77891-30-6Relevant articles and documents
Selective photocatalytic C-C bond cleavage under ambient conditions with earth abundant vanadium complexes
Gazi, Sarifuddin,Hung Ng, Wilson Kwok,Ganguly, Rakesh,Putra Moeljadi, Adhitya Mangala,Hirao, Hajime,Soo, Han Sen
, p. 7130 - 7142 (2015/11/24)
Selective C-C bond cleavage under ambient conditions is a challenging chemical transformation that can be a valuable tool for organic syntheses and macromolecular disassembly. Herein, we show that base metal vanadium photocatalysts can harvest visible light to effect the chemoselective C-C bond cleavage of lignin model compounds under ambient conditions. Lignin, a major aromatic constituent of non-food biomass, is an inexpensive, accessible source of fine chemical feedstocks such as phenols and aryl ethers. However, existing lignin degradation technologies are harsh and indiscriminately degrade valuable functional groups to produce intractable mixtures. The selective, photocatalytic depolymerization of lignin remains underexplored. In the course of our studies on lignin model compounds, we have uncovered a new C-C activation reaction that takes place under exceptionally mild conditions with high conversions. We present our fundamental studies on representative lignin model compounds, with the aim of expanding and generalizing the substrate scope in the future. Visible light is employed in the presence of earth-abundant vanadium oxo catalysts under ambient conditions. Selective C-C bond cleavage leads to valuable and functionally rich fine chemicals such as substituted aryl aldehydes and formates. Isotope labeling experiments, product analyses, and intermediate radical trapping, together with density functional theory studies, suggest a unique pathway that involves a photogenerated T1 state during the C-C bond cleavage reactions. Our study demonstrates a sustainable approach to harvest sunlight for an unusual, selective bond activation, which can potentially be applied in organic transformations and biomass valorization.
Alkoxyl- and carbon-centered radicals as primary agents for degrading non-phenolic lignin-substructure model compounds
Ohashi, Yasunori,Uno, Yukiko,Amirta, Rudianto,Watanebe, Takahito,Honda, Yoichi,Watanabe, Takashi
experimental part, p. 2481 - 2491 (2011/05/14)
Lignin degradation by white-rot fungi proceeds via free radical reaction catalyzed by oxidative enzymes and metabolites. Basidiomycetes called selective white-rot fungi degrade both phenolic and non-phenolic lignin substructures without penetration of extracellular enzymes into the cell wall. Extracellular lipid peroxidation has been proposed as a possible ligninolytic mechanism, and radical species degrading the recalcitrant non-phenolic lignin substructures have been discussed. Reactions between the non-phenolic lignin model compounds and radicals produced from azo compounds in air have previously been analysed, and peroxyl radical (PR) is postulated to be responsible for lignin degradation (Kapich et al., FEBS Lett., 1999, 461, 115-119). However, because the thermolysis of azo compounds in air generates both a carbon-centred radical (CR) and a peroxyl radical (PR), we re-examined the reactivity of the three radicals alkoxyl radical (AR), CR and PR towards non-phenolic monomeric and dimeric lignin model compounds. The dimeric lignin model compound is degraded by CR produced by reaction of 2,2′-azobis(2-amidinopropane) dihydrochloride (AAPH), which under N2 atmosphere cleaves the α-β bond in 1-(4-ethoxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-1,3-propanediol to yield 4-ethoxy-3-methoxybenzaldehyde. However, it is not degraded by the PR produced by reaction of Ce4+/tert-BuOOH. In addition, it is degraded by AR produced by reaction of Ti3+/tert-BuOOH. PR and AR are generated in the presence and absence of veratryl alcohol, respectively. Rapid-flow ESR analysis of the radical species demonstrates that AR but not PR reacts with the lignin model compound. Thus, AR and CR are primary agents for the degradation of non-phenolic lignin substructures.
The immobilized porphyrin-mediator system Mn(TMePyP)/clay/HBT (clay-PMS): A lignin peroxidase biomimetic catalyst in the oxidation of lignin and lignin model compounds
Crestini, Claudia,Pastorini, Alessandra,Tagliatesta, Pietro
, p. 4477 - 4483 (2007/10/03)
A biomimetic system for lignin peroxidase (LiP) was designed by using a cationic porphyrin, [Mn(TMePyP)OAc5], supported on the smectitic clay montmorillonite [Mn(TMe-PyP)/clay]. The natural role of the polypeptidic pocket of LiP was mimicked by the clay. The possibility to use low-molecular-weight redox mediators as active readily diffusible oxidizing species has been investigated. This assembly - a sort of "synthetic enzyme" - can be defined as an immobilized porphyrin-mediator system (clay-PMS). The clay-PMS was found to be a stable, recyclable, and efficient catalyst for the environmentally friendly H2O2-catalyzed oxidation of different lignins and representative lignin model compounds. The clay-PMS showed a higher reactivity than Mn(TMePyP)/clay alone due to an effective role of the redox mediator on the oxidation. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.
