Heterogeneous Diels-Alder catalysis for biomass-derived aromatic compounds

In this tutorial review, we provide an overview of heterogeneous Diels-Alder catalysis for the production of lignocellulosic biomass-derived aromatic compounds. Diels-Alder reactions afford an extremely selective and efficient route for carbon-carbon cycloadditions to produce intermediates that can readily undergo subsequent dehydration or dehydrogenation reactions for aromatization. As a result, catalysis of Diels-Alder reactions with biomass-derived dienes and dienophiles has seen a growth of interest in recent years; however, significant opportunities remain to (i) tailor heterogeneous catalyst materials for tandem Diels-Alder and aromatization reactions, and (ii) utilize biomass-derived dienes and dienophiles to access both conventional and novel aromatic monomers. As such, this review discusses the mechanistic aspects of Diels-Alder reactions from both an experimental and computational perspective, as well as the synergy of Bronsted-Lewis acid catalysts to facilitate tandem Diels-Alder and aromatization reactions. Heterogeneous catalyst design strategies for Diels-Alder reactions are reviewed for two exemplary solid acid catalysts, zeolites and polyoxometalates, and recent efforts for targeting direct replacement aromatic monomers from biomass are summarized. Lastly, we point out important research directions for progressing Diels-Alder catalysis to target novel, aromatic monomers with chemical functionality that enables new properties compared to monomers that are readily accessible from petroleum.