Stella Chapelle

Configurational structures of lactic acid stereocopolymers as determined by 13C{1H} n.m.r.

Abstract Investigations are made of the 13C{1H} n.m.r. spectra of a series of poly(lactic acid) stereocopolymers obtained by different means, i.e. ring-opening polymerization of l -, rac- and meso-lactides initiated by powdered Zn, or copolymerization of l - and rac-lactides in different proportions, or condensation polymerization of rac-lactic acid. Resonances arising from the stereosensitivity of the carbonyl carbon atoms have been resolved using the resolution enhancement technique. The fine structures thus obtained are discussed in terms of configurational sequences. Comparison is made of the peak intensities with theoretical stereosequence distributions obtained when assuming single and pair additions of repeating units according to the Bernoullian statistics. Neither of the models for triad, tetrad and pentad stereosequences agrees with the area of the experimental lines. It is shown that redistribution of stereosequences in poly(lactic acid) occurs because of transesterification at the ester bonds in the melt. The final stereosequence distribution in the polymers obtained by ring-opening polymerization of lactides is discussed in relation to the pair-addition mechanism and to the redistribution due to transesterification reactions.

A 11b and 13c nmr determination of the structures of borate complexes of pentoses and related sugars

Abstract A 11B NMR study demonstrated that the four pentoses, like hexoses, form concurrently 1:1 and 1:2 borate complexes in aqueous solution, contrary to earlier reports. It also showed that in both species, the borate ion was bound to a vicinal diol site. The corresponding stability constants were determined by a potentiometric method. The structures of most 1:2 complexes were precised by 13C NMR spectroscopy, since the carbon atoms which bear the chelating diol group were clearly deshielded and had increased 1JCH values. The results were rationalised by grouping the sugars in series of related configurations. All sugars were complexed in furanose form. Those of the arabino - galacto - fructo series and of the xylo - gluco - sorbo series gave the same type of complex involving the anomeric hydroxyl group and the nearest ring CHOH. The very stable 1:2 complex of D-ribose was shown to be a mixture of two species in constant ratio, borate being bound either at C-1, C-2 or at C-2, C-3. The same result was obtained with the sugars of the lyxo - manno - tagato series, which also possessed two cis CHOH groups.

A 125Te NMR study of the exchange reaction between diarylditellurides

Abstract 125 Te NMR is used to study the chemical exchange between diarylditellurides. It is shown that the exchange is slow, and that is a thermal process. NMR parameters (δ, T 1 , 1 J (TeTe)) of 125 Te of ditellurides are given.

The synthesis and spectroscopic examination of telluronium salts based on the 1-organo-3,4-benzo-1-telluracyclopentane cation

Abstract The syntheses of a new range of telluronium salts based on the 1-organo-3,4-benzo-1-telluracyclopentane cation are reported (C8H8TeRX: R  CH3, CH2CH3, X  I; R  CH3, X  PhCOO; R  CH3, X  ClO4; R  CH3, X  BPh4; R  Ph, X  BPh4; R  CH2Ph, CH2CHCH2, CH2COPh, X  Br.) Solution studies using 1H, 13C and 125Te NMR spectroscopy in conjunction with conductivity and molecular weight measurements show the salts to be associated via weak “ionic” bonds in solvents of lower polarity such as chloroform but to be more ionic and strongly solvated in DMSO. Solvents such as DMF provide an intermediate and more complex situation. Mass spectra indicate that association can extend to the gas phase and some novel features in the spectra are discussed. Infrared studies indicate polymorphism for 1-methyl-3,4-benzo-1-telluracyclopentane perchlorate with one form containing perchlorate ions interacting with tellurium.

Relationship between structures and stabilities of molybdate complexes of alditols: A potentiometric, 13C and 95Mo nmr study

Abstract Dinuclear molybdate complexes of alditols have been studied in aqueous acidic solution by 95 Mo and 13 C NMR. Their formation constants were determined by potentiometry. In all complexes, the dimolybdate group was chelated by four vicinal hydroxyls. Two types of complexes were characterized by NMR, depending on the erythro or threo configuration of the central diol group. Their structures were precised using literature crystal data. A single asymmetrical complex of the first type was formed by symmetrical erythro ligands (erythritol, galactitol), while the asymmetrical d -arabinitol and D -mannitol formed pairs of isomeric species of this type. Ligands with complexing sites involving a central threo group ( DL -threitol, xylitol) were complexed in a symmetrical manner. D -Glucitol formed four complexes as pairs of each type. Rules relating the stabilities of the complexes to the structures of the ligands are presented.

Synthesis and spectroscopic investigations of 1-organo-1-halo-1-telluracyclopentane

Abstract A new class of cyclic telluronium salts has been prepared. All the salts are stable in solution in CHCl 3 or dimethylsulphoxide (DMSO). Conductivity measurements in DMSO and dimethylformamide (DMF) have shown that considerable ion pairing occurs in solution. Infrared, 1 H, 13 C, 125 Te NMR, and mass spectra are reported and discussed.

Evidence for molybdate complexes of ketoses and aldosesin the furanose form: a 13C and 1H NMR study

Abstract The formation of dimolybdate complexes of ketoses of the ribo and lyxo series and of aldoses of the lyxo series was investigated by 13 C and 1 H NMR spectroscopy. All these sugars possess two cis hydroxyl groups adjacent to the anomeric centre and form two different series of complexes involving the furanose forms. The tridentate complexes of 2-ketoses involve the ring HO-3,4 and the cis exocyclic HO-1 of the β ( ribo series) or of the α ( lyxo series) anomer, whereas the trans anomeric HO-2 is not bound to molybdenum. In the lyxo series, the β anomers form tetradentate complexes involving HO-1,2,3,5 (aldoses) or HO-2,3,4,6 (ketoses), that appear similar to a β - d -lyxofuranose complex already characterized in the solid state. A mixture of tridentate and tetradentate species was obtained in the case of d -tagatose. The relative stabilities of the complexes are discussed.

Structures of the borate complexes of D-allose, D-talose, and D-psicose in aqueous solution: an 11B- and 13C-N.M.R. study

Abstract The formation of 1:1 or 1:2 borate-sugar complexes by sugars having ribo configurations has been studied by 11B- and 13C-n.m.r. spectroscopy. Two 1:2 complexes can be formed, depending on whether the sugar is α or β. The main species involved cis-HO-1,2. d -Psicose formed a single complex at HO-2,3. A second species was formed by d -talose (10%), d -ribose (30%), and d -allose (30%), which involved cis-HO-2,3 with HO-1,2 trans. The order of stabilities of the complexes was d -psicose > d -ribose > d -talose > d -allose. The high affinity of ribo sugars towards borate is discussed. There was no correlation between the stability constants and the relative proportions of 1:2 complexes.

Acyclic molybdate complexes of aldoses in the arabino and xylo series and their application to the determination of the proportion of acyclic forms in aqueous solution

Abstract Aldoses of the arabino and xylo series with molybdate ions in aqueous acidic medium, form tetradentate acyclic complexes which are much weaker than those of the related alditols. The formation constants ( K f ) of these complexes were obtained by two independent methods, potentiometry and UV spectrophotometry, which gave values in good agreement. The structures of the complexes with aldoses were shown by 13 C NMR spectroscopy to involve the ligand in its acyclic form. A study of the complexes of carbohydrate derivatives proved that the magnitude of K f was mainly dependent on the configuration of the site of chelation. The lower stabilities of the aldose complexes were due to the endergonic opening of the pyranose heterocycle in the first step of the complex formation. An application to the determination of the equilibrium constant for the ring-opening reaction of aldopyranoses is described. It ultimately allowed the calculation of the proportion of acyclic forms (aldehyde and hydrate) in aqueous solutions of the aldoses in the arabino and xylo series.

A 13C and 183W NMR study of the structures of tungstate and molybdate complexes of volemitol

Abstract 1 H, 13 C, and 183 W NMR spectroscopic methods were used for the structural characterization of the tungstate and molybdate complexes of volemitol ( d - glycero - d - manno -hepitol) in aqueous solution. The major species (type E) are a pair of isomeric complexes formed at the arabino site in reversed orientation, i.e., HO-1,2,3,4 and HO-4,3,2,1. A single, minor complex is formed at the HO-3,4,5,6 altro site and is shown to be isostructural with the known single complexes of ribitol and d -altritol (type E′). The present NMR results support the hypothesis that complexes of type E′ are weaker than those of type E because they are destabilized by a steric strain due to the interaction of the side chain of the ligand with the site of chelation.