Transition metal-catalyzed oxidative cleavage of lignin and lignin β-O-4 model compounds

The research aim of the dissertation “Transition metal-catalyzed oxidative cleavage of lignin and lignin β-O-4 model compounds” was to study the oxidative cleavage of lignin with transition metal catalysts. Lignin is one of three main components of lignocellulosic biomass and it is considered a potential feedstock for chemicals and fuels. Due to the complexity of lignin and its challenging analytics every initial catalyst screening was performed with lignin model compounds. In the majority of these studies erythro 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-1,3-propanediol (1) was employed as model substrate.A superior protocol for the synthesis of 1 and other diastereomerically pure 1,3-dilignol model compounds was developed. The key step in this synthesis is the 1,2-addition of a tert-butyl aryloxy ester enolate to a benzaldehyde derivative. The corresponding erythro and threo β-hydroxy esters could completely be separated from each other by column chromatography which had been one of the main challenges in previous protocols. Two iron catalyzed homogeneous reaction systems were studied. Nonheme iron catalysts displayed high activity for the conversion of 1 but afforded low selectivity for the formation of cleavage products. FeCl3-derived catalysts also showed high activity but were significantly more selective than the tested nonheme iron catalysts. 2-Methoxyphenol and veratraldehyde were the main products in 42% and 35% yield. The reaction likely involves the formation of methyl radicals that are generated from dimethyl sulfoxide and H2O2.Furthermore, heterogeneous transition metal catalysts were screened. Supported gold nanoparticles displayed distinctly different activity for the cleavage of 1 depending on the support and the reduction method. The most active catalyst in the oxidative cleavage was 1-phenylethanol reduced gold on hydrotalcite. In reactions performed in dimethyl carbonate it was observed that basic supports could function as catalysts for the base-catalyzed cleavage of 1.Transition metal-containing hydrotalcite-like catalysts were employed in the cleavage of lignin model compounds and different lignin samples. Copper-vanadium hydrotalcite-like catalysts (HTc-Cu-V) showed high activity and good selectivity for the cleavage of 1. Veratric acid and veratraldehyde were the main products in a combined yield of over 70%. Leaching experiments with HTc-Cu-V revealed that it acts to a significant degree as dispenser of catalytically active homogeneous copper and vanadium species that are continuously released. Lignin cleavage studies with organosolv and kraft lignin showed that HTc-Cu-V cleaved the different lignin samples to dimer and trimer size products.During the leaching experiments conducted with HTc-Cu-V, V(acac)3 and Cu(NO3)2·3H2O were tested as homogeneous vanadium and copper sources. V(acac)3/Cu(NO3)2·3H2O showed high activity and good selectivity for the cleavage of 1 affording veratric acid and veratraldehyde in a combined yield of over 80%. Studies with organosolv and kraft lignin showed that V(acac)3/Cu(NO3)2·3H2O displays similar activity as HTc-Cu-V for the cleavage of lignin furnishing dimer and trimer size products

Mechanistic Investigation of the Catalyzed Cleavage for the Lignin β-O-4 Linkage: Implications for Vanillin and Vanillic Acid Formation

The depolymerization of kraft lignin with a copper–vanadium hydrotalcite-like catalyst (HTc-Cu-V) and V­(acac)₃/Cu­(NO₃)₂·3H₂O mixtures to monomeric aromatic aldehydes and aromatic acids such as vanillin and vanillic acid using molecular oxygen as oxidant is reported. The obtained products correlate to model-based studies with <i>erythro</i>-1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-1,3-propanediol (<b>1a</b>). Kinetic investigations with <b>1a</b> demonstrate that there is a combined effect of V­(acac)₃ together with Cu­(NO₃)₂·3H₂O that enhances the catalytic activity and increases the selectivity and yield for the cleavage products veratric acid and veratraldehyde. Veratric acid is formed through reaction pathways involving either the transient oxidation products 1-(3,4-dimethoxyphenyl)-3-hydroxy-2-(2-methoxyphenoxy)­propan-1-one (<b>2</b>) and 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)­prop-2-en-1-one (<b>5</b>) or through overoxidation of veratraldehyde. The formation of veratraldehyde likely proceeds through C–C bond cleavage involving either a retro-aldol or a single electron transfer mechanism