This study proposes a photoredox-based strategy for the selective degradation of lignin model substrates. By utilizing photoredox conditions, the researchers successfully triggered a reverse biosynthetic pathway that degrades the β-O-4 and β-5-β-O-4 linkages found in lignin. This approach offers high yields of monolignol precursors, which are important for the production of flavoring and fragrance compounds. The study presents the optimization of synthesis methods for model compounds and explores the reactivity of redox auxiliaries in initiating deoxygenative radical formation.
Summary
- 🔬 Lignin is a component of lignocellulose biomass and is not fully utilized, with over 40 million tons discarded and incinerated annually.
- 🌱 Lignin is naturally synthesized through the oxidation of phenylpropanoid monomers to phenolic radicals, followed by dimerization to form C-O and C-C linkages.
- ⚛️ The low reactivity of lignin’s ether C-O bonds and its irregular structure pose challenges for selective conversion into functional products.
- 🔄 The study proposes a reverse biosynthetic approach inspired by the natural biosynthesis of lignin, involving oxygen-atom abstraction and subsequent β-scission of the β-O-4 linkage.
- 🧪 Photoredox conditions using redox auxiliaries, such as dihydropyridine carboxylic acid (DHP-CO2H) and oxalyl chloride (COCl)2, are employed to initiate deoxygenative radical formation.
- 🎯 The degradation of β-O-4 model substrates resulted in high yields of monolignol precursors, while the dimeric β-5-β-O-4 model substrate exhibited lower yields.
- 🌐 The study provides insights into lignin depolymerization mechanisms and the synthesis of lignin model substrates, contributing to the development of selective lignin conversion methods.
Lignocellulose biomass, which includes lignin, is a valuable renewable resource that can be utilized for various applications such as fuels, chemicals, and energy. However, lignin, which accounts for 15-30% of lignocellulose biomass by weight, is not fully utilized, leading to the disposal and incineration of over 40 million tons annually. Old ways of breaking down lignin have problems such as losing important chemical features and creating difficult-to-separate mixtures of complex products.
In this study, researchers propose a novel strategy for lignin degradation based on a reverse biosynthesis pathway. Inspired by the natural biosynthesis of lignin, they hypothesize that deoxygenative radical formation can occur at the α-position of the β-O-4 linkage, triggering a reverse biosynthetic pathway. This pathway involves the fragmentation of the β-O-4 and β-5-β-O-4 linkages, leading to the formation of monolignol precursors. The researchers utilize photoredox conditions to initiate the deoxygenative radical formation and subsequent fragmentation.
The study presents the results of experiments using photoredox conditions with two redox auxiliaries, dihydropyridine carboxylic acid (DHP-CO2H) and oxalyl chloride (COCl)2. The photoredox-based degradation of a β-O-4 model substrate resulted in high yields of phenol and 3,4-dimethoxycinnamyl acetate. Furthermore, an optimized synthesis method for a β-5-β-O-4 model compound was developed, allowing for the assessment of the reverse biosynthetic degradation.
Selective Cleavage of Lignin Model Compounds via a Reverse Biosynthesis Mechanism
Sub Title
- Sang Mi Suh, Subramanian Jambu, Mason T. Chin, and Tianning Diao*
- Publication Date:June 9, 2023
- Published by American Chemical Society
