An Introduction to Model Compounds of Lignin Linking Motifs; Synthesis and Selection Considerations for Reactivity Studies

Abstract

The development of fundamentally new valorization strategies for lignin plays a vital role in unlocking the true potential of lignocellulosic biomass as sustainable and economically compatible renewable carbon feedstock. In particular, new catalytic modification and depolymerization strategies are required. Progress in this field, past and future, relies for a large part on the application of synthetic model compounds that reduce the complexity of working with the lignin biopolymer. This aids the development of catalytic methodologies and in‐depth mechanistic studies and guides structural characterization studies in the lignin field. However, due to the volume of literature and the piecemeal publication of methodology, the choice of suitable lignin model compounds is far from straight forward, especially for those outside the field and lacking a background in organic synthesis. For example, in catalytic depolymerization studies, a balance between synthetic effort and fidelity compared to the actual lignin of interest needs to be found. In this Review, we provide a broad overview of the model compounds available to study the chemistry of the main native linking motifs typically found in lignins from woody biomass, the synthetic routes and effort required to access them, and discuss to what extent these represent actual lignin structures. This overview can aid researchers in their selection of the most suitable lignin model systems for the development of emerging lignin modification and depolymerization technologies, maximizing their chances of successfully developing novel lignin valorization strategies.

Hydrogenolysis of Lignin-Derived Aromatic Ethers over Heterogeneous Catalysts

“…However, the structure of lignin is more complex due to the presence of additional functionalities, and hence studies with more representative model compounds are desirable. Figure illustrates model compounds with increasing complexity that more closely mimic the most abundant β-O-4 linkage in lignin . Phenolic groups at the R 2 position in these model compounds represent β-O-4 linkages at the end of the lignin chain, while a methoxy group at R 2 position represents internal β-O-4 linkages.…”

Selective modification of hydroxyl groups in lignin model compounds by ruthenium-catalyzed transfer hydrogenation

“…The percentage of these linkage types depends on the lignin source, with β-O-4 type being the most abundant. 19 Consequently, also in our study, β-O-4 fragments and methoxy groups typical for lignin were introduced to the test reaction substrates. Furthermore, a lignin model monomer containing a 5-5' linkage was prepared and tested.…”

Exploiting Nature's Most Abundant Polymers: Developing New Pathways for the Conversion of Cellulose, Hemicellulose, Lignin and Chitin into Platform Molecules (and Beyond)

“…Some of the key ways in which such phenylpropanoid residues are incorporated into the polymeric lignin framework are shown in Figure 13. A knowledge of these has informed studies of model compounds undertaken in efforts to define conditions for the controlled depolymerisation of lignins [48p,49] . So, for example, Hartwig and co‐workers have devised hydrogenolytic methods for the cleavage of biaryl ethers (see 73 ), and so affording the corresponding mixtures of aryls and phenols, [49b,d] while Ford et al have reported [49a] related means for cleaving dihydrobenzofurans (see 74 ).…”