Co(salen)-Catalyzed Oxidation of Lignin Models to Form Benzoquinones and Benzaldehydes: A Computational and Experimental Study

Lignin
is a highly abundant polyphenolic polymer that imparts mechanical
strength to plant biomass. Transition-metal complexes can catalyze
lignin oxidation to produce value-added products, but low catalytic
efficiency has hampered their use in industry. Identifying the chemical
and structural factors that govern catalytic activity is a prerequisite
to rational design of catalysts with improved activity. Here, we combine
computational and experimental approaches to investigate the mechanism
of Co­(salen)-catalyzed oxidation of the monomeric lignin models syringyl
(S), vanillyl (G), and 4-hydroxybenzyl alcohol (H) to produce benzoquinone
and benzaldehyde products. Experimentally, S oxidation to form dimethoxybenzoquinone
proceeded efficiently with a Co­(salen) catalyst coordinated by a pyridine
ligand, but G and H did not undergo oxidation. Density functional
theory calculations reveal that catalyst regeneration is energetically
unfavorable in the presence of H, which prevents oxidation. In contrast,
S readily facilitates catalyst regeneration. Formation of methoxybenzoquinone
from G was achieved experimentally by adding bulky, noncoordinating
bases. These findings provide a fundamental baseline for enhancing
the activity of Co-Schiff base catalysts toward lignin-like molecules
by adding sterically hindered nitrogenous bases or potentially by
including a cocatalyst that promotes catalyst regeneration.