- One-Pot Transformation of Lignin and Lignin Model Compounds into Benzimidazoles
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It is a challenging task to simultaneously achieve selective depolymerization and valorization of lignin due to their complex structure and relatively stable bonds. We herein report an efficient depolymerization strategy that employs 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as oxidant/catalyst to selectively convert different oxidized lignin models to a wide variety of 2-phenylbenzimidazole-based compounds in up to 94 % yields, by reacting with o-phenylenediamines with varied substituents. This method could take full advantage of both Cβ and/or Cγ atom in lignin structure to furnish the desirable products instead of forming byproducts, thus exhibiting high atom economy. Furthermore, this strategy can effectively transform both the oxidized hardwood (birch) and softwood (pine) lignin into the corresponding degradation products in up to 45 wt% and 30 wt%, respectively. Through a “one-pot” process, we have successfully realized the oxidation/depolymerization/valorization of natural birch lignin at the same time and produced the benzimidazole derivatives in up to 67 wt% total yields.
- Guo, Tao,He, Jianghua,Liu, Tianwei,Zhang, Yuetao
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- A multicentre synergistic polyoxometalate-based metal-organic framework for one-step selective oxidative cleavage of β-: O -4 lignin model compounds
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A novel mixed-valence polyoxovanadate-based copper-organic framework, [CuI(bbi)]2{[CuI(bbi)]2VIV2VV8O26}·2H2O (NENU-MV-5, bbi = 1,1′-(1,4-butanediyl)bis(imidazole)), was facilely synthesized from routine reagents under mild hydrothermal conditions. Using NENU-MV-5 as a heterogeneous catalyst without any co-catalyst, one-step oxidative cleavage of β-O-4 lignin into phenols and aromatic acids with high catalytic activity and selectivity was realized under an oxygen atmosphere. No obvious decrease in activity was observed after five cycles, which indicates the excellent stability and sustainability of NENU-MV-5. The perfect catalytic performance of NENU-MV-5 can be attributed to the multi-site synergistic effect of the mixed-valence VV-O-VIV sites on polyoxovanadate for the oxidation of β-O-4 alcohol to β-O-4 ketone and the Cu(i) sites on the framework for the rapid cleavage of the Cα-Cβ bond of β-O-4 ketone. This system represented the first co-catalyst-free example for the one-step selective degradation of lignin catalyzed by a well-defined crystalline catalyst with definite composition and structure in a single solvent.
- Tian, Hong-Rui,Liu, Yi-Wei,Zhang, Zhong,Liu, Shu-Mei,Dang, Tian-Yi,Li, Xiao-Hui,Sun, Xiu-Wei,Lu, Ying,Liu, Shu-Xia
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supporting information
p. 248 - 255
(2020/01/13)
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- Visible-light-induced C-C bond cleavage of lignin model compounds with cyanobenziodoxolone
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The catalytic degradation of lignin to value-added chemicals has received considerable attention over the past decade. Photocatalysis provides promising approaches to enable previously inaccessible transformations. However, examples of the visible-light promoted degradation of lignin are still limited. In this work, the visible-light-induced selective C-C bond cleavage of β-O-4 lignin model compounds has been disclosed via β-scission of in situ generated alkoxy radical intermediates. With cyanobenziodoxolone as the oxidant, a variety of substrates could be transformed into aldehydes in moderate to good yields. In addition, unexpected acetal esters which could conveniently furnish formaldehyde and phenols by alcoholysis were observed.
- Zheng, Ming,Huang, Yan,Zhan, Le-Wu,Hou, Jing,Li, Bin-Dong
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supporting information
(2020/10/02)
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- Cleavage of lignin model compounds and ligninox using aqueous oxalic acid
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Aqueous oxalic acid cleaves oxidised β-O-4 lignin model compounds by two distinct mechanisms that are dependent on the presence of the hydroxymethyl substituent. Various β-O-4 phenoxyacetophenones that do not contain the hydroxymethyl substituent undergo oxidative cleavage upon exposure to aqueous oxalic acid in the presence of air, likely through concerted ring opening of a dioxetane intermediate to give the corresponding benzoic acid and phenyl formate. Importantly, detrimental side reactions arising from singlet oxygen and hydroperoxy radicals (from both O2 and oxalic acid) are minimal when the cleavage is run under air compared to neat oxygen. When oxidised β-O-4 lignin model compounds bearing the hydroxymethyl group are cleaved by aqueous oxalic acid, the resulting diketone and phenol products arise from a redox neutral cleavage that is analogous to the formic acid-sodium formate mediated lignin cleavage process reported by Stahl. Aqueous oxalic acid also cleaves lignin itself, with oxidised milled wood lignin (MWLox) from Pinus radiata giving a 14% yield of ethyl acetate soluble aromatics with good selectivity for vanillin. Aqueous oxalic acid appears to be a promising lignin cleavage system given the benign, bio-based reagents, absence of metals and organic solvents and a simple extraction procedure that enables oxalic acid recycling.
- Lindsay, Ashley C.,Kudo, Shinji,Sperry, Jonathan
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supporting information
p. 7408 - 7415
(2019/08/15)
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- Self-hydrogen transfer hydrogenolysis of β-O-4 linkages in lignin catalyzed by MIL-100(Fe) supported Pd-Ni BMNPs
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A MIL-100(Fe) supported Pd-Ni BMNP catalyst has been fabricated, and the catalyst exhibits superior catalytic performance toward the intramolecular transfer hydrogenolysis of lignin model compounds and organosolv lignin. Alcoholic groups (CαH-OH) of lignin were exploited as the hydrogen source, and selective cleavage of β-O-4 linkages in lignin was realized without an extra hydrogen donor. This protocol was suitable for organosolv lignin as well as model compounds; several phenols and functionalized acetophenones were detected when extracted lignin was treated in our system. The catalyst exhibits outstanding catalytic stability during the reaction process, which can be ascribed to the porous structure and the strong water stability of MIL-100(Fe). The excellent catalytic performance of Pd1Ni4/MIL-100(Fe) highlights the "synergistic effect" between the BMNPs and the functional synergy between MNPs and MOFs, and our work shows the bright future of BMNPs and MOFs in the development of catalysts for sustainable chemistry.
- Zhang, Jia-Wei,Lu, Guo-Ping,Cai, Chun
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p. 4538 - 4543
(2017/10/13)
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- New protocol of copper-catalyzed oxidative C(CO)[sbnd]C bond cleavage of aryl and aliphatic ketones to organic acids using O2 as the terminal oxidant
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Catalytic oxidation of C[sbnd]C bond is a key technology to transform petroleum-based as well as sustainable biomass feedstock into more valuable oxygenates. We herein describe a convenient and useful oxidation strategy of converting ketones into carboxylic acids using homogeneous copper catalyst without additives and with O2as the terminal oxidant. A wide range of aryl and aliphatic ketones as well as β–O–4 lignin models were selectively oxidized to acids via C[sbnd]C bond cleavage. Mechanism studies by EPR and in situ NMR elucidate the principles of Cu/O2reactivity that involves C[sbnd]H bond and O2activation via a peroxide species. This provides an important foundation for expanding the scope of useful aerobic oxidation reactions using copper catalysts.
- Liu, Huifang,Wang, Min,Li, Hongji,Luo, Nengchao,Xu, Shutao,Wang, Feng
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p. 170 - 179
(2017/01/22)
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- Fine Tuning the Redox Potentials of Carbazolic Porous Organic Frameworks for Visible-Light Photoredox Catalytic Degradation of Lignin β-O-4 Models
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We report a facile approach to fine tune the redox potentials of π-conjugated porous organic frameworks (POFs) by copolymerizing carbazolic electron donor (D) and electron acceptor (A) based comonomers at different ratios. The resulting carbazolic copolymers (CzCPs) exhibit a wide range of redox potentials that are comparable to common transition-metal complexes and are used in the stepwise photocatalytic degradation of lignin β-O-4 models. With the strongest oxidative capability, CzCP100 (D:A = 0:100) exhibits the highest efficiency for the oxidation of benzylic β-O-4 alcohols, while the highly reductive CzCP33 (D:A = 66:33) gives the highest yield for the reductive cleavage of β-O-4 ketones. CzCPs also exhibit excellent stability and recyclability and represent a class of promising heterogeneous photocatalysts for the production of fine chemicals from sustainable lignocellulosic biomass.
- Luo, Jian,Zhang, Xiang,Lu, Jingzhi,Zhang, Jian
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p. 5062 - 5070
(2017/08/17)
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- Promoting Lignin Depolymerization and Restraining the Condensation via an Oxidation-Hydrogenation Strategy
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For lignin valorization, simultaneously achieving the efficient cleavage of ether bonds and restraining the condensation of the formed fragments represents a challenge thus far. Herein, we report a two-step oxidation-hydrogenation strategy to achieve this goal. In the oxidation step, the O2/NaNO2/DDQ/NHPI system selectively oxidizes CαH-OH to Cα=O within the β-O-4 structure. In the subsequent hydrogenation step, the α-O-4 and the preoxidized β-O-4 structures are further hydrogenated over a NiMo sulfide catalyst, leading to the cleavage of Cβ-OPh and Cα-OPh bonds. Besides the transformation of lignin model compounds, the yield of phenolic monomers from birch wood is up to 32% by using this two-step strategy. The preoxidation of CαH-OH to Cα=O not only weakens the Cβ-OPh ether bond but also avoids the condensation reactions caused by the presence of Cα+ from dehydroxylation of CαH-OH. Furthermore, the NiMo sulfide prefers to catalyze the hydrogenative cleavage of the Cβ-OPh bond connecting with a Cα=O rather than catalyze the hydrogenation of Cα=O back to the original CαH-OH, which further ensures and utilizes the advantages of preoxidation.
- Zhang, Chaofeng,Li, Hongji,Lu, Jianmin,Zhang, Xiaochen,Macarthur, Katherine E.,Heggen, Marc,Wang, Feng
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p. 3419 - 3429
(2017/06/09)
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- Facile and selective hydrogenolysis of β-O-4 linkages in lignin catalyzed by Pd-Ni bimetallic nanoparticles supported on ZrO2
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The β-O-4 linkage in lignin can be selectively cleaved by Pd-Ni bimetallic nanoparticles supported on ZrO2 using hydrogen gas as the hydrogen donor under ambient pressure and neutral conditions. Conspicuous enhancement in activity is observed compared with single nickel and palladium catalysts based on the results of experiments and characterization. Moreover, hydrogenation of the produced phenols is tuned by adjusting the amount of NaBH4. The catalyst can be reused over ten times in the model reaction and over five times in the hydrogenolysis of lignin without an obvious change in activity and selectivity.
- Zhang, Jia-Wei,Cai, Yao,Lu, Guo-Ping,Cai, Chun
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p. 6229 - 6235
(2016/12/03)
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- Aerobic Oxidation of Olefins and Lignin Model Compounds Using Photogenerated Phthalimide-N-oxyl Radical
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A metal-free protocol to generate phthalimide-N-oxyl (PINO) radicals from N-hydroxyphthalimide (NHPI) via a photoinduced proton-coupled electron transfer process is reported. Using donor-substituted aromatic ketones, such as 4,4′-bis(diphenylamino)benzophenone (DPA-BP), PINO radicals are efficiently produced and subsequently utilized to functionalize olefins to afford a new class of alkyl hydroperoxides. The DPA-BP/NHPI/O2 photocatalytic system exhibits high efficiency toward the aerobic oxidation of β-O-4 lignin models.
- Luo, Jian,Zhang, Jian
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p. 9131 - 9137
(2016/10/17)
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- Two-Step, Catalytic C-C Bond Oxidative Cleavage Process Converts Lignin Models and Extracts to Aromatic Acids
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We herein report a two-step strategy for oxidative cleavage of lignin C-C bond to aromatic acids and phenols with molecular oxygen as oxidant. In the first step, lignin β-O-4 alcohol was oxidized to β-O-4 ketone over a VOSO4/TEMPO [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl)] catalyst. In the second step, the C-C bond of β-O-4 linkages was selectively cleaved to acids and phenols by oxidation over a Cu/1,10-phenanthroline catalyst. Computational investigations suggested a copper-oxo-bridged dimer was the catalytically active site for hydrogen-abstraction from Cβ-H bond, which was the rate-determining step for the C-C bond cleavage.
- Wang, Min,Lu, Jianmin,Zhang, Xiaochen,Li, Lihua,Li, Hongji,Luo, Nengchao,Wang, Feng
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p. 6086 - 6090
(2016/09/09)
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- The Photochemistry of α-Phenoxyacetophenones Investigated by Flash-CIDNP-spectroscopy
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The photochemistry of 14 different substituted α-phenoxyacetophenones (1) was studied by CIDNP-spectroscopy using XeCl excimer laser (308 nm) and a 250 MHz 1H-NMR spectrometer.To characterize the excited states the electronic absorption and emission spectra as well disappearance quantum yields were investigated also. - Two reaction channels were established: 1) β-cleavage from the singlet state; and 2) the formation of acetophenones via the respective enol forms where the origin of the polarization remains unclear.It is concluded that these are the main reaction channels, since all products formed show polarizations in the CIDNP-spectr a. - In contrast to the current assumptions for the typical acetophenone reaction of (1) leading to the β-cleavage, we are led to interprete the CIDNP-spectra of (1) as a result of a reaction due to the phenoxy part with all characteristics of a photo-Claisen reaction. Keywords: Luminescence / Photochemistry / Radicals / Spectroscopy, CIDNP / Spectroscopy, Nuclear Magnetic Resonance / Spectroscopy, Ultraviolet
- Palm, W.-U.,Dreeskamp, H.,Bouas-Laurent, H.,Castellan, A.
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