Copper- and Vanadium-Catalyzed Oxidative Cleavage of Lignin using Dioxygen.

Transition-metal-containing hydrotalcites (HTc) and V(acac)3 /Cu(NO3 )2 ⋅3 H2 O (acac=acetylacetonate) mixtures were tested for their catalytic activity in the cleavage of the lignin model compound erythro-1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-l,3-propanediol (1) with molecular oxygen as oxidant. Both catalytic systems displayed high activity and good selectivity and afforded veratric acid as the main product. The catalyst behavior was studied by EPR spectroscopy, XRD, and Raman spectroscopy. After the catalysts were established for the model system, lignin depolymerization studies were performed with various organsolv and kraft lignin sources. The oxidative depolymerization and lignin bond cleavage were monitored by gel permeation chromatography (GPC), MALDI MS, and 2D-NMR (HSQC). Irrespective of the lignin pretreatment, both HTc-Cu-V and V(acac)3 /Cu(NO3 )2 ⋅3 H2 O were able to cleave the ß-O-4 linkages and the resinol structures to form dimeric and trimeric products.

Ruthenium-catalyzed C-C bond cleavage in lignin model substrates.

Ruthenium-triphos complexes exhibited unprecedented catalytic activity and selectivity in the redox-neutral C-C bond cleavage of the ß-O-4 lignin linkage of 1,3-dilignol model compounds. A mechanistic pathway involving a dehydrogenation-initiated retro-aldol reaction for the C-C bond cleavage was proposed in line with experimental data and DFT calculations.

From gene towards selective biomass valorization: bacterial ß-etherases with catalytic activity on lignin-like polymers.

Microbial ß-etherases, which selectively cleave the ß-O-4 aryl ether linkage present in lignin, hold great promise for future applications in lignin valorization. However, very few members have been reported so far and little is known about these enzymes. By using a database mining approach, four novel bacterial ß-etherases were identified, recombinantly produced in Escherichia coli, and investigated together with known ß-etherases in the conversion of various lignin and non-lignin-type model compounds. The resulting activities revealed the significant influence of the substituents at the phenyl ring adjacent to the ether bond. Finally, ß-etherase activity on polymeric substrates, measured by using a fluorescently labeled synthetic lignin, was also proven; this underlined the applicability of the enzymes for the conversion of lignin into renewable chemicals.

Mild copper-mediated direct oxidative cross-coupling of 1,3,4-oxadiazoles with polyfluoroarenes by using dioxygen as oxidant.

CH(3)CN and O(2) do the trick! A copper-mediated direct oxidative cross-coupling of 2-(het)aryl-1,3,4-oxadiazoles with polyhaloarenes under mild reaction conditions has been developed (see scheme). The process provides a concise access to biaryl structures containing polyhaloarenes, which are of interest in the fields of pharmaceuticals and functional materials. Acetonitrile and oxygen play crucial roles.

Transition metal-free direct C-H bond thiolation of 1,3,4-oxadiazoles and related heteroarenes.

A convenient transition metal-free procedure for the direct thiolation of 1,3,4-oxadiazole C-H bonds using diaryl disulfides has been developed. Other substrates including indole, benzothiazole, N-phenylbenzimidazole, and caffeine were also thiolated in this manner, providing the corresponding products in good to excellent yields.

Preparation of diastereomerically pure dilignol model compounds.

A gram-scale synthetic access to diastereomerically pure dilignol ß-O-4 type model compounds, which represent valuable candidates for studies of lignin cleavage and valorization, is described. Following a straightforward procedure both diastereoisomers of 1,3-dilignols can be prepared. In the key-step, tert-butyl aryloxy esters are used as enolate precursors for additions on aldehydes. After separation, the resulting erythro and threo ß-hydroxy esters are independently reduced to afford the target compounds in high yields.