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.     

A Mechanistic Study of the Fischer–Tropsch Synthesis Using Transient Isotopic Tracing. Part-1: Model Identification and Discrimination

A steady state isotopic transient kinetic analysis (SSITKA) of the Fischer–Tropsch synthesis over Co/Ru/TiO₂ catalyst is reported by using ¹³CO and D₂. Besides a qualitative interpretation of the transients, model identification and discrimination are mainly based on numerical modeling. From this, it is concluded that two single-C species are present on the catalyst surface, C_α,ads and C_β,ads. These species both participate in the formation of methane and of higher hydrocarbons. The heterogeneity of the catalyst surface is limited to these two single-C species. Only one type of chain-growth site is present in a low concentration compared to the surface concentrations of CO_ads, C_α,ads, and C_β,ads. The H-content of C_α,ads and C_β,ads is assessed by comparing the simulated transient for the incorporation of the D-labeling into methane with experiments. In case the stepwise hydrogenations of C_ads to CH₄ are irreversible, C_α,ads and C_β,ads are H-free species. In case these reactions are reversible, the H-exchange between C_ads, CH_ads, CH_2,ads, and CH_3,ads is fast compared to the net formation of methane, and the H-content of C_α,ads and C_β,ads cannot be assessed. The most probable mechanism for the Fischer–Tropsch reaction resulting from this study is used in a next paper (Part 2) for the quantification of the kinetic parameters. In that paper, a systematic analysis of these parameters yields detailed mechanistic insight into the Fischer–Tropsch synthesis reaction.     

Reaktionen von β-Chlorvinylaldehyden. V [1]. Zur Bildung von 2,2′-Thiopyrylotrimethinfarbsalzen aus 2-(α-Formylalkyliden)-2H-thiopyranen

Reactions of β-Chlorovinylaldehydes. V. The Formation of 2,2′-Thiopyrylocyanine Dyes from 2-(α-Formylalkylidene)-2H-thipyranes Substituted β-chlorocrotonaldehydes react with Na₂S · 9H₂O to form 2-(α-formyl-alkylidene)-2H-thiopyranes 1 . The corresponding thiopyrylium salts 2 which are easily available from 1 and strong acids exist in the enolic form (2-β-hydroxyvinyl-thiopyrylium salts). They are converted at room temperature in methanol solution to dark green compounds, which are identified as 2,2′-thiopyrylotrimethincyanine dyes 3 . The mechanism of formation of compounds 3 is discussed.     

Selective Depolymerization of Lignin: Assessment of Yields of Monomeric Products

A method is developed to assess the limits of selective depolymerization, that is, splitting of aryl ether bonds to yield monomeric aromatic products, of native and technical lignins. The method and respective formula is based on measuring the difference between a sum of aryl ether bonds (α-О-4 and β-О-4) and sum of 5-5 and 4-O-5 bonds. The values calculated based on the proposed formula are correlated with published experimental data on catalytic degradation of lignin. In all cases, wood species and experimental conditions notwithstanding, the yields of monomeric aromatic products are found to be close to theoretically calculated for both softwood (23%) and hardwood (51%). Notice that in the actual experiments the yield in hardwood is twice as much as in softwood. In technical lignins (kraft, organosolv) the yields of monomeric products are lowed due to splitting of α-О-4 and β-О-4 bonds during pulping. The developed method is helpful in evaluation of technologies aiming to bring value-added products from lignin through selective degradation.     

Pulping Catalysts From Lignin (7). Nitrogen Dioxide Oxidation Of A Lignin Model Dimer

Oxidation of a lignin model disyringyl dimer with nitrogen dioxide (NO₂) in the presence of air and N-hydroxysuccinimide led to C₁-C_αcleavage with the formation of approximately equal amounts of 2,6-dimethoxy-p-benzoquinone (DMBQ) and glyceraldehyde-2-syringyl ether type structures. The result indicates that only the phenolic end syringyl units of a lignin polymer will be converted to DMBQ upon treatment with the current NO₂ reaction conditions. Internal (non-phenolic) lignin units, bonded by β-O-4 linkages, will resist oxidation.     

Constituents of human meconium: II. Identification of steroidal acids with 21 and 22 carbon atoms

Monohydroxylated acid fraction isolated from human meconium was found to contain, in addition to C₂₀ and C₂₄ acids identified previously, three C₂₂ bile acids-(20S)-3α-hydroxy-23,24-bisnor-5β-cholan-22-oic, (20S)- and (20R)-3β-hydroxy-23,24-bisnor-chol-5-en-22-oic, and one C₂₁ acid-3β-hydroxypregn-5-en-21-oic. These compounds were identified by capillary gas chromatography-mass spectrometry and by comparison with standards. It is postulated that these C₂₂ acids, as well as the two monohydroxylated C₂₄ bile acids (lithocholic and 3β-hydroxychol-5-enoic) are produced in the maternal intestine by microbial flora and transferred to the fetus through the placenta.     

Effect of moulding pressure on crystallinity in powder processing of semicrystalline polymers

The crystalline content in high pressure compacted polymeric powders has been investigated as a function of moulding pressure. It has been noted that the crystallinity C_p varied with the pressure as [C₀ — βP^α] where α and β were material dependent constants. There also appears a correlation between α, β, and an empirically defined factor which is related to the glass transition temperature, melting point, and compressive yield strength for all the polymers studied. The results have been discussed in terms of various processes taking place during the high pressure compacting step.     

BEHAVIOR OF EUCALYPTUS GLOBULUS LIGNIN DURING KRAFT PULPING. II. ANALYSIS BY NMR,ESI/MS,AND GPC

ABSTRACT

Residual and dissolved lignin from different phases of kraft delignification of Eucalyptus globulus wood were isolated and characterized by 1D and 2D ¹H NMR, ¹³C NMR, Electrospray Ionization Mass Spectrometry (ESI/MS), and gel permeation chromatography (GPC). During the temperature rise period, below 70°C, about 20% of the lignin was dissolved without significant structural changes. Above 70°C, the lignin suffered significant degradation/fragmentation in the cell wall prior to dissolution. The lignin ether-linked syringyl units were the most susceptible to alkaline degradation. Through the course of pulping, the residual lignin (RL) revealed a gradual increase of aliphatic moieties of unknown structure, as well as a decrease of native structures such as phenylcoumaran and pino-/syringaresinol lignin units. A significant decrease of the β-O-4 structures content in RL was detected only at the final cooking temperature. The lignin dissolved in the black liquor (BL) consisted of highly branched oligomers with rather low molecular weight (average mass 800–1000 u). A part of BL (about 30%) was chemically linked to carbohydrates and possessed a large molecular weight distribution (500–4000 u). BL showed a progressive decrease in β-O-4 and pino-/syringaresinol structures and formation of enol ether and stilbene structures. The GPC analyses showed a continuous decrease of the molecular weight of both the residual and dissolved lignins during the pulping process, particularly in the residual stage.     

A study of the structure of poly(hexene-1) prepared by nickel(α-diimine)/MAO catalyst using high resolution NMR spectroscopy

Homopolymerization of hexene-1 with nickel(α-diimine) catalyst forms poly(hexene-1) containing different types of branches and varying number of methylene units in the backbone. Types of branches identified by NMR spectroscopy include, butyl (C₄), longer than butyl (>C₄) and methyl (isolated and meso and racemic head-to-head). Formation of ethyl (C₂) and propyl (C₃) branches were not observed. The migration of the nickel during polymerization not only causes the formation of branches other than C₄, but also the runs of methylene units in the backbone due to complete 1,6-enchainment (chain running). The formation of regio-irregular methyl branches required an insertion of hexene-1 in the nickel-secondary carbon. Formation of methyl branches and (1, ω) enchainment has also been observed during the polymerization of octene-1, decene-1, tetradecene-1 using the same catalyst system. The extent formation of these subunits in the polymer depends on polymerization temperature. A plausible explanation for the formation of different branches is discussed.     

The Characteristics and Reactivity of Acid-Labile Aryl Ether Units in Wood Lignin

Wood lignin contains significant amounts of acid-labile aryl ether units, which play a significant role in lignin modification or delignification processes. We have evaluated the rate and reaction kinetics on the acid-catalyzed cleavages of aryl ether structures for wood lignin in situ based on the formation of phenolic hydroxyl groups. The content of acid-labile aryl ether units was quite uniform for a variety of softwood wood lignins (～4% per C₉ unit) and it varied appreciably among hardwood species, ranging from 4% for aspen to 9% for beech wood lignin. These variations, however, appear to be related to the content of syringyl units in wood lignin. The reactivity of these reactive aryl ether structures was noticeably higher for the spruce than for the aspen wood lignin. This difference in reactivity, based on the behavior of lignin model compound reactions, can be attributed to the influence of syringyl moieties in aspen wood lignin. It appears that most of the acid-labile aryl ether units in hardwood were associated with the syringy moiety being present as a benzyl unit, which is much less reactive than the corresponding guaiacyl moiety.     

β-Ketoester — a Rearranged Product of Epoxidation of α, β-Unsaturated Methyl Ester

Attempted epoxidation of long-chain α,β-unsaturated esters resulted in the formation of the rearranged products of the corresponding epoxyesters. It was observed that reaction of peracids with esters of C₁₆, C₁₈ and C₂₂ trans-2-enoic acids, instead of yielding the expected epoxides, gave the isomerized products characterized as β-ketoesters. The structure of β-ketoester as saturated 3-oxoester was unambiguously established by chemical methods as well as by IR, NMR and Mass spectrometry. The selectivity of this rearrangement provides a useful synthetic pathway for the preparation of β-ketoesters.     

Delignification Mechanism During High-Boiling Solvent Pulping. IV. Effect of a Reducing Sugar on the Degradation of Guaiacylglycerol-β-guaiacyl Ether

Abstract

A phenolic β-O-4 type lignin model compound, guaiacylglycerol-β-guaiacyl ether was treated with 70 wt% aqueous 1,3-butanediol solution in the presence of glucose at 160–200°C to investigate the effect of reducing sugars on the degradation of lignin during high-boiling solvent (HBS) pulping. Addition of glucose increased the formation of guaiacol, coniferyl alcohol, and its γ-ethers, and decreased the formation of radical coupling compounds dramatically. These results suggest that reducing sugars may stabilize phenoxy radicals formed by homolysis of phenolic β-ethers. The kinetic studies also revealed that the disappearance of the β-ether model compound was enhanced substantially by the presence of glucose, which suggests that in addition to homolysis of the β-ether, a reducing sugar-assisted β-ether cleavage may be involved under the conditions used.     

Reactions of ethylene oxide and ethanol in the presence of amberlyst 15: Products and product rate modeling

The reactions of ethylene oxide and ethanol catalyzed by Amberlyst 15 (H⁺) were studied at 60°, 90°, and 120°C. The concentration ranges investigated at 90°C were 0.9–5 and 0.3–6 mol/1 for the oxide and ethanol, respectively, while the mass of resin taken was in the range of 60 to 28 g. It was found that ethylene oxide reacts with the sulfonic acid groups of the resin to form polymeric ester species which cause catalyst deactivation. Despite the deactivation several reactions occur at 90°C yielding 2-ethoxyethanol, 2-(2-ethoxyethoxy)-ethanol and 1,4-dioxane. Two parallel reaction paths and the overall rate model r = r = C₁C₂C₃/(β₁C + β₂C₂) are proposed for the formation of 2-ethoxyethanol. A back-biting mechanism is tentatively proposed for the formation of 1,4-dioxane from the polymer ester species and the model r = βC₃ for its rate of formation.     

Absolute Configuration of β- and α-Asymmetric Carbons within β-O-4-Structures in Hardwood Lignin

In this study, we investigated the proportion of erythro- and threo-forms of β-O-4-ether structures and their enantiomeric compositions in hardwood lignin by applying the ozonation method to birch wood meal. Optical activity was not substantially observed in either the erythronic or threonic acids obtained as the ozonation products of β-O-4-structures in birch wood meal. The proportions of the four stereoisomeric forms {(αS,βR)-erythro, (αR,βR)-threo, (αS,βS)-threo, and (αR,βS)-erythro forms} were estimated to be 37-38%, 13-14%, 12-13%, and 36-37% based on the yields of erythronic and threonic acids, and on their optical activities. The proportions suggest that the entire components of β-O-4-ether structures in birch wood lignin have R- and S-configurations at the β-carbon in approximately the same quantities {(βR)-β-O-4-structure: (βS)-β-O-4-structure = 50–52:48–50}; i.e., that the β-ether structures are essentially racemic. This estimation implies that, during lignin biosynthesis, an equal number of enantiomeric forms of β-O-4-bonded quinone methides were formed by radical coupling reactions.     

Interaction between nickel hydroxy phthalocyanine derivatives with p-chlorophenol: Linking electrochemistry experiments with theory

In this work the interaction between peripherally (β) substituted nickel tetrahydroxyphthalocyanines (β-NiPc(OH)₄ and β-Ni(O)Pc(OH)₄) with p-chlorophenol is theoretically rationalised by performing calculations at B3LYP/6-31G(d) level. Density functional theory (DFT) and molecular orbital theory are used to calculate the condensed Fukui function for phthalocyanine derivatives and p-chlorophenol, in order to determine the reactive sites involved when p-chlorophenol is oxidized, and to compare theoretically predicted reactivity to experimentally determined electrocatalytic activity. Electrocatalytic activities of adsorbed NiPc derivatives: ads-α-NiPc(OH)₈-OPGE (OPGE = ordinary poly graphite electrode), ads-α-NiPc(OH)₄-OPGE and ads-β-NiPc(OH)₄-OPGE are compared with those of the polymerized counterparts: poly-α-Ni(O)Pc(OH)₈-OPGE, poly-α-NiPc(OH)₄-OPGE and poly-β-NiPc(OH)₄-OPGE, respectively.     

Lead-free 0.7BiFeO3-0.3BaTiO3 high-temperature piezoelectric ceramics: Nano-BaTiO3 raw powder leading to a distinct reaction path and enhanced electrical properties

Herein, nano-BaTiO₃/Bi₂O₃/Fe₂O₃ and BaCO₃/TiO₂/Bi₂O₃/Fe₂O₃ were respectively used to prepare 0.7BiFeO₃-0.3BaTiO₃ (0.7BF-0.3BT) ceramics by conventional solid-state method and clarify the reaction path, phase structure, microstructure, ferroelectric, and piezoelectric properties. 0.7BF-0.3BT ceramic using nano-BaTiO₃ with cubic phase (C_BT) undergoes the formation of rhombohedral α-phase (R_α) and the transition from R_α and C_BT to rhombohedral β-phase (R_β) and pseudo-cubic (PC) phases, while the counterpart using BaCO₃/TiO₂ directly generates R_β and PC. Nano-BaTiO₃ can decrease pores and oxygen vacancies in the resultant ceramics comparing to BaCO₃/TiO₂, which is due to an inhibited decomposition of R_α and a weaker reduction of Fe³⁺ to Fe²⁺, leading to increased density and reduced leakage current density. Benefitting from a proper phase ratio, increased density and reduced leakage current density, the enhanced piezoelectric properties d₃₃?=?210?pC/N, k_p?=?0.34, P_r?=?31.2?μC/cm² and T_C?=?514?°C are obtained in 0.7BF-0.3BT ceramic using nano-BaTiO₃. Our work reveals the importance of raw materials and the potential of BF-BT as a high-temperature lead-free piezoelectric ceramic material.     

Intramolecular 1,2- and 1,3-Hydrogen Transfer Reactions of Thiyl Radicals

This review article summarizes recent experiments on 1,2- and 1,3-hydrogen transfer reactions in thiyl radicals from cysteine and related compounds. Pulse radiolysis in combination with time-resolved UV spectroscopy was applied to monitor the equilibration of initial thiyl radicals with carbon-centered radicals at both the C_α and C_β positions of cysteine. Experiments with thiyl radicals from penicillamine and cysteamine confirmed the formation of carbon-centered radicals at these positions. Complementary evidence for the intermediary formation of carbon-centered radicals was obtained from mass spectrometry and ¹H NMR spectroscopy experiments, both of which indicated covalent H/D exchange at original C H bonds when thiyl radicals were generated in D₂O. The 1,2- and 1,3-hydrogen transfer reactions can have profound consequences for the integrity of proteins when Cys residues are oxidized to Cys thiyl radicals, which subsequently equilibrate with carbon-centered radicals.     

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.     

Reaktivität und Selektivität bei der Oxidation von Styrolderivaten. IV. Untersuchungen zur Oxidation von substituierten β,β-Dimethylstyrolen

Reactivity and Selectivity in the Oxidation of Styrene Derivatives. IV. Studies on the Oxidation of Substituted β,β-Dimethylstyrenes The liquid phase oxidation of substitued (p-MeO-, p-Cl-, m-CF₃-) 2-aryl-3-methyl-but-2-enes, of 1,1-diphenyl-2-methyl-propene, of 1-ethoxy-2-methyl-1-phenyl-propene and of 9-isopropylidene-fluorene with pure oxygen was investigated in chlorobenzene solution and in presence of cumene and of cumene hydroperoxide in the temperature range 65-125 °C. The product yields were determined gaschromatographically. The differences of the activation energies of epoxide formation and the parallel reactions were calculated. They amount to 19–48 kJ/mol. The epoxide selectivity increases with increasing temperature and increasing concentration of olefin. The relative chain propagation constants (k_{pC C}) were determined by competitive oxidation with cumene. The k_{pC C} values of substitued β,β-dimethylstyrenes can be correlated by a LFE-relationship with the ionisation energies of the olefins.     

Photoluminescent cuprous iodide polymorphs generated via in situ organic reactions

Two cuprous iodide polymorphs formulated as [N(CH₃)₄][Cu₂I₃] α and β were synthesized, in which N(CH₃)₄⁺ template that never existed in the reaction materials are in situ generated via cleavage of CN bonds and N-alkylation reactions. Solvothermal reaction of copper source, hexamethylenetetramine and HI in methanol solution at 150 °C led to α and β polymorphs. Two polymorphs have been synthesized via subtle modification of the experimental condition, which indicates that the small force is also sufficient for energy difference for polymorphs. α polymorph contains one-dimensional Cu₂I₃⁻ chain in which the copper atoms are arranged into a zigzag lattice; β polymorph is composed of novel ladder-like chains which can be considered as being built up by Cu₁₂I₁₈ and Cu₂I₃ two types of secondary building units. The UV–vis absorption spectra for α and β polymorphs were investigated in reflectance mode in the solid state at room temperature, exhibiting that they belong to potential semiconductors. The excitation and emission in α and β are very distinct, which highlight that the molecular packing in polymorphs is critical to photophysical properties.