto a support or to develop heterogeneous mediators will facilitate the product separation and purification processes, especially for large-scale applications.

Methods

General electrochemical methods. Experiments were conducted using a Gamry Interface 1010B potentiostat. Electrochemical measurements were performed in an H-shaped three-electrode electrochemical cell with a Nafion membrane separating the working and counter compartments. 0.1 M LiBF₄, 0.1 M pyridine and 0.1 M acetic acid (HOAc) acetonitrile solution served as the electrolyte unless otherwise stated. The bulk electrolysis was performed in constant current mode. For all experiments, a leakless Ag/AgCl electrode was used as reference electrodes. All experiments were performed at ambient temperature, (21 ± 1 °C), except for the reaction temperature optimization experiments.

Characterizations. The products of bulk electrolysis were identified using an Agilent Model 7820A GC equipped with an Agilent 5977B single quadrupole MS detector. The GC column is 30 m × 250 μm × 0.25 μm (HP-5ms Ultra Inert, Agilent). The products of bulk electrolysis were quantified using an Agilent Model 7890A GC equipped with an FID. The GC column is 30 m × 250 μm × 0.25 μm (DB-1701, Agilent). The electrolyte of polystyrene depolymerization was analyzed by Agilent 1100 GPC with three 5 μm PLgel Agilent GPC columns in series arranged from larger pore size to smaller pore size from 104 to 103 to 50 Å.

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