Advanced Model Compounds for Understanding Acid-Catalyzed Lignin Depolymerization: Identification of Renewable Aromatics and a Lignin-Derived Solvent

The
development of fundamentally new approaches for lignin depolymerization
is challenged by the complexity of this aromatic biopolymer. While
overly simplified model compounds often lack relevance to the chemistry
of lignin, the direct use of lignin streams poses significant analytical
challenges to methodology development. Ideally, new methods should
be tested on model compounds that are complex enough to mirror the
structural diversity in lignin but still of sufficiently low molecular
weight to enable facile analysis. In this contribution, we present
a new class of advanced (β-O-4)-(β-5) dilinkage models
that are highly realistic representations of a lignin fragment. Together
with selected β-O-4, β-5, and β–β structures,
these compounds provide a detailed understanding of the reactivity
of various types of lignin linkages in acid catalysis in conjunction
with stabilization of reactive intermediates using ethylene glycol.
The use of these new models has allowed for identification of novel
reaction pathways and intermediates and led to the characterization
of new dimeric products in subsequent lignin depolymerization studies.
The excellent correlation between model and lignin experiments highlights
the relevance of this new class of model compounds for broader use
in catalysis studies. Only by understanding the reactivity of the
linkages in lignin at this level of detail can fully optimized lignin
depolymerization strategies be developed.