Light-Driven Depolymerization of Native Lignin Enabled by Proton-Coupled Electron Transfer

Abstract

Here we report a catalytic, light-driven method for the redox-neutral depolymerization of native lignin biomass at ambient temperature. This transformation proceeds via a proton-coupled electron-transfer (PCET) activation of an alcohol O-H bond to generate a key alkoxy radical intermediate, which then drives the β-scission of a vicinal C-C bond. Notably, this depolymerization is driven solely by visible light irradiation, requiring no stoichiometric chemical reagents and producing no stoichiometric waste. This method exhibits good efficiency and excellent selectivity for the activation and fragmentation of β-O-4 linkages in the polymer backbone, even in the presence of numerous other PCET-active functional groups. DFT analysis suggests that the key C-C bond cleavage reactions produce non-equilibrium product distributions, driven by excited-state redox events. These results provide further evidence that visible-light photocatalysis can serve as a viable method for the direct conversion of lignin biomass into valuable arene feedstocks. Figure 1. (a) Stephenson's photocatalytic lignin depolymerization. (b) C-C bond cleavage of a model lignin dimer via O-H PCET. (c) Photocatalytic depolymerization of native lignin via O-H PCET.

Photocatalytic Conversion of Lignin into Chemicals and Fuels

“…A proton‐coupled electron transfer (PCET) process was also reported for the cleavage of C−C bonds in lignin β‐O‐4 linkages . PCET refers to the reaction in which protons and electrons are transferred at the same time.…”

Catalytic generation of alkoxy radicals from unfunctionalized alcohols

“…The PCET strategy described here for C-C bond scission was adapted independently by the groups of Zhang and Knowles for the depolymerization of lignin models and biomass-derived polymeric lignin. 88,89 Following their 2019 report, the Knowles lab developed a strategy for the catalytic ring expansion of cyclic alcohols to n + 1 or n + 2 expanded ketones by taking advantage of the propensity of alkoxy radicals to undergo C-C cleavage. 90 By employing a similar series of elementary steps as in their prior work, O-H PCET is leveraged to generate an alkoxy radical intermediate from cyclic allylic alcohols (Scheme 32).…”

Catalytic Hydroetherification of Unactivated Alkenes Enabled by Proton‐Coupled Electron Transfer

“…With the success of model substrate 1, we found that a range of other 1,2-disubstituted olefins gave excellent yields of the desired cyclic ether products (3)(4)(5)(6)(7)(8)(9). Hydroetherification of disubstituted olefins with secondary alcohols also proved to be viable, albeit with moderate reactivity (10). Notably, this method provides direct access to a variety of bicyclic structures from alcohol precursors, forming fused rings (11,12) and bridged ethers (13) with high diastereoselectivity.…”

“…Our group has developed methods for the homolytic activation of alcohol O À H bonds to access alkoxy radical intermediates, focusing exclusively on CÀC bond bscission reactions. [10,11] Because the rates of b-scission and 1,5hydrogen atom transfer (1,5-HAT) are often comparable to the rate of cyclization, [12, 8b,i] we wondered whether alkoxy radicals could be leveraged instead for productive CÀO bond formation, thereby generating cyclic ethers directly from readily accessible alkenols while outcompeting other pathways. [13] Within this context, we endeavored to find catalysts and reaction conditions that could not only effectively bias the reactivity of alkoxy radicals to favor olefin addition but to do so with a broad scope and functional-group tolerance.…”

“…Based on prior work, we envisioned that PCET activation of the alcohol O À H bond of the substrate would form an alkoxy radical intermediate through the concerted action of an Ir III -based visible-light photooxidant and a weak Brønsted base catalyst. [10,11] The resulting O-centered radical would undergo addition to a pendent olefin, forming a new C À O bond and an adjacent alkyl radical. Hydrogen-atom transfer from a thiol-derived co-catalyst to the C-centered radical would furnish the desired cyclic ether.…”