Application of gel-phase 13c-nmr to monitor solid phase peptide synthesis

The results presented in this article show the general applicability of gel-phase ¹³C-NMR to monitor solid phase peptide synthesis on the most commonly used polystyrene-based resins. The optimal conditions to acquire gel-phase ¹³C-NMR spectra of copoly(styrene-l%-divinylbenzene) has been determined. The technique proved to be applicable to characterize polystyrene-based starting supports as well as to the determination of their degree of functionality and purity. The stage-by-stage ¹³C-NMR characterization of the growing peptide chain during the synthesis of tripeptide H-Asn-(N-Me)Ala-Thr-NH₂ on a benzhydrylamine resin is described. The structural information, derived from this technique is relevant for the synthetic process as the spectra can be acquired under similar conditions. The characteristics of the spectra facilitate monitoring of coupling and deprotection reactions.     

Ring dehydration mechanisms and kinetics of polyamic acid models in solution and in the solid state

We have synthesized and studied the ring dehydration mechanisms and kinetics of polyamic acid models in solution and in the solid state using ¹³C-NMR (solid and liquid), HPLC, FTIR, and x-ray diffraction. Results obtained in solution show the role of temperature, catalysts, and the basicity of the amine in ring dehydration mechanisms and kinetics, as well as conformation and intramolecular bonds in the amic acid bond in the solid state. © 1993 John Wiley & Sons, Inc.     

Iron(II) complexes of 2,6-bis(5-alkyl-1,5-dimethyl-4,5-dihydro-1H-imidazol-4-on-2-yl)pyridine ligands. Synthesis, characterisation and solvolytic stability

New 2,6-bis(5-alkyl-1,5-dimethyl-4,5-dihydro-1H-imidazol-4-on-2-yl)pyridines have been prepared and characterized by ¹H-, ¹³C-NMR and elemental analysis. These ligands form complexes with FeCl₂, which were isolated and characterized by elemental analysis and MS. Both in the solid phase and in aqueous and methanolic solutions the complexes have two ligand molecules per one ferrous ion. By means of the kinetic stopped-flow method, the rate constants of acid catalyzed decomposition, which proceeds as a system of two consecutive reactions, were determined. It was found that increasing steric demands of 5-alkyl substituent in the 4,5-dihydro-1H-imidazol-4-one cycle significantly retards the acid catalyzed solvolysis. The methyl derivative is solvolyzed cca two orders faster than the corresponding t-Bu derivative.     

Diether polyphenylquinoxalines. Monomers via nitro displacement–13C-NMR analysis of monomers and polymers

Bisbenzils have been prepared by nucleophilic aromatic nitro displacement reactions. These materials, some model compounds, and some polyphenylquinoxaline polymers prepared from them were rigorously analyzed by ¹³C-NMR spectroscopy. The observed chemical shifts were in excellent agreement with calculated values based on model systems. This served as the strongest evidence to support the proposed structures and gave additional intimate structural information on the polyphenylquinoxalines.     

Investigation of a resorcinol–formaldehyde resin by 13C-NMR spectroscopy and intrinsic viscosity measurement

Various molecular weight fractions and oligomers isolated from a novolac type resorcinol–formaldehyde (RF) wood adhesive resin were studied by ¹³C-NMR, gel permeation chromatography (GPC), and intrinsic viscosity measurements. The ¹³C-NMR results indicate that methylene groups occur mostly between C₄–C′₄ and C₂–C′₄ carbons and in minor amounts between C₂–C′₂ carbons of resorcinol rings. By suppressing the nuclear Overhauser effect (nOe) and using a long delay time for ¹³C-NMR measurements, reasonable methylene/aromatic carbon ratios were obtained from the aromatic C₁ carbon patterns that have resulted from different numbers of methylene groups bonded on the aromatic ring. This analysis results indicate that the RF resin has an appreciable amount of branch structures and compare favorably with the gel permeation chromatography and intrinsic viscosity measurement results. © 1993 John Wiley & Sons, Inc.     

13C-NMR.-Substituenteninkremente der N3-Gruppe in Azido-1,4-benzochinonen

¹³C-NMR. Substituent Increments of the Azido-group in 1,4-Benzoquinones The empirical substituent increments of the azido-group in 1,4-benzoquinones have been derived from the ¹³C-NMR. chemical shifts of 2-azido-5-chloro-1,4-benzoquinone. The validity of the obtained values was tested by comparison of the empirical and computed ¹³C-NMR. chemical shifts of other azido-chloro-1,4-benzoquinones.     

Investigation of the formation of poly(ethylene terephthalate) with model molecules: Kinetics and mechanisms of the catalytic esterification and alcoholysis reactions. II. Catalysis by metallic derivatives (monofunctional reactants)

The reactions of model molecules were considered to study the kinetics and the mechanism of the esterification and the alcoholysis reactions catalyzed by various metal compounds (Li, Na, K, Zn, Co, Mn, Ti). Ti was found to be the most active catalyst for both reactions and to be acting via a different mechanism (concerted). The possible structure of the active titanium intermediate was investigated by means of ¹H- and ¹³C-NMR spectroscopy, FT–IR spectroscopy, and electroconductivity.     

Kinetic study of 2‐furanaldehyde, 3‐furanaldehyde,and 5‐methyl‐2‐furanaldehyde reactions initiated by Cl atoms

Rate coefficients for the gas‐phase reactions of chlorine atoms with a series of furanaldehydes have been determined at 298 ± 2 K and atmospheric pressure (708.5 ± 0.1). The experiments were performed using the relative technique combined with solid‐phase microextraction (SPME) sampling and gas chromatography with flame ionization detection (GC‐FID). Rate constants were determined relative to the reaction of Cl with n‐nonane and 2‐ethylfuran. The absolute rate coefficients k (in units of 10^?10 cm³ molecule^?1 s^?1) obtained were 2.61 ± 0.27 for 2‐furaldehyde, 3.15 ± 0.27 for 3‐furaldehyde, and 4 ± 0.5 for 5‐methyl‐2‐furaldehyde. This study shows that the reactions of furanaldehydes and Cl are very fast with little influence of the position of the aldehyde group or the presence of other substituent on the reactivity. The results seem to indicate a mechanism involving two main reaction channels, addition of chlorine atom to the double bond of the aromatic ring, and the abstraction of the aldehydic hydrogen. Further product studies are necessary to determine the mechanism of these reactions in more detail. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 670–678, 2008     

Identification of microbial inhibitory functional groups in corn stover hydrolysate by carbon-13 nuclear magnetic resonance spectroscopy

Dilute-acid biomass hydrolysates contain biomass degradation products that are inhibitory to cell growth and fermentation. Overliming with Ca(OH)₂ has been found to be one of the most effective methods for detoxifying the dilute-acid hydrolysate for ethanol production. However, the mechanism of overliming is not well understood. Carbon-13 nuclear magnetic resonance (¹³C-NMR) spectroscopy was used to elucidate the functional groups involved in the overliming reaction. The ¹³C-NMR spectra showed that the major functional groups removed during the overliming process were aliphatic and aromatic acids or esters, and other aromatic and aliphatic compounds. Ketone and aldehyde functionalities were not detected in the spectra. This is the first time that ¹³C-NMR has been used to elucidate the overliming reaction.     

An “Ortho Effect” in Electrophilic Aromatic Nitrations: Theoretical Analysis and Experimental Validation

Usually, a π‐donor substituent acts as an ortho/para directing group in an electrophilic aromatic substitution reaction, and a π‐acceptor substituent acts as a meta directing group. Interestingly, when a π‐acceptor substituent is meta to a π‐donor substituent, certain electrophilic aromatic nitration occurs ortho to the acceptor substituent rather than para. The “ortho effect”, highlighted in various text books, has been tentatively analyzed here based on ab initio calculations. The reliability of the calculations was verified by the corresponding experimental data, including a new‐designed electrophilic aromatic nitration that also gave reasonable product distributions.     

Preparation and characterization of cellulose β-ketoesters prepared by homogeneous reaction with alkylketene dimers: comparison with cellulose/fatty acid esters

Cellulose was dissolved in lithium chloride/1,3-dimethyl-2-imidazolidinone (LiCl/DMI), and reacted with alkylketene dimers (AKDs) under non-aqueous and homogeneous conditions to prepare cellulose/AKD β-ketoesters with high degrees of substitution (DS). Six AKDs synthesized from octanoic, decanoic, dodecanoic, tetradecanoic, hexadecanoic and octadecanoic acids via their fatty acid chlorides were used in this study. The cellulose/AKD β-ketoesters obtained were gummy solid at room temperature, and had DS values ranging from 1.9 to 2.9. Cellulose/fatty acid esters with DS 2.5–2.9 were also prepared as references. ¹³C-NMR spectra of the cellulose/AKD β-ketoesters showed that cellulose carbons and substituent carbons close to cellulose chains were restricted in motion and behaved like solid in solutions. In contrast, the cellulose/fatty acid esters did not demonstrate such anomalous ¹³C-NMR spectra. The unique ¹³C-NMR patterns are characteristic for the cellulose/AKD β-ketoesters, which have long and branched alkyl substituents in each anhydroglucose unit. Size-exclusion chromatography furnished with multi-angle laser light scattering (SEC-MALLS) revealed, on the other hand, that all cellulose/AKD β-ketoesters and cellulose/fatty acid esters prepared had flexible or random-coil conformations in tetrahydrofuran (THF). There were no clear differences in conformation or stiffness of cellulose chains between cellulose/AKD β-ketoesters and cellulose/fatty acid esters.     

1H- und 13C-NMR.-Spektren von 6-(p-X-Phenyl)fulvenen. Kurzmitteilung

¹H- and ¹³C-NMR.-spectra of 6-(p-X-phenyl)fulvenes The ¹H- and ¹³C-NMR.-spectra of a series of 6-(p-X-phenyl)fulvenes 3 , measured at 9.39 T (93.9 kgauss), have been analyzed. In these compounds, electronic effects due to the substituent X clearly exert changes in chemical shifts as well as in coupling constants in the 5-membered ring. Small changes in bond length are observed by comparison of the vicinal ¹H, ¹H-coupling constants, whereas changes in charge densities linearly influence the chemical shifts of C(5), C(2) and C(3).     

Résonance Magnétique Nucléaire de 17O. Aldéhydes et cétones aliphatiques: additivité des effets de substitution et corrélation avec la 13C-RMN

¹⁷O-NMR. Aliphatic aldehydes and ketones, additivity of substituent effects and correlation with ¹³C-NMR. The ¹⁷O-chemical shifts of 9 aldehydes, 22 aliphatic and 4 alicyclic ketones, in the natural abundance FT.-NMR. spectra followed a good correlation with the ¹³C-chemical shifts of the terminal C-atoms of corresponding methylene compounds. An additivity relation involving 6 parameters represents the ¹⁷O-shifts of 28 of the measured products with a standard deviation of 2.5 ppm. The additivity parameters are discussed with respect to the modifications of the polarity of the carbonyl group induced by the hyperconjugative interaction of π and π* orbitals with the πCH ₃ orbitals of the alkyl substituent groups.     

Hyperfine shifts of the 13C-NMR. in low spin iron(3) porphyrin complexes

The ¹³C-NMR. in Zn(II)(Porphin), Fe(III)(Porphin)(CN₂), Zn(II)(Tetraphenyl-porphin), and Fe(III)(Tetraphenylporphin)(CN₂) have been identified, and the ¹³C hyperfine shifts in the iron complexes evaluated. It was found that dipole-dipole coupling with the electron spin localized in the π-orbitals of the aromatic carbon atoms makes an important contribution, to the ¹³C hyperfine shifts. In a preliminary analysis the experimental spin density distribution obtained from the combined ¹H- and ¹³C-NMR.-data is compared with theoretical models of the iron porphyrin complexes.     

Amine‐blocked polyisocyanates. I. Synthesis of novel N‐methylaniline‐blocked polyisocyanates and deblocking studies using hot‐stage fourier transform infrared spectroscopy

A series of substituted N‐methylaniline‐blocked polyisocyanates based on 4,4′‐methylenebis(phenyl isocyanate) and poly(tetrahydrofuran) were prepared and characterized thoroughly with FTIR, ¹H NMR, and ¹³C NMR spectroscopy methods. Compared with unsubstituted N‐methylaniline, a blocking agent with an electron‐releasing substituent at the para position took a shorter time, whereas those with an electron‐releasing substituent at the ortho position or an electron‐withdrawing substituent at the ortho and para positions took longer times for the blocking reaction. The thermal dissociation reactions of blocked polyisocyanates were carried out with an FTIR spectrophotometer attached to hot‐stage accessories under dynamic and isothermal conditions. The dynamic method was used to determine the deblocking temperature, and the isothermal method was used to calculate the deblocking kinetics and activation parameters. The cure times of blocked polyisocyanates with hydroxyl‐terminated polybutadiene were also determined. The deblocking temperatures, the results of cure‐time studies, and the kinetic parameters revealed that the thermal dissociation of the N‐methylaniline‐blocked polyisocyanates was retarded by electron‐donating substituents and facilitated by electron‐withdrawing substituents. The action of N‐methylanilines as blocking agents for isocyanate was explained by the formation of a four‐center, intramolecularly hydrogen‐bonded ring structure during the thermal dissociation of the blocked polyisocyanates. The formation of such a hydrogen‐bonded ring structure was confirmed and supported by variable‐temperature ¹H NMR studies and entropy parameters, respectively. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1557–1570, 2007     

1H- and 13C-NMR. Studies of some metal complexes of o,o′-dihydroxyazobenzenes

The aromatic ¹H- and ¹³C-NMR. spectra of some metal complexes of o, o′-dihydroxyazobenzenes are shown to be useful in distinguishing the two possible isomers (acolar and discolar) stemming from the non equivalence of the two ligating azo nitrogen atoms. The ortho aromatic carbon atoms, C(6) and C(12) experience relatively large upfield shifts between 12.8 and 15.7 ppm when the adjacent nitrogen atom is coordinated. The protons attached to these carbon atoms are shifted downfield. The values ⁿJ (¹⁵N, ¹³C) for the ligand 2,2′-dihydroxy-3-methyl-4′-chloro-5-(t-butyl)-¹⁵N-azobenzene are reported.     

Mechanism and kinetics of the polymerization of thermoplastic polyimides. I. Study of the pyromellitic anhydride/aromatic amine system

We have synthesized model compounds representing the repeating units of a polyamic acid prepared from pyromellitic anhydride and an aromatic amine. They were characterized with HPLC, ¹H and ¹³C liquid and solid NMR and FTIR. The Kinetic study of the ring dehydration reaction of model compounds showed that the reaction paths were very different, depending on the experimental conditions (isotherms, liquid, or solid states). They could lead to a deterioration of the polyamic of the polyamic acid by the formation of side reactions. These reactions could predominate as a result of the existence of an equilibrium between the amic acid group and starting monomers.     

Ester derivatives of α-hydroxymethylacrylates: Itaconate isomers giving high molecular weight polymers

New ester derivatives of ethyl α-hydroxymethylacrylate were synthesized using acid chlorides (traditional solution reactions), sodium salts of acids (with phase transfer catalysis), and trifluoroacetic anhydride (trifluoroacetate). The interfacial process gave high yields of clean products under very mild conditions. Derivatives obtained include the formate, acetate, hexanoate, stearate, benzoate, trifluoroacetate, and adamantanoate. Bulk polymerizations with 2,2′-azobis (isobutyronitrile) gave high molecular weight polymers with intrinsic viscosities of over 2 dL/g and molecular weights of several million [based on size-exclusion chromatography (SEC) comparison to polystyrene standards]. These high molecular weights were the result of autoacceleration in the bulk as shown by monitoring molecular weight with respect to conversion. Solution polymerization in benzene gave more typical polymer, e.g., the acetate derivative showed an SEC molecular weight of 52,000. Glass transition temperatures for the n-alkyl esters decreased from the formate (77°C) to the hexanoate (15°C); the stearate showed a side-chain melting point of 40°C but no T_g. Glass transitions were observed for the trifluoroacetate, benzoate, and adamantanoate polymers at 69, 130, and 214°C, respectively. Solution ¹³C-NMR showed evidence of tacticity information for the formate and acetate derivatives with appaent preference for syndiotactic polymer formation similar to that of methyl methacrylate. FTIR and solid-state ¹³C-NMR analysis gave spectra with functional group peaks and chemical shift values expected based on composition. The stearate monomer and polymer gave solid-state ¹³C chemical shifts of 34 and 33 ppm, respectively, for the central CH₂ units consistent with monoclinic and orthorhombic crystal packing. © 1994 John Wiley & Sons, Inc.     

Einfluss der Orientierung von Substituenten auf die chemische Verschiebung von 13C. II. 13C-NMR.-spektroskopische Untersuchung von substituierten Spiro [cyclopropan-1′,9-fluorenen]

Effect of the Orientation of Substituents on the Chemical Shift of ¹³ II. ¹³C-NMR. Study of Substituted Spiro [cyclopropane-1′, 9-fluorenes] The ¹³C-NMR. spectra of spiro[cyclopropane-1′, 9-fluorenes] substituted at the cyclopropane ring have been assigned, largely with the aid of selective proton decoupling. The effects of the substituents upon the chemical shifts of the aromatic carbons are orientation dependent. the effects of cis-substituents on γ-carbon atoms are large and negative (inducing high field shifts), those of trans-substituents smaller and dependent on the electron donor/acceptor properties of the substituents. The effects on δ-carbon atoms in close proximity of the substituents are large and positive; they are interpreted in terms of electric field effects.     

Aza-2-diènes-1,3. 2. Synthèse et structure d'un ylure d'azométhine stable

2-Aza-1,3-dienes. 2. Synthesis and Structure of a Stable Azomethine-ylid. Thermal treatment of 2-(2′-cyano-2′-methoxycarbonyl)methylide-amino-1-morpholino-cyclohexene results in the formation of a stable isomere. Its azomethine-ylid structure is demonstrated from the spectroscopic data (UV., IR., ¹H- and ¹³C-NMR.). 1,3-dipolar cycloaddition reactions of this ylid leads to spiropyrrolines.