Selectivity Control of C-O Bond Cleavage for Catalytic Biomass Valorization

Increasing fossil fuels consumption and global warming have driven the global revolution towards renewable energy sources. Lignocellulosic biomass is the main source of renewable carbon-based fuels. The abundant intermolecular linkages and high oxygen content between cellulose, hemicellulose, and lignin limit the use of traditional fuels. Therefore, it is a promising strategy to break the above linkages and remove oxygen by selective catalytic cracking of C–O bond to further transform the main components of biomass into small molecular products. This mini-review discusses the significance of selectivity control in C–O bond cleavage with well-tailored catalytic systems or strategies for furnishing biofuels and value-added chemicals of high efficiency from lignocellulosic biomass. The current challenges and future opportunities of converting lignocellulose biomass into high-value chemicals are also summarized and analyzed.

Organogel Assisted Porous Organic Polymer Embedding Cu Nanoparticles for Selectivity Control Semi Hydrogenation of Alkynes

Heteroatom-rich porous-organic-polymers (POPs) comprising of highly cross-linked robust skeletons with high physical and thermal stability, high surface area, and tunable pore size distribution have garnered significant research interests for their...

NHCs and Visible Light-mediated Photoredox Co-catalyzed Radical 1,2-Dicarbonylation of Alkenes for 1,4-Diketones

Alkenes are ubiquitous, and radical difunctionalization of alkenes represents one of the most practical approaches to constructing value-added compounds. Dicarbonylation of alkenes provides direct access to value-added 1,4-dicarbonyl compounds. However, selectivity control for unsymmetric 1,2-dicarbonylation is an unclosed challenge. We herein describe NHCs and photocatalysis co-catalyzed three competent radical 1,2-dicarbonylation of alkenes by distinguishing two carbonyl groups, providing structurally diversified 1,4-diketones. Mechanistic studies indicated that NHCs-stabilized ketyl-type radicals originate from aroyl fluorides via oxidative quenching process of excited photocatalysis, and acyl radicals are generated from single-electron-oxidation of α-keto acids. Distinct properties of acyl radical and NHCs-stabilized ketyl radical contributed to selectivity control. Transient acyl radicals are rapidly added to alkenes delivering alkyl radicals, which undergo subsequent radical-radical cross-coupling with ketyl-type radicals, affording 1,2-dicarbonylation products. This transformation features mild reaction conditions, broad substruct scope, and excellent selectivity, providing a general and practical approach for the dicarbonylation of olefins.

Selectivity descriptors for the direct hydrogenation of CO2 to hydrocarbons during zeolite-mediated bifunctional catalysis

Cascade processes are gaining momentum in heterogeneous catalysis. The combination of several catalytic solids within one reactor has shown great promise for the one-step valorization of C1-feedstocks. The combination of metal-based catalysts and zeolites in the gas phase hydrogenation of CO2 leads to a large degree of product selectivity control, defined mainly by zeolites. However, a great deal of mechanistic understanding remains unclear: metal-based catalysts usually lead to complex product compositions that may result in unexpected zeolite reactivity. Here we present an in-depth multivariate analysis of the chemistry involved in eight different zeolite topologies when combined with a highly active Fe-based catalyst in the hydrogenation of CO2 to olefins, aromatics, and paraffins. Solid-state NMR spectroscopy and computational analysis demonstrate that the hybrid nature of the active zeolite catalyst and its preferred CO2-derived reaction intermediates (CO/ester/ketone/hydrocarbons, i.e., inorganic-organic supramolecular reactive centers), along with 10 MR-zeolite topology, act as descriptors governing the ultimate product selectivity.

Slug flow as tool for selectivity control in the homogeneously catalysed solvent-free epoxidation of methyl oleate

Catalytic oxidation of sustainable raw materials like unsaturated fats and oils, or fatty acids and their esters, lead to biobased, high-value products. Starting from technical grade methyl oleate, hydrogen peroxide as a green oxidant produces only water as by-product. A commercially available, cheap water-soluble tungsten catalyst is combined with Aliquat® 336 as a phase-transfer agent in solvent-free reaction conditions. In this study, we first report the transfer of this well-known batch system into continuous mode. The space–time yield is improved from 0.08 kg/L.h in batch to 1.29 kg/L.h in flow mode. The improved mass transfer and reduced back mixing of the biphasic liquid–liquid slug flow allows for selectivity control depending on physical parameters of slug flow namely volumetric phase ratio, volumetric flow rate, and slug length. Even though the product, methyl 9,10-epoxystearate is obtained at a maximum selectivity of only 58% in flow mode, higher space time yield combined with possible reactant recycling in flow mode offers a promising avenue of research. This work analyses the use of slug flow parameters as tools for controlling selectivity towards oxidation products of methyl oleate.

Organocatalytic enantioselective dearomatization of thiophenes by 1,10-conjugate addition of indole imine methides

Catalytic asymmetric dearomatization (CADA) is a powerful tool for the rapid construction of diverse chiral cyclic molecules from cheap and easily available arenes. This work reports an organocatalytic enantioselective dearomatization of substituted thiophenes in the context of a rare remote asymmetric 1,10-conjugate addition. By suitable stabilization of the thiophenyl carbocation with an indole motif in the form of indole imine methide, excellent remote chemo-, regio-, and stereocontrol in the nucleophilic addition can be achieved with chiral phosphoric acid catalysis under mild conditions. This protocol can be successfully extended to the asymmetric dearomatization of other heteroarenes including selenophenes and furans. Control experiments and DFT calculations demonstrate a possible pathway in which hydrogen bonding plays an important role in selectivity control.