Electrochemical Activation of C−C Bonds through Mediated Hydrogen Atom Transfer Reactions

Abstract

Activating inert sp 3 -sp 3 carbon-carbon (CÀ C) bonds remains a major bottleneck in the chemical upcycling of recalcitrant polyolefin waste. In this study, redox mediators are used to activate the inert CÀ C bonds. Specifically, N-hydroxyphthalimide (NHPI) is used as the redox mediator, which is oxidized to phthalimide-N-oxyl (PINO) radical to initiate hydrogen atom transfer (HAT) reactions with benzylic CÀ H bonds. The resulting carbon radical is readily captured by molecular oxygen to form a peroxide that decomposes into oxygenated CÀ C bond-scission fragments. This indirect approach reduces the oxidation potential by > 1.2 V compared to the direct oxidation of the substrate. Studies with model compounds reveal that the selectivity of CÀ C bond cleavage increases with decreasing CÀ C bond dissociation energy. With NHPI-mediated oxidation, oligomeric styrene (OS 510 ; M n = 510 Da) and polystyrene (PS; M n � 10 000 Da) are converted into oxygenated monomers, dimers, and oligomers.

Recent Advances in Catalytic Chemical Recycling of Polyolefins

“…[168,169] Regardless, there are examples of using electrical energy, favorably cheap and generated from renewable sources, to enable otherwise difficult reactions or to drive more established processes under milder conditions. Once again, there were some attempts to apply electrocatalysis in plastic degradation focused on PET, [141,170] and a recent example for PS, [171] however, there is one example for HDPE, which will be mentioned shortly even though the temperature exceeds the threshold set for this review. In 2020, Jiang et al published an article on a solar thermo-coupled electrochemical process (STEP) for polypropylene degradation.…”

Electrochemically Driven Hydrogen Atom Transfer Catalysis: A Tool for C(sp3)/Si–H Functionalization and Hydrofunctionalization of Alkenes

“…To help researchers make informed decisions on the selection of appropriate HAT mediators, we have summarized the types of electrochemical mediators and their physical properties ,,,− ,,− (Scheme ) which directly affect their catalytic performance. Based upon our description of electrochemically driven HAT mechanisms, we have classified HAT mediators into three types, direct HAT mediator, photoelectrochemical HAT mediator, and donor HAT mediator.…”

A Horseradish Peroxidase–Mediator System for Benzylic C–H Activation

“…Benzylic substrate radicals can also react with O 2 , , leading to formation of benzylic ketones or aldehydes that can be subsequently converted to an array of different functional groups, including amines . Redox mediator-based oxidation methods using transition metal catalysts typically require high temperatures and O 2 pressures (Figure b). ,− More recently, photocatalytic , and electrochemical − methods have enabled the oxidation of redox mediators at room temperature under air for functionalization of diverse substrates (Figure c).…”

“…Compounds bearing phenol or aniline functionalities tend to be oxidized by HRP directly, leading to a complex mixture of products, including substrate dimers or oligomers. Additionally, certain alkylbenzene substituents are prone to C–C bond cleavage, consistent with previous reports . Compounds with tertiary benzylic carbons, such as cumene, are also prone to C–C bond cleavage (Figure S3).…”

“…Additionally, certain alkylbenzene substituents are prone to C−C bond cleavage, consistent with previous reports. 21 Compounds with tertiary benzylic carbons, such as cumene, are also prone to C−C bond cleavage (Figure S3). However, careful control of reaction temperature and solvent identity can minimize C−C bond cleavage in PINO-based reactions.…”