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

Abstract

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.

The base-catalyzed aerobic oxidation of hydroquinones to benzoquinones under metal-free conditions

“…Benzoquinones (BQs) are valuable precursors in the synthesis of pharmaceuticals and fine chemicals. 1,2 Among the many approaches to prepare BQs, the aerobic oxidation of hydroquinones (HQs) is an attractive method. 3 For example, Li and Trush reported that Cu( ii ) strongly mediates the oxidation of HQ producing BQ and H 2 O 2 with a Cu( ii )/Cu( i ) redox mechanism.…”

Sustainable Production of Benzylamines from Lignin

“…Lignin transformation has therefore become a major topic in biomass conversion area and would play an important role in improving the economics of the overall biorefinery associated with addressing the problem of environmental impacts. So far, great efforts have been devoted to selective cleavage of C−O or C−C linkages with the aim to obtain phenols, [1a, 4] arenes, [5] cycloalkanes, [6] aldehydes, [7] ketones, [3a, 8] acids, [9] and benzoquinones [10] . The products are limited to C, H, O containing compounds, while value‐added heteroatom‐containing products are rarely reported.…”

Ion‐in‐Conjugation: A Promising Concept for Multifunctional Organic Semiconductors

“…[ 50 ] By further comparing experimental evidence and density functional theory (DFT) studies, the mechanism of catalysis for asymmetric reactions can be understood in detail (Figure 14d). [ 114–117 ]…”

“…[50] By further comparing experimental evidence and density functional theory (DFT) studies, the mechanism of catalysis for asymmetric reactions can be understood in detail (Figure 14d). [114][115][116][117] Type I IIC materials, owing to multiple hydrogen bonds to guest molecules, can activate the organic molecule for further reaction in principle because the lowest unoccupied molecular orbital of the electrophile can be lowered by cooperative 10)-TpPa. Reproduced with permission.…”