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Transition metal catalyzed C-O bond activation in biomass derived building blocks

par De Smet, Gilles
Président du jury Cunha, Ana
Promoteur Evano, Gwilherm ;Maes, Bert U. W.
Publication Non publié, 2023-10-26

Thèse de doctorat

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Résumé :

The increasing CO2 levels and associated climate change are drivers to move towards a circular chemical industry. Therefore, there is besides recyclable carbon an urgent need for means to access renewable carbon. Two main strategies have been put forward: (i) carbon capture and utilization (CCU) and (ii) use of biomass as feedstock. One particularly interesting biorenewable resource is lignin, part of lignocellulose biomass, giving access to phenolics, i.e. guaiacols, syringols, catechols and pyrogallols. These substrates are often referred to as platform molecules with the potential to serve as renewable feedstock for the production of commodity chemicals as an alternative to oil. However, lignin derived aromatics have a high oxygen content, which is in sharp contrast to benzene, toluene, xylene (BTX) obtained from current petroleum refining. Therefore, instead of an oxidative approach to introduce functional groups required for oil, a reductive approach is needed for bioaromatics. One way to allow a controlled decrease in oxygen content of lignin derived aromatics while maintaining aromaticity is chemoselective hydrodeoxygenation. A particular interesting subclass of HDO reactions is selective hydrodehydroxylation allowing removal of hydroxy groups in presence of methoxy groups. However, this transformation requires pre-activation of the hydroxy group by installment of a leaving group. Acetate is introduced as a renewable leaving group for selective hydrodeacetoxylation of aryl acetates using a homogeneous Ni-NHC catalyst and pinacolborane reductant in green dimethyl carbonate solvent. Proof-of-concept for oil derived substrates was demonstrated using renewable 4-propylguaiacol obtained from pine wood via reductive catalytic fractionation (RCF). Furthermore, overcoming the limitations of air- and water-sensitive Ni catalysts, a heterogeneous Ni-catalyzed hydrodeoxygenation method was developed for methyl aryl carbonates and catechol carbonates. This methodology was also shown applicable to renewable 4-propylguaiacol and 4-propylcatechol obtained from RCF of pine wood. Next to aromatic substrates, hydrodeoxygenation of aliphatic hydroxy groups, pre-activated as methyl carbonates, using air-stable Ni(cod)(dq) as catalyst giving the corresponding alkane products is also described.