A Characteristic Reaction of Lignin in Ionic Liquids; Glycelol Type Enol-Ether as the Primary Decomposition Product of β-O-4 Model Compound

Abstract

Guaiacylglycerol-β-guaiacyl ether (GG), which contains a predominant inter-unit linkage of lignin, could be converted into a corresponding glycerol type enol-ether (EE), 3-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-2-propenol, by the heat treatment in ionic liquids. EE is believed to be the unstable intermediate of the lignin decomposition process under acidic and alkaline conditions. By contrast, EE could be isolated as a relatively stable compound from the reaction mixture of ionic liquids. EE was formed as a primary reaction product in all ionic liquids used in this research under the temperature conditions of 120°C, although the decomposition rate and secondary decomposition products of GG varied with the ionic liquid used. NMR data suggested that dehydration reaction of GG progressed stereospecifically and [Z] isomer was predominantly formed (stereoselectivety of [Z] is higher than 90%).     

Effects of Cα -Oxidation in the Fungal Metabolism of Lignin

C_α-Oxidation (benzyl alcohol oxidation) is a prominent reaction in the degradation of lignin by white-rot fungi. This study showed that such oxidation markedly retards metabolism of a nonphenolic β-O-4 model compound, 1-(3-methoxy-4-ethoxyphenyl)-2-(o-methoxyphenoxy)propane-1,3-diol, by cultures of Phanerochaete chrysosporium Burds. Surprisingly, however, selective chemical C_α -oxidation of spruce lignins enhanced their depolymerization by the cultures. Thus the decrease in intrinsic degradability of substructures is more than compensated by another effect of C_α-oxidation in lignin. One possibility is that the oxidation increases the accessibility of the lignin to enzymes by decreasing its steric complexity. This study also revealed that the β-O-4 model, like lignin in wood, is degraded in part via C_α-oxidation by P. chrysosporium. Reduction of the α-carbonyl groups thus formed does not occur. Addition of L-glutamate to ligninolytic cultures completely suppresses their competence to degrade the model compound, as it does their ability to oxidize lignin to CO₂. This result strengthens past evidence indicating that substructure models are metabolized by the same enzyme system as lignin.     

Revisiting the Mechanism of β-O-4 Bond Cleavage During Acidolysis of Lignin. Part 3: Search for the Rate-Determining Step of a Non-Phenolic C6-C3 Type Model Compound

Abstract

The rate-determining step of a C₆-C₃ dimeric non-phenolic β-O-4 type lignin model compound, 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)propane-1,3-diol (veratrylglycerol-β-guaiacyl ether, VG), was evaluated under acidolysis conditions (0.2 mol/l HBr in 82% aqueous 1,4-dioxane at 85°C) by comparing the disappearances between VG and the corresponding compound labeled at the β-position of VG, 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)(2–²H)propane-1,3-diol. The disappearance of VG occurred more rapidly than that of the latter compound, and a primary kinetic isotope effect was clearly observed. This result indicates that the C-H bond at the β-position of VG is broken in the rate-determining step. Two possible mechanisms are presented as the rate-determining step: (1) A base abstracts the β-proton of a benzyl cation-type intermediate produced from VG affording an enol ether compound, 2-(2-methoxyphenoxy)-3-(3,4-dimethoxyphenyl)prop-2-en-1-ol; (2) The hydride transfers from the β- to the α-position of the benzyl cation. It was confirmed that both mechanisms certainly exist and that the latter seems to contribute more than has generally been considered.     

Revisiting the Mechanism of β-O-4 Bond Cleavage During Acidolysis of Lignin. Part 6: A Review

ABSTRACT

This paper reviews our results on the acidolysis of dimeric non-phenolic β-O-4-type lignin model compounds in aqueous 82% 1,4-dioxane containing an acid at 85°C. It was shown that the mechanism of a C₆-C₂-type model compound, 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)ethanol IX, is fairly different from that of a C₆-C₃ analogue, 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)propane-1,3-diol XX, suggesting the significance of the presence of the γ-hydroxymethyl group. It was confirmed that the hydride transfer mechanism exists as a reaction route of the benzyl-cation-type intermediates derived from both compounds, and the contribution of this mechanism is greater than expected in the acidolysis of compound XX. An enol ether compound, 2-(2-methoxyphenoxy)-3-(3,4-dimethoxyphenyl)prop-2-en-1-ol, was first detected in the acidolysis of compound XX using the DBr/D₂O/1,4-dioxane system. It was confirmed in the acidolysis of compound XX using HBr that the mechanism is different from that in the system using either HCl or H₂SO₄ and an unknown mechanism contributes to the reaction. This unknown mechanism surprisingly contributed more with decreasing concentration of HBr or Br^?, provided that Br^? still existed in the system.     

Formation of Carbon-Linked Anthrone-Lignin and Anthrahydroquinone-Lignin Adducts

The quinone methide from guaiacylglycol-β-guaiacyl ether [l-(3-methoxy-4-hydroxyphenyl)-2-(2-methoxyphenoxy)ethanol] formed carbon-carbon bonded adducts with both anthrone and anthrahydroquinone (AHQ). It was found that the anthrone adduct [l-(3-methoxy-4-hydroxyphenyl)-1-(anthracen-9-one-10-yl)-2-(2-methoxyphenoxy)ethane] was much more resistant to alkaline degradation than the corresponding AHQ-adduct Jl-(3-methoxy-4-hydroxyphenyl)-l-(10-hydroxyanthracen-9-one-10-yl)-2-(2-methoxyphenoxy)ethane]. It was demonstrated, by NMR spectroscopy that analogous anthrone and AHQ adducts formed with milled wood lignin and had relative alkaline stabilities consistent with those observed with the model adducts. At 10° C the AHQ-lignin adduct partially decomposed to the Y-monoacetates of coniferyl alcohol and p_-coumaryl alcohol which subsequently reacted with excess AHQ to give the novel adducts, trans-1-(3-methoxy-4-hydroxyphenyl)-3-(10-hydroxyanthracen-9-one-10-yl)propene, and trans-1-(4-hydroxyphenyl)-3-(10-hydroxyanthracen-9-one-10-yl) propene, respectively.     

Revisiting the Mechanism of β-O-4 Bond Cleavage during Acidolysis of Lignin. Part 2: Detailed Reaction Mechanism of a Non-Phenolic C6-C2 Type Model Compound

Abstract

The detailed reaction mechanism of a C₆-C₂ dimeric non-phenolic β-O-4 type lignin model compound, 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)ethanol (V′G), was examined under acidolysis conditions (mainly 0.2 mol/l HBr in 82% aqueous 1,4-dioxane at 85°C), and was suggested to be as follows. The initial elementary reaction step is protonation of the α-hydroxyl group, followed by the release of water to afford a benzyl cation intermediate (BC′). The latter step is relatively slow but reversible. The β-proton abstraction from BC′ by the solvents affords an enol ether compound, 1-(2-methoxyphenoxy)-2-(3,4-dimethoxyphenyl)ethene (EE′). This step is practically irreversible, and is the rate-determining step in the disappearance of V′G. The stereoisomers of EE′ are rapidly converted into each other, accompanied by protonation of the double bond. Complete protonation affords a β-oxymethylene cation intermediate (OMC′), which is also formed via hydride transfer from the β- to α-position of BC′ as a minor route. OMC′ preferentially undergoes the addition of water at the β-position, and the consequent β-O-4 bond cleavage affords 2-methoxyphenol and a Hibbert's monomer, 3,4-dimethoxyphenylacetaldehyde.     

Effect of Counter Cation on Alkaline Reaction of β-O-4-Type Substructure in Lignin

This study aimed to clarify the effects of counter cations on the alkaline-induced β-O-4 bond cleavage and further reactions of β-O-4-type substructures in lignin. For this purpose, a non-phenolic β-O-4-type lignin model compound, the erythro isomer of 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)propane-1,3-diol (veratrylglycerol-β-guaiacyl ether), was treated in a 100% water solution or an aqueous methanol, ethanol, or 1,4-dioxane solution containing LiOH, NaOH, or CsOH as an alkaline source at 150?°C. The rates of β-O-4 bond cleavage were in the order of CsOH?>?NaOH?>?LiOH in all solvents. This order can rationally be attributed to the strength of the interactions between HO^– and the counter cations. Because Cs⁺ has the lowest positive charge density among the counter cations and hence interacts with HO^– most weakly, HO^– can exert its reactivity most actively in the reactions using CsOH. We also discuss how the counter cations affect the profile of reaction products.     

Decomposition of 1,2‐dichloroethane in an RF plasma environment

A radio frequency (RF) plasma system has been used to decompose 1,2‐dichloroethane (DCE). Final products were identified by a Fourier transform infrared (FTIR) spectrometer. The main products of DCE decomposition in O₂/Ar plasma were CO₂, CO, and HCl. Other minor chlorinated products were CCl₄, C₂HCl₃, C₂H₃Cl, C₂Cl₄, CHCl₃, C₂HCl₅, and COCl₂. Nonchlorinated products were C₂H₂, C₂H₄, C₂H₆, and HCOOH. The plasma reactor with a brass electrode had a higher decomposition fraction of DCE [η, (C_in ? C_out)/C_in × 100%] than that obtained with other materials (Au, Ni, and Cr). Different electrode configurations (inner and outer) were also evaluated for the decomposition of DCE. Argon plus oxygen was found to be the most suitable carrier/auxiliary gas for DCE decomposition. In addition, operational parameters for DCE decomposition in RF plasma including concentration, operational pressure, and total gas flow rate were evaluated. Copyright © 2003 Society of Chemical Industry     

Characteristics of Dioxane Lignins Isolated at Different Ages of Nalita Wood (Trema orientalis)

Abstract

Nalita (Trema orientalis) is one of the fastest growing trees in the tropical countries. The structural characteristics of lignin isolated at different ages of Nalita wood (Trema orientalis) by acidolytic dioxane method were examined by UV, FTIR, ¹H‐NMR and ¹³C‐NMR spectroscopy, alkaline nitrobenzene oxidation, molecular weight determination, elemental and methoxyl analysis. The data were compared with aspen lignin. The structural analysis revealed that Nalita wood lignin is syringyl‐guaiacyl type. The methoxyl content in Nalita wood lignin was lower than aspen lignin. The C₉ formulas for 30‐months‐old Nalita was C₉H_9.31O_3.13(OCH₃)_1.27, whereas that of aspen was C₉H_8.94O_3.15(OCH₃)_1.47. The weight average molecular weight of Nalita wood lignin was decreased from 36,500 to 25,500 with increasing tree age from 12 to 30 months, whereas weight average molecular weight of aspen was 20,000. Both alcoholic and phenolic hydroxyl group in Nalita wood lignin is lower than aspen lignin.     

Mechanochemistry of Syringylglycerol-β-Guaiacyl Ether and Its Similarity to the Conventional Bleaching of Mechanical Pulps

Syringylalcohol (1), α-methyl syringylalcohol (2), 3, 4, 5-trime-thoxyphenyl methylcarbinol (3) were dispersed onto filter paper pulp or linter pulp, and treated, respectively, with a ceramic ball mill (CBM) or a vibration ball mill (VBM-1 or -2) under air for 1h. Mechanical treatment of VBM-2, having the more rigid surface of linter pulp, furnished the p-carbonylphenols (5), (6), and 3, 5-dimethoxy-p-benzoquinone (7). Mechanical treatments of syringylgl-ycerol-β-guaiacyl ether (4) with CBM, VBM, and a laboratory refiner provided α-(2-methoxyphenoxy)-β-hydroxypropiosyringone (8) in the highest yield and a less yield of p-quinone derivative (7) and others as shown in TABLE 2 and FIGURE 3. When treated the resultant mixture with alkaline H₂0₂, the chromophore (III) can be decomposed, but remarkable amounts of the leucochromophre (IV) are produced as shown in FIGURE 4.     

Anthraquinone Losses During Alkaline Pulping

Extensive loss of anthraquinone (AQ) or the active catalyst anthrahydroquinone (AHQ) from the AQ –- AHQ catalytic cycle has been explained in part by side reactions leading to the reaction product anthrone (anthracen-9-one), followed by subsequent formation of adducts with lignin quinone methides. Degradation of an adduct between anthrone and the quinone methide of guaiacylglycerol-β-guaiacyl ether, under soda pulping conditions, resulted in a complex mixture of products. The mixture included 3-guaiacylbenzanthrone, bianthronyl, bianthrone, guaiacol, AQ, trans-coniferyl alcohol, trans-coniferylaldehyde, cis- and trans-1-(3-methoxy-4-hydroxyphenyl)-2-(2-methoxyphenoxy)ethene, vanillin, and 2-methoxy-4-vinylphenol. C-13 NMR studies of lignins isolated from soda/AQ spent liquors indicated the presence of residual anthrone adducts and a significant content of chemically attached AQ.     

Contribution of the γ-hydroxy group to the β-O-4 bond cleavage of lignin model compounds in a basic system using tert-butoxide

Abstract

The most common non-phenolic β-O-4-type lignin model compounds with or without the γ-hydroxymethyl group (C₆-C₃- or C₆-C₂-type, respectively) were treated in a 0.5?mol/L potassium tert-butoxide in DMSO solution at 30?°C to examine the effects of presence of the group. The β-O-4 bond of the C₆-C₃-type cleaved more rapidly than the C₆-C₂-type, indicating that the γ-hydroxy group contributes to the cleavage, in contrast to their reactions in alkaline pulping processes. Furthermore, the β-O-4 bond of the threo isomer of the C₆-C₃-type cleaved more rapidly than that of the erythro isomer. This result can be attributed to the fact that the erythro isomer has the hydrogen bond between a generated alkoxide and the other hydroxy group at its α- and γ-positions in its most-preferential conformer, interfering with the β-O-4 bond cleavage. It was also suggested in treatments of their methyl-etherified derivatives at the α- or γ-hydroxy group that the contribution of the γ-hydroxy group of the threo isomer is greater than that of the erythro isomer. Detailed examination of the distribution profile of reaction products supported this greater contribution of the γ-hydroxy group of the threo isomer.     

Microwave assisted selective oxidation of lignin model phenolic monomer over SBA-15

Microwave assisted oxidation of the lignin model monomer apocynol, 1-(4-hydroxy-3-methoxyphenoxy)-ethanol was carried out over mesoporous SBA-15 catalyst using H₂O₂ as oxidant to produce acetovanillone, vanillin and 2-methoxybenzoquinone. Reaction conditions were optimised in order to obtain acetovanillone selectively, which serves as a raw material for 3,4-dimethoxybenzoic acid, a building block for synthesis of the smooth muscle relaxant mebeverin. Under conventional heating reactions were sluggish and gave poor yields of a mixture of products. The use of increased amount of catalyst resulted in lowering of acetovanillone selectivity. The possibility of recycling the catalyst was also studied.     

Novel methoxylated FA from the Caribbean sponge Spheciospongia cuspidifera

The Δ⁶ monoenoic methoxylated FA (6Z)-2-methoxy-6-heptadecenoic acid and (6Z)-2-methoxy-6-octadecenoic acid were identified for the first time in nature in the phospholipids from the uncommon Caribbean sponge Spheciospongia cuspidifera. These findings expand the occurrence of Δ⁶ 2-methoxylated FA to C₁₇−C₁₈ chain lengths and establish a new FA biosynthetic possibility for these marine organisms. The novel methoxylated FA also could have originated from the phospholipids of a bacterium in symbiosis with the sponge.     

Copper Catalysts for Selective C?C Bond Cleavage of β‐O‐4 Lignin Model Compounds

The reactivity of homogeneous copper catalysts towards the selective C C bond cleavage of both phenolic and non‐phenolic arylglycerol β‐aryl ether lignin model compounds has been explored. Several copper precursors, nitrogen ligands, and solvents were evaluated in order to optimize the catalyst system. Using the optimized catalyst system, copper(I) trifluoromethanesulfonate [Cu(OTf)]/L/TEMPO (L=2,6‐lutidine, TEMPO=2,2,6,6‐tetramethyl‐piperidin‐1‐yl‐oxyl), aerobic oxidation of the non‐phenolic β‐O‐4 lignin model compound proceeded with good selectivity for C_α C_β bond cleavage, affording 3,5‐dimethoxybenzaldehyde as the major product. Aerobic oxidation of the corresponding phenolic β‐O‐4 lignin model proceeded with different selectivity, affording 2,6‐dimethoxybenzoquinone and α,β‐unsaturated aldehyde products resulting from cleavage of the C_α C_aryl bond. At low catalyst concentrations, however, a change in selectivity was observed as oxidation of the benzylic secondary alcohol predominated with both substrates.

     

Physicochemical characterization of lignin fractions sequentially isolated from bamboo (Dendrocalamus brandisii) with hot water and alkaline ethanol solution

Nine lignin fractions from bamboo (Dendrocalamus brandisii) were sequentially isolated with hot water at 80, 100, and 120°C for 3 h and 60% aqueous ethanol containing 0.25, 0.5, 1.0, 2.0, 3.0, and 5.0% NaOH at 80°C for 3 h. Molecular weight and purity analysis revealed that the lignin fractions isolated by hot water (L₁, L₂, and L₃) had lower weight-average molecular weights (between 1350 and 1490 g mol⁻¹) and contained much higher amounts of associated hemicelluloses (between 9.26 and 22.29%), while the lignin fractions isolated by alkaline aqueous ethanol (L₄, L₅, L₆, L₇, L₈, and L₉) had higher weight-average molecular weights (between 2830 and 3170 g mol⁻¹) and contained lower amounts of associated hemicelluloses (between 0.63 and 1.66%). Spectroscopy (UV, FTIR, ¹³C-NMR, and HSQC) analysis showed that the bamboo (Dendrocalamus brandisii) lignin was typical grass lignin, consisting of p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units. The major interunit linkages presented in the alkaline aqueous ethanol extractable bamboo lignin were β-O-4′ aryl ether linkages (about 74.3%), followed by β-β′ resinol-type linkages and β-1′ spirodienone-type linkages (both for 7.8%), together with small amounts of β-5′ phenylcoumaran (6.8%) and p-hydroxycinnamyl alcohols end groups (3.1%). In addition, a small percentage (1.0%) of the lignin side-chain was found to be acetylated at the γ-carbon, predominantly over syringyl units. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

High Activity Solid Base Catalysts for Alkenyl Aromatic Isomerisation

The isomerisation of allylbenzene, dimethoxyallylbenzene (4-allyl-1,2-dimethoxybenzene), eugenol (2-methoxy-4-(2-propenyl)phenol), and estragole (4-allylanisole) into their respective cis/trans internal alkenyl aromatics has been studied over a supported solid base catalysts (K₂CO₃/alumina). The catalyst was active at 305 K with conversions as high as 36%. However all reactants cause deactivation of the catalyst.     

Reaction of Aryl Iodides with (PCP)Pd(II)—Alkyl and Aryl Complexes: Mechanistic Aspects of Carbon—Carbon Bond Formation

The reaction of the PCP-type complex Pd(Me){2,6-(ⁱPr₂PCH₂)₂C₆H₃}( 3 ) with phenyl iodide results in the formation of Pd(I){2,6-(ⁱPr₂PCH₂)₂C₆H₃} ( 5 ), methyl iodide, toluene, and biphenyl. Formation of Pd(Ph){2,6-(ⁱPr₂PCH₂)₂C₆H₃}( 4 ) is observed during the reaction by ³¹P NMR. Reaction of 4 with aryl iodides results in the formation of 5 and Ph–Ph, Ph–Ar, and Ar–Ar, products indicative of a radical reaction. Under pseudo-first-order conditions, the rates of the reactions follow the order p-OMe > p-Me > H > p-NO₂ > m-Cl. The reaction is likely to involve electron transfer from 4 to the aryl iodide followed by fast decomposition of a postulated radical cation [Pd(Ph){2,6-(ⁱPr₂PCH₂)₂C₆H₃}]^+. ( 4 ^+.) to give a phenyl radical and [Pd{2,6-(ⁱPr₂PCH₂)₂C₆H₃}]⁺ ( 6 ⁺). Facile decomposition of the aryl iodide radical anion generates an aryl radical and I⁻. Recombination of aryl radicals gives rise to mixed biaryls, and 6 ⁺ combines with I⁻ to give 5 .     

Thermal Decomposition Study of HNIW by Synchrotron Photoionization Mass Spectrometry

Thermal decomposition of hexanitrohexaazaisowurtzitane (HNIW) was investigated through tuneable vacuum ultraviolet photoionization with molecular‐beam sampling mass spectrometry (MBMS). According to photoionization efficiency (PIE) spectroscopic results, the initial decomposition products of HNIW were identified including HCN, CO, NO, HNCO, N₂O, CO₂ (a little), NO₂, C₂H₂N₂, C₃H₃N₃, C₄H₃N₃, C₃H₄N₄, C₅H₄N₄, C₅H₅N₅ and C₆H₆N₆. The possible ionization energies of C₂H₂N₂, C₄H₃N₃, C₃H₄N₄ and C₆H₆N₆ were analyzed on basis of the PIE spectra. The data were compared with those of thermogravimetry‐mass spectrometry (TG‐MS) and thermogravimetry‐Fourier transform‐infrared spectroscopy (TG‐FT‐IR). The kinetic parameters for the formation of HNCO, HCN and CO₂ were calculated from the current curves of species by TG‐FT‐IR spectroscopy, typically the apparent activation energy (E_a) and prefactor (A) for HNCO were E_a=161.3 ± 2.5 kJ mol⁻¹ and A=38.9 ± 0.6 s⁻¹ with an optimal mechanism function f(α)=(1−α). Global thermal decomposition reaction and Arrhenius equation of HNIW were suggested at the end.     

Synthese und Strukturaufklärung von 19-Nor-pregnanderivaten mit stickstoffhaltigen Vierringen in 14,15-Stellung

Synthesis and Structural Elucidation of 19-Nor-pregnane Derivatives with Nitrogen-Containing Four-Membered Rings in 14,15-Position The introduction of a C₂ side chain in the 17-position of 3-methoxy-14β, 15β-(3′,4′-azetidine)-estra-1,3,5(10)-trien-17β-ol ( 1 ) is described. Hydroboration of the 17-ethylidene compound 4 gives a mixture of the 17αH and 17α-pregnane derivatives 6 and 6a in 70% and 10% yields, respectively. Birch reaction of 6 , followed by oxidation yields the 14β,15β-(3′,4′-azetidine)-19-nor-pregn-4-ene-3,20-dione ( 12 ). The new compounds were characterized by ¹H-n.m.r. The structure of product 13 was determined by X-ray crystallography.