Substituent Effects on 13C Chemical Shifts of Aromatic Carbons in β-O-4 and β-5 type Lignin Model Compounds

Abstract

Substituent effects on the chemical shifts of aromatic carbons in lignin model compounds have been elucidated from ¹³C NMR spectra of guaiacyl and syringyl type monomerio and β-O-4 model compounds and guaiacyl type β-5 model compounds. Evaluation of the observed values of substituent chemical shift (SCS) for the aromatic carbons leads to elucidation of a generalized SCS additivity rule, for estimation of the chemical shifts of aromatic carbons in ring A of β-O-4 and β-5 type substructures in model compounds and in ring B of β-O-4 substructures in lignin preparations, with errors of less than 1 ppm. The rule is applicable to substructures of both guaiacyl and syringyl types, using an appropriate parent compound as reference instead of benzene. Signals in the aromatic region of the ¹³C NMR spectra of β-O-4 and β-5 type model compounds are reassigned on the basis of the observed SCS's as well as APT spectra of the compounds.     

Evidence for Increased Steric Compression in Anti Compared to Syn Lignin Model Quinone Methides

Two new lignin model quinone methides (QMs) have been characterised by ¹H and ¹³C NMR techniques. It was shown that the QMs existed in solution as non-interconverting syn and anti isomers, in a ratio of 2:1. Evaluation of chemical shift data and nuclear Overhauser effects from these and previously synthesised QMs showed that the anti isomer was under greater steric strain than the syn isomer. Additional evidence for the increased steric strain was obtained from a relative kinetic study of the addition of primary amines to the QMs.     

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.     

Reactions of Metal Ion Complexes with Lignin Model Compounds Part I. Co(TSPP) as a Single Electron Transfer Catalyst and Implications for the Mechanism of AQ Pulping

Cleavage of the β-aryl ether bonds of the lignin model compounds guaiacylglycol β-guaiacyl ether (1) and guaiacylglycerol β-guaiacyl ether (11) to form guaiacol was found to be catalyzed by the water soluble porphyrin complex trisodium meso-tetra-4-sulfonatophenvlporphinecobalt(III), (Co(TSPP)). The resulting product distributions were remarkably similar to those obtained for the anthraquinone (AQ) catalyzed reactions under the same conditions. Studies of the reactions of CofTSPP) with proposed intermediate compounds indicate that the major route to guaiacol formation involves reaction of the reduced catalyst, Co^II(TSPP), with the quinone methides 9 and 13 which are derived from the phenolic model compounds 1 and 11 respectively. It is proposed that the mechanisms of these reactions involve single electron transfer to the quinone methide as the first step. The implications of this proposal for the AQ catalyzed reactions of 1 are discussed.     

Separation and spectral data of the six isomeric ozonides from methyl oleate

The structures of the products obtained on ozonation of methyl oleate have been re-examined. The assignments for the six isomeric ozonides of methyl oleate have been made by¹H and¹³C nuclear magnetic resonance (NMR), which were consistent with the retention times observed in high-performance liquid chromatography; the assignments were confirmed by mass and infrared spectroscopy. Two triplets for the ozonide ring protons of thecis andtrans isomers in the normal (MOO1) and the two cross ozonides (MOO1 and MOO3) can be resolved by 400 MHz NMR. For MOO1 and MOO3, where the two ring carbons are equivalent, two peaks for the ring carbons of each cross ozonides are resolved in the¹³C NMR spectra, one for thecis and one for thetrans isomer. For MOO2, four peaks for the ring carbons are resolved in the¹³C NMR spectra, two for thecis and two for thetrans isomer.     

Synthesis of Conjugated Polymers Containing Phosphole with the 5-Member Fused Carbocycle

Summary Novel conjugated polymers 9a–c possessing the phosphole ring with the 5-member fused carbocycle were synthesized by palladium catalyzed coupling reaction of 1-phenyl-2,5-bis(p-bromophenyl)phosphole 7 with diethynylarenes 8a–c. The obtained polymers 9a–c had the regio-regulated 2,5-substituted phosphole ring in the polymer main chain, and were characterized by ¹H, ¹³C, ³¹P NMR, and FTIR. The ³¹P NMR spectra revealed that the extension of π-conjugation changed the phosphole ring angles. The optical properties of polymers 9a–c were independent of the size of the fused carbocycle.     

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.     

Thermally activated polymerization behavior of bisphenol‐S/methylamine‐based benzoxazine

A bifunctional benzoxazine monomer, 6,6′‐bis(3‐methyl‐3,4‐dihydro‐2H‐benzo[e] [1,3]oxazinyl) sulfone (BS‐m), was synthesized from bisphenol‐S, methylamine, and formaldehyde via a solution method. The chemical structure of BS‐m was characterized with ¹H and ¹³C‐nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and elemental analysis. The ring‐opening polymerization reaction of BS‐m monomer was studied by FTIR, ¹³C solid‐state NMR, and differential scanning calorimetry. With the polymerization reaction proceeding, the intensities of the FTIR absorption peaks of CH₂, C? O? C, and C? N? C of the oxazine ring decreased gradually, and some of these absorption peaks disappeared. The shapes and intensities of the absorption peaks associated with benzene ring, sulfone group, and aromatic C? S bond changed in various ways. The changes in the solid‐state ¹³C‐NMR pattern, including chemical shifts, intensity of resonances, and line‐width, were observed from the spectra of BS‐m and the corresponding polybenzoxazine. The melting process of BS‐m overlapped with the beginning of the ring‐opening polymerization reaction. The polymerization kinetic parameters were evaluated for nonisothermal and isothermal polymerization of BS‐m. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

NMR characterization of dihydrosterculic acid and its methyl ester

Cyclopropane FA occur in nature in the phosphoplipids of bacterial membranes, in oils containing cyclopropene FA, and in Litchi sinensis oil. Dihydrosterculic acid (2-octyl cyclopropaneoctanoic acid) and its methyl ester were selected for ¹H and ¹³C NMR analysis as compounds representative of cyclopropane FA. The 500 MHz ¹H NMR spectra acquired with CDCl₃ as solvent show two individual peaks at −0.30 and 0.60 ppm for the methylene protons of the cyclopropane ring. Assignments were made with the aid of 2D correlations. In accordance with previous literature, the upfield signal is assigned to the cis proton and the downfield signal to the trans proton. This signal of the trans proton is resolved from the peak of the two methine protons of the cyclopropane ring, which is located at 0.68 ppm. The four protons attached to the two methylene carbons α to the cyclopropane ring also show a split signal. Two of these protons, one from each methylene moiety, display a distinct shift at 1.17 ppm, and the signal of the other two protons is observed at 1.40 ppm, within the broad methylene peak. The characteristic peaks in the ¹³C spectra are also assigned.     

Crowned polyacetylene. 4. Solid state NMR studies on molecular motions and cis-trans isomerization of poly(4'-ethynylbenzo-15-crown-5)

Summary Chain conformations and molecular motions of poly(4'-ethynylbenzo-15-crown-5) (PEB15C5) and its annealed sample (AN-PEB15C5) in the solid state were investigated by ¹H NMR and CP-MAS ¹³C NMR measurements. The chain conformation was determined to be cis-transoidal for PEB15C5 and 70°-deflected trans-transoidal for AN-PEB15C5. The exothermic peak at 180°C observed in the DSC thermogram was due to the cis-trans isomerization. Motional modes of each chain were observed in the three temperature regions of γ, β and α with an increase of temperature, which are attributed to the sterically hindered oscillation of the side chains and the vibrational and free rotations of the main chain with the oscillating side chain. The free rotation of the trans chain in the α-region occured in a lower temperature region by 15°C, comparing with that of cis chain. Received: 1 December 1997/Revised version: 5 January 1998/Accepted: 12 January 1998     

Synthesis and Characterization of π-Conjugated Polymers with a 2,5-Substituted Phosphole Skeleton

Summary Novel π-conjugated polymers 10a-c having the phosphole ring were prepared by PdCl₂(PPh₃)₂/CuI catalyzed coupling of 1-phenyl-2,5-bis(p-bromophenyl)phosphole 8 with fluorine-substituted diethynylbenzene 9a-c. The obtained polymers 10a-c were regio-regulated with the 2,5-substituted phosphole ring in the polymer main chain, and characterized by ¹H, ³¹P NMR, and FTIR. Polymers 10a-c were found to be an extended π-conjugated system according to the results of UV-Vis absorption spectra. In the fluorescence emission spectra of 10a-c, moderate emission peaks were observed in the visible blue to green region.     

Design,Synthesis and Optical Properties of Tetrahedrally Silicon-Cored Phenyl-Linked Pyrazole Ligands

A novel series of tetrahedrally silicon-centered pyrazole derivatives, including bis(4-(pyrazol-1-yl)phenyl)dimethylsilane (1), bis(3-(pyrazol-1-yl)phenyl)dimethylsilane (2) and tri(4-(pyrazole-1-yl)phenyl)methylsilane (3) have been designed, synthesized and determined by FT-IR, ¹H NMR, ¹³C NMR, ²⁹Si NMR and mass spectroscopy. These compounds are stable in moisture and have high thermal stability. The optical properties reveal that these compounds show a fluorescent emission in the blue-violet region, which could be potentially applied as blue emitters for OLEDs and utilized to construct functional MOFs with luminescent properties.     

The thermal Aromatization of Methyl-1,3-cyclohexadienes – an important argument against commonly accepted sigmatropic 1,7-H-shift reactions

It has been demonstrated that the methyl and ring C-atoms of methyl-1,3-cyclohexadienes interchange their positions intramolecularly during the thermal conversion to toluene at temperatures above 600°C in a quartz flow system. Gas phase pyrolysis of double ¹³C-labeled methyl-1,3-cyclohexadienes with ¹³C-labels for the primary and the tertiary C-atom gave definite ¹³C-distribution patterns in the aromatic ring systems of the formed toluene as well as benzene with far-reaching similarities. The NMR data of the [¹³C₂]toluene isotopomers definitely rule out that the observed integration of the methyl C-atom into the ring system proceeds via the hitherto well-established sequence: electrocyclic ring opening of the 5-methyl-1,3-cyclohexadiene to 1,3,5-heptatriene, its sigmatropic 1,7-H shift and ensuing recyclization of the 1,3,5-heptatriene to methyl-1,3-cyclohexadienes.     

Ultrasound-promoted one-pot four-component synthesis of novel biologically active 3-aryl-2,4-dithioxo-1,3,5-triazepane-6,7-dione and their toxicity investigation

We have described a novel four-component reaction (4MCR) between oxalyl chloride 1, anilines 2a–h, and two molecules of ammonium thiocyanate 3 in acetone under ultrasound irradiation to give 3-aryl-2,4-dithioxo-1,3,5-triazepane-6,7-diones. A synthetically useful ultrasound effect was observed and title products were obtained in high yields after 15–35?min sonication. Our procedure compared to the conventional heating method has the benefit of higher reaction yields and shorter reaction times. The IR spectra showed the presence of N–H, C═O and C═S groups in these compounds and ¹H, ¹³C NMR and Mass spectral results verified their structures. Cellular investigations showed that these compounds are toxic toward cancer cells.     

The Potential of 13C Cp/Mas Nmr in the Study of Kraft Pulping Kinetics

Abstract

Solid state ¹³C CP/MAS nuclear magnetic resonance spectra were obtained for unbleached kraft pulps prepared from white spruce with yields ranging from 47.6 to 96.1%. The observed trends of the relative peak intensities were found to correspond to established facts on kraft kinetics. These results suggest that ¹³C CP/MAS NMR of wood and pulp could be used to study kinetics in situ.     

31P NMR Spectroscopy in Wood Chemistry. I. Model Compounds

The quantitative reaction in an NMR tube of 1,3,2-dioxaphospholanyl chloride (I) with compounds bearing active hydrogens was explored as a simple method for derivatizing the labile centres known to occur in lignins. Derivatives of phenols, alcohols and carboxylic acids with (I) gave ³¹P chemical shifts which appeared in different ranges of the NMR spectra. Any ortho substitution onto the aromatic ring of phenols significantly affected the magnitude of the ³¹P NMR chemical shifts, while para and/or meta substituents had a much smaller effect. A clear distinction between guaiacyl, syringyl and unsubstituted phenolic hydroxyls can thus be made in mixtures of model compounds. Primary alcohols were clearly distinguished from secondary and tertiary alcohols in their derivatives with (I), while different ³¹P NMR signals for derivatives of erythro and threo forms of lignin-like model compounds were identified. While alcohols, phenols and simple carboxylic acids on reaction with (I) gave derivatives that were substitution products, aldehydes reacted via a distinctly different addition mechanism; ketones did not react at all.     

The Influence of Pulping Conditions on the Structure of Acetosolv Eucalyptus Lignins

Acetosolv lignins obtained under a variety of pulping conditions as regards acetic acid concentration, catalyst concentration, temperature and pulping time were characterized structurally by elemental analysis, FTIR and ¹H and ¹³C NMR spectroscopy, and determination of methoxyl group content. Increasing catalyst concentration and pulping time favours both the dissolution of lignin (by cleavage of α and β-O-4 bonds) and the occurrence of condensation reactions. Increasing temperature favours the degradation of syringyl units more than of guaiacyl units.     

Structural analysis of cellulose triacetate polymorphs by two-dimensional solid-state C-C and H-C correlation NMR spectroscopies

For the elucidation of the crystal structures of the two crystalline allomorphs of cellulose triacetate (CTA), namely CTA I and CTA II, two-dimensional (2D) solid-state through-bond ¹³C-¹³C and ¹H-¹³C correlations NMR techniques applied to the two crystalline allomorphs of CTA. As a result, the ¹³C and ¹H chemical shifts of the glucopyranose ring of CTA I and CTA II were completely assigned by the 2D NMR spectra of these allomorphs. On the 2D ¹³C-¹³C correlation spectrum of CTA II, two sets of the ¹³C-¹³C correlations from C1 to C6 were observed. This indicated that the CP/MAS ¹³C NMR spectrum of CTA II can be characterized by its overlapping of the ¹³C subspectra of two kinds of 2,3,6-triacetyl-anhydroglucopyranose units and that there are two magnetically non-equivalent sites in the unit cell of CTA II. In the case of CTA I, the numbers of respective ¹³C and ¹H shifts of CTA I agreed with the those of the glucopyranose residue in the allomorph, strongly suggesting that the asymmetric unit of CTA I is only one glucose residue. In addition, conformational differences in the exocyclic C5-C6 bonds between CTA I and CTA II were strongly suggested by the notable differences in the ¹H and ¹³C chemical shifts at the C6 sites of these allomorphs.     

Synthesis of 3′‐Fluoro‐tRNA Analogues for Exploring Non‐ribosomal Peptide Synthesis in Bacteria

Aminoacyl‐tRNAs (aa‐tRNAs) participate in a vast repertoire of metabolic pathways, including the synthesis of the peptidoglycan network in the cell walls of bacterial pathogens. Synthesis of aminoacyl‐tRNA analogues is critical for further understanding the mechanisms of these reactions. Here we report the semi‐synthesis of 3′‐fluoro analogues of Ala‐tRNA^Ala. The presence of fluorine in the 3′‐position blocks Ala at the 2′‐position by preventing spontaneous migration of the residue between positions 2′ and 3′. NMR analyses showed that substitution of the 3′‐hydroxy group by fluorine in the ribo configuration favours the S‐type conformation of the furanose ring of terminal adenosine A76. In contrast, the N‐type conformation is favoured by the presence of fluorine in the xylo configuration. Thus, introduction of fluorine in the ribo and xylo configurations affects the conformation of the furanose ring in reciprocal ways. These compounds should provide insight into substrate recognition by Fem transferases and the Ala‐tRNA synthetases.     

Preparation and characterization of poly(2,6-dimethyl-1,4-phenylene oxide) and Nylon 6 graft copolymer

Summary A poly(2,6-dimethyl-1,4-phenylene oxide) (PPO, 1) macroinitiator having carbonylcaprolactam groups was prepared through metalation of methyl group of PPO and subsequent modification to introduce carboxyl group, acid chloride group, and finally carbonylcaprolactam group. Anionic ring opening copolymerization of -caprolactam took place onto the macroinitiator to give a graft copolymer of PPO and Nylon 6. The structure of intermediate materials and the graft copolymer were characterized by ¹H NMR, ¹³C NMR, and IR spectroscopy. Glass transition temperatures and melting temperatures of these materials are also reported. It is found that the copolymer has a microphase-separated morphology even with segment molecular weight as low as 3000.