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

Veronika D Badazhkova,Risto Savela,Reko Leino

doi:10.1039/d2dt00267a

Veronika D Badazhkova, Risto Savela + Show 1 more

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https://doi.org/10.1039/d2dt00267a

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Abstract

Selective ruthenium-catalyzed oxidation of lignin diol model compounds and lignin was accomplished by a transfer hydrogenation methodology. The developed procedure allows us to selectively oxidize benzylic secondary alcohols in model diols and spruce milled wood lignin in the presence of a commercially available Shvo catalyst under aerobic conditions. Six ketoalcohols were obtained in 70-92% yields from the model compounds, which also included lignin monomers containing 5-5' and β-O-4 linkages. The developed method can be used as an intermediate step for the introduction of new functional groups into lignin-type structures and lignin to allow their further modifications.

Highlights

We have developed a feasible methodology for selective functionalization of diol fragments in lignin model compounds using a ruthenium-catalyzed hydrogen borrowing procedure
It was demonstrated that benzylic hydroxyl groups in several model compounds could be selectively oxidized in the presence of the Shvo catalyst under aerobic conditions via the transfer hydrogenation pathway
After screening of the model compounds, this methodology was applied to native lignin, resulting in selective modification of the benzylic hydroxyl groups

Summary

Introduction

The search for innovative processes based on renewable energy sources and the development of environmentally benign methods for producing modern materials and base chemicals are significant global challenges.[1]. Due to its aromatic structure and high carbon content, lignin is a highly potential renewable source for the synthesis of a wide range of aromatic compounds and materials for various industrial applications. It was demonstrated that the hydroxyl groups in lignin can be selectively oxidized using organocatalytic methods under an oxygen atmosphere,[6] iridium catalysts under oxidant-free conditions,[7] or in the presence of radical catalysts.[8]. Another approach for modification of lignin is based on the functionalization of the phenolic and aliphatic hydroxyl groups by hydrogen borrowing reactions.[9,10]. Another approach for modification of lignin is based on the functionalization of the phenolic and aliphatic hydroxyl groups by hydrogen borrowing reactions.[9,10] Compared to other oxidation processes, hydrogen transfer is highly selective, safe and eco-friendly and takes place in the presence of a wide range of metal catalysts.[11]

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