Chromophores from hexeneuronic acids: identification of HexA-derived chromophores

© 2017, The Author(s). Hexeneuronic acids (HexA) have long been known as triggers for discoloration processes in glucuronoxylan-containing cellulosic pulps. They are formed under the conditions of pulping from 4-O-methylglucuronic acid residues, and are removed in an “A stage” along the bleaching sequences, which mainly comprises acidic washing treatments. The chemical structures of HexA-derived chromophoric compounds 4–8, which make up 90% of the HexA-derived chromophores, are reported here for the first time. The compounds are ladder-type, mixed quinoid-aromatic oligomers of the bis(furano)-[1,4]benzoquinone and bis(benzofurano)-[1,4] benzoquinone type. The same chromophoric compounds are generated independently of the starting material, which can be either a) HexA in pulp, b) the HexA model compound methyl 1- 13 C-4-deoxy-β-L-threo-hex-4-enopyranosiduronic acid (1) or c) a mixture of the primary degradation intermediates of 1, namely 5-formyl-furancarboxylic acid (2) and 2-furancarboxylic acid (3). Isotopic labeling ( 13 C) in combination with NMR spectroscopy and mass spectrometry served for structure elucidation, and final confirmation was provided by X-ray structure analysis. 13 C-Isotopic labeling was also used to establish the formation mechanisms, showing all the compounds to be composed of condensed, but otherwise largely intact, 2-carbonylfuran and 2-carbonylfuran-5-carboxylic acid moieties. These results disprove the frequent assumption that HexA-derived or furfural-derived chromophores are linear furanoid polymers, and might have a direct bearing on structure elucidation studies of “humins”, which are formed as dark-colored byproducts in depolymerization of pentosans and hexosans in different biorefinery scenarios

Degradation of 2,5-Dihydroxy-1,4-benzoquinone by Hydrogen Peroxide under Moderately Alkaline Conditions Resembling Pulp Bleaching: A Combined Kinetic and Computational Study

2,5-Dihydroxy-1,4-benzoquinone (DHBQ) is one of the key chromophores occurring in all types of aged cellulosics. This study investigates the mechanism of H₂O₂ degradation of DHBQ under conditions relevant to pulp bleaching (3.0% H₂O₂, NaOH, pH 10), to obtain insights useful for improved pulp processing. DHBQ is degraded quantitatively into malonic acid with an activation energy (<i>E</i>_a) of 16.1 kcal/mol and activation entropy (Δ^⧧<i>S</i>°) of ∼28 cal/mol·K. Higher concentrations of sodium cations increase the reaction rate. Theoretical computations indicate the formation of an intermediate <b>I</b>_<b>O</b> having an O–O bridge between C-2 and C-5 of the 1,4-cyclohexadione structure. <b>I</b>_<b>O</b> undergoes O–O homolysis to form a biradical <b>Bt</b>, which is fragmented into malonate anions. The calculated <i>E</i>_a (17.8 kcal/mol) agrees well with the experimental one. Coordination of Na⁺ to <b>I</b>_<b>O</b> and <b>Bt</b> decreases their energies and enhances the O–O homolysis rate, which is consistent with the acceleration by sodium cation and the negative Δ^⧧<i>S</i>°. The homolysis of <b>I</b>_<b>O</b> is much favored over that of the neutral counterpart, with the unpaired electrons of <b>Bt</b> being stabilized by the geminal anionic oxygen. This difference in the stability of the intermediates translates into significant variations in the reaction rate and the product distribution between pH 10 and neutral/acidic conditions

Degradation of 2,5-Dihydroxy-1,4-benzoquinone by Hydrogen Peroxide: A Combined Kinetic and Theoretical Study

2,5-Dihydroxy-1,4-benzoquinone (DHBQ) is one of the key chromophores formed upon aging in cellulosic materials. This study addresses the degradation mechanism of DHBQ by hydrogen peroxide to provide a solid knowledge base for optimization of bleaching sequences aiming at DHBQ removal. Kinetic analysis provided an activation energy (<i>E</i>_a) of 20.4 kcal/mol. Product analyses indicated the product mixture to contain malonic acid, acetic acid, and carbon dioxide. DFT­(B3LYP) computation presented a plausible mechanism for the formation of these products from DHBQ. DHBQ forms intermediate <b>I2k</b>, having an intramolecular O–O bridge between C-2 and C-5 of the 1,4-cyclohexadione structure. This O–O bond is homolytically cleaved, and the subsequent β-fragmentation of the resulting radical forms ketene and oxaloacetic acid. While ketene yields acetic acid, oxaloacetic acid then gives malonic acid and carbon dioxide through further attack of hydrogen peroxide via an intermediate that is oxidatively decarboxylated. The calculated <i>E</i>_a value (23.3 kcal/mol) in the rate-determining step, i.e., the homolysis of <b>I2k</b>, agreed well with the experimental value. There is also a minor pathway in which the spin state changes to triplet during the homolysis of <b>I2k</b>; in this way two malonyl radicals are formed that are converted to two molecules of malonic acid

A General, Selective, High-Yield N-Demethylation Procedure for Tertiary Amines by Solid Reagents in a Convenient Column Chromatography-like Setup

A traditional preparative chromatographic column can be used to achieve quantitative N-demethylation of tertiary N-methylamines and alkaloids. The filling is the crucial part and is loaded with different solid reagents in three reaction zones. The parent compound is charged on the column, and the neat N-demethylated secondary amine leaves the column some minutes later

Irradiation of Cellulosic Pulps: Understanding Its Impact on Cellulose Oxidation

Different pulp samples were irradiated by three energy sources: plasma, electron beaming, and γ radiation. The effect of increased exposure to irradiation was studied by multidetector gel permeation chromatography with fluorescence labeling of carbonyl groups to quantify changes of the cellulose. Whereas plasma treatment had no effect, for gamma and electron beam the degradation primarily affects the high molar mass area. Kinetic calculations based on DP_w were performed. They show close-to-linear relations with slopes in the same order of magnitude, suggesting that wood-derived pulps degrade slower than pulps from annual plants. The rise in carbonyl group content is linear with increasing dose. In particular, in pulps from annual plants, most detected carbonyl structures originate from the new reducing end groups. Therefore, oxidative modification of cellulose molecules by means of radiation appears to be viable for pulps produced from wood. Here the increase in oxidized functionalities is partially disconnected from chain scission

Vitamin E Chemistry. Nitration of Non-α-tocopherols: Products and Mechanistic Considerations

In contrast to the α-form permethylated at the aromatic ring, non-α-tocopherols possess free aromatic ring positions which enable them to act as potent scavengers of electrophiles in vivo and in vitro. In preparation of enzymatic studies involving peroxynitrite and other nitrating systems, the behavior of non-α-tocopherols under nitration conditions was studied. The nitration products of β-, γ-, and δ-tocopherol were identified, comprehensively analytically characterized, and their structure was supported by X-ray crystal structure analysis on truncated model compounds. Even under more drastic nitration conditions, no erosion of the stereochemistry at 2-C occurred. The nitrosation of γ-tocopherol and δ-tocopherol was re-examined, showing the slow oxidation of the initial nitroso products to the corresponding nitro derivatives by air to be superimposed by a fast equilibrium with the tautomeric ortho-quinone monoxime, which only in the case of γ-tocopherol released hydroxyl amine at elevated temperatures to afford the stable ortho-quinone. Mononitration of δ-tocopherol selectively proceeded at position 5. This selectivity can be explained by the theory of strain-induced bond localization (SIBL) to the quinoid nitration intermediates. Bisnitration was only insignificantly disfavored by the first nitro group, so that under normal nitration conditions offering an excess of nitrating species only the bisnitration product was found

Theoretical Foundation for the Presence of Oxacarbenium Ions in Chemical Glycoside Synthesis

Glycoside formation in organic synthesis is believed to occur along a reaction path involving an activated glycosyl donor with a covalent bond between the glycosyl moiety and the leaving group, followed by formation of contact ion pairs with the glycosyl moiety loosely bound to the leaving group, and eventually solvent-separated ion pairs with the glycosyl moiety and the leaving group being separated by solvent molecules. However, these ion pairs have never been experimentally observed. This study investigates the formation of the ion pairs from a covalent intermediate, 2,3,4,6-tetra-<i>O</i>-methyl-α-d-glucopyranosyl triflate, by means of computational chemistry. Geometry optimization of the ion pairs without solvent molecules resulted in re-formation of the covalent α- and β-triflates but was successful when four solvent (dichloromethane) molecules were taken into account. The DFT­(M06-2X) computations indicated interconversion between the α- and β-covalent intermediates via the α- and β-contact ion pairs and the solvent-separated ion pairs. The calculated activation Gibbs energy of this interconversion was quite small (10.4–13.5 kcal/mol). Conformational analyses of the ion pairs indicated that the oxacarbenium ion adopts ⁴H₃, ²H₃/E₃, ²H₃/²S₀, E₃, ^2,5B, and B_2,5 pyranosyl ring conformations, with the stability of the conformers being strongly dependent on the relative location of the counteranion

Vitamin E Chemistry. Nitration of Non-α-tocopherols: Products and Mechanistic Considerations

In contrast to the α-form permethylated at the aromatic ring, non-α-tocopherols possess free aromatic ring positions which enable them to act as potent scavengers of electrophiles in vivo and in vitro. In preparation of enzymatic studies involving peroxynitrite and other nitrating systems, the behavior of non-α-tocopherols under nitration conditions was studied. The nitration products of β-, γ-, and δ-tocopherol were identified, comprehensively analytically characterized, and their structure was supported by X-ray crystal structure analysis on truncated model compounds. Even under more drastic nitration conditions, no erosion of the stereochemistry at 2-C occurred. The nitrosation of γ-tocopherol and δ-tocopherol was re-examined, showing the slow oxidation of the initial nitroso products to the corresponding nitro derivatives by air to be superimposed by a fast equilibrium with the tautomeric ortho-quinone monoxime, which only in the case of γ-tocopherol released hydroxyl amine at elevated temperatures to afford the stable ortho-quinone. Mononitration of δ-tocopherol selectively proceeded at position 5. This selectivity can be explained by the theory of strain-induced bond localization (SIBL) to the quinoid nitration intermediates. Bisnitration was only insignificantly disfavored by the first nitro group, so that under normal nitration conditions offering an excess of nitrating species only the bisnitration product was found

Dissolution Behavior of Different Celluloses

Celluloses from different origins were dissolved stepwise in N,N-dimethylacetamide/lithium chloride (9% v/w; DMAc/LiCl) with the aim to study the time course of the dissolution process, completeness of dissolution in the dissolved fractions, possible discrimination effects, and differences between the celluloses. Cellulosic pulps from both annual plants and different wood species were analyzed. The obtained fractions were subject to gel permeation chromatography (GPC) with multiple detection to monitor the development of molecular mass distribution (MMD), molecular mass, and recovered mass. The dissolution behavior of accompanying xylans was followed by quantitative analysis of the uronic acids by fluorescence labeling − GPC. The morphological changes at the remaining fibers in the stepwise dissolution were addressed by SEM. The time needed to dissolve completely the cellulosic pulp differed from species to species, mainly between pulps from annual plants and pulps from wood. Annual plants generally needed much longer to dissolve completely. In the beginning of the dissolution, the dissolved fractions of annual plants showed a distinct discrimination effect because they were enriched in hemicellulose. By contrast, wood pulps dissolve fast and without distinct changes in the MMD of the dissolved fractions over time. Bagasse pulp is an exception to the observation for annual plants and rather resembled the behavior of wood celluloses. Prolonged dissolution times, as often practiced in cellulose GPC, do not lead to any improvements regarding the determination of molecular mass, MMD, and recovered mass of injected sample, so that the dissolution times required for reliable GPC analysis can be significantly shortened, which will be important for biorefinery analytics with high numbers of samples