1,057 research outputs found

    Solid-phase synthesis of recyclable diphosphine ligands

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    The authors thank the European Union (Marie Curie ITN SusPhos, Grant Agreement No. 317404 and COST action PhoSciNet cm08602) for financial support.An efficient solid-phase synthetic approach towards diphosphine ligands is demonstrated. This modular method offers facile access to this important class of ligands, in quantitative yield, providing huge potential for ligand fine-tuning. These supported ligands can be efficiently applied in asymmetric catalysis. Moreover, the immobilized catalysts can successfully be recycled multiple times addressing several synthetic and work-up challenges in the field of catalytic chemistry.PostprintPostprintPeer reviewe

    Catalytic Activity and Fluxional Behavior of Complexes Based on RuHCl(CO)(PPh<sub>3</sub>)<sub>3</sub> and Xantphos-Type Ligands

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    With RuHCl(CO)(PPh3)3 as the starting material, the complexes RuHCl(CO)(PPh3)(L) were prepared for L = Xantphos and closely related ligands. Their catalytic activity in the direct amination of cyclohexanol showed large differences depending on the different backbone structures. In those complexes the Xantphos-type ligand backbones are slightly bent and display fluxionality, studied by VT-NMR. This was assigned to the "flipping" of the backbone via the bridging atoms in the xanthene backbone. Via line shape analysis of the peaks, the Gibbs free energy of activation of the flipping movement was found to be around 56 kJ/mol in all cases. However, the activation enthalpy and entropy differed considerably. Employing RuCl2(PPh3)3 as the precursor resulted in the trans-coordinated complexes RuCl2(PPh3)(L) for L = Xantphos, Sixantphos. Fluxionality was no longer observed, due to the fact that in these complexes the O atom in the backbone also coordinates to the Ru

    Electronic and bite angle effects in catalytic C-O bond cleavage of a lignin model compound using ruthenium xantphos complexes

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    The authors would like to thank the EPSRC (Global Engagement grant EP/K00445X/1 and critical mass grant EP/J018139/1) and the European Union (Marie Curie ITN ‘SuBiCat’ PITN-GA-2013-607044) for financial support. NMSF-Swansea and Mr. Stephen Boyer are kindly acknowledged for mass spectrometry and elemental analysis, respectively.Bite angle and electronic effects on the ruthenium-diphosphine catalysed ether bond cleavage of the lignin β-O-4 model compound 2-phenoxy-1-phenethanol were tested. Enhanced conversion of the substrate was observed with increasing σ-donor capacity of the ligands. Kinetic and thermodynamic data suggest oxidative addition of the dehydrogenated model compound to the diphosphine Ru(0) complex to be rate-limiting.PostprintPeer reviewe

    Artificial Metalloenzymes as Catalysts for Oxidative Lignin Degradation

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    We report novel artificial metalloenzymes (ArMs), containing tris­(pyridylmethyl)­amine (TPA), for the atom economic oxidation of lignin β-O-4 model compounds, using hydrogen peroxide. The protein scaffold alters the selectivity of the reaction from a low yielding cleavage reaction when using the parent Fe-tpa complex to a high yielding benzylic alcohol oxidation when using the complex incorporated into a protein scaffold, SCP-2L A100C. Engineering the protein scaffold to incorporate glutamic acid was found to improve the ArM activity, showing that rational design of the protein environment using metal binding amino acids can be a first step toward improving the overall activity of an artificial metalloenzyme

    Organocatalytic chemoselective primary alcohol oxidation and subsequent cleavage of lignin model compounds and lignin

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    This research was supported by the European Union (Marie Curie ITN “SuBiCat” PITN-GA-2013-607044, S.D.) and by the Distinguished Professorship Program at RWTH Aachen University funded by the Excellence Initiative of the German federal and state governments.A one-pot two-step degradation of lignin β-O-4 model compounds initiated by preferred oxidation of the primary over the secondary hydroxyl groups with a TEMPO/DAIB system has been developed [TEMPO=2,2,6,6-tetramethylpiperidine-N-oxyl, DAIB=(diacetoxy)iodobenzene]. The oxidised products are then cleaved by proline-catalysed retro-aldol reactions. This degradation methodology produces simple aromatics in good yields from lignin model compounds at room temperature with an extension to organosolv beech-wood lignin ( L1 ) resulting in known cleavage products.PostprintPostprintPeer reviewe

    Metal triflates for the production of aromatics from lignin

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    This work was funded by the European Union (Marie Curie ITN ‘SuBiCat’ PITN-GA-2013-607044, PJD, CWL, NJW, PCKL, KB, JGdeV) as well as EP/J018139/1, EP/K00445X/1 grants (NJW and PCJK) and an EPSRC Doctoral Prize Fellowship (CSL).The depolymerization of lignin into valuable aromatic chemicals is one of the key goals towards establishing economically viable biorefineries. In this contribution we present a simple approach for converting lignin to aromatic monomers in high yields, under mild reaction conditions. The methodology relies on the use of catalytic amounts of easy to handle metal triflates (M(OTf)x). Initially, we evaluated the reactivity of a broad range of metal triflates using simple lignin model compounds. More advanced lignin model compounds were also used to study the reactivity of different lignin linkages. The product aromatic monomers were either phenolic C2-acetals obtained by stabilization of the aldehyde cleavage products by reaction with ethylene glycol, or methyl aromatics obtained by catalytic decarbonylation. Notably, when the former method was ultimately tested on lignin, especially Fe(OTf)3 proved very effective and the phenolic C2-acetal products were obtained in an excellent, 19.3±3.2 wt % yield.PostprintPeer reviewe

    Revealing the fate of the phenylcoumaran linkage during lignin oxidation reactions

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    This work was funded by the EP/J018139/1, EP/K00445X/1 grants (N.J.W. and P.C.J.K.), an EPSRC Doctoral Prize Fellowship (C.S.L.), and the European Union (Marie Curie ITN “SuBiCat” PITN-GA-2013-607044, C.W.L., N.J.W., P.C.J.K.).The fate of most lignin linkages, other than the β-O-4, under selective oxidation conditions is largely unknown. In this work we use advanced β-5 lignin model compounds to identify the fate of phenylcoumaran units in a softwood lignin during oxidation with DDQ. By using model compounds combined with detailed characterisation of the oxidised lignin polymer using HSQC and HMBC NMR we show that phenylcoumarones are a major product, and therefore constitute a novel non-native β-5 linkage in oxidised lignins. Additionally, the reactivity of these units in lignin led us to further investigate their connectivity in lignin, showing that they are found as both phenolic and etherified units. The findings and approach developed here will help improve the efficiency of selective oxidative lignin depolymerisation processes, particularly those aimed at the upgrading of softwood lignin in which phenylcoumarans are a major linkage.PostprintPeer reviewe
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