Polyfluorothiolate derivatives of rhodium(I). Crystal and molecular structure of [Rh(μ-SC6HF4)(C8H12)]2

Summary The thiolato-bridged dinuclear compounds [Rh(-SR)-(COD)]₂, where R=p-C₆HF₄ (1),p-C₆H₄F (2) and CF₃ (3), are obtained from the chloro-bridged analogue by ligand exchange.Compound (1) crystallizes in the space group P₁ with a=9.740(3)Å, b=11.642(4)Å, c=13.997(6)Å, =103.87(3)°, =106.98(3)° and =105.10(2)°; z=2. In this dinuclear molecule both Rh atoms have a square planar coordination sharing one edge, namely the two sulphur bridging atoms. The Rh—Rh separation of 2.96 Å is consistent with at most a very weak metal-metal interaction. Upon addition of CO the dimeric [Rh(-SR)(CO)₂]₂ (4), (5) and (6) are obtained, but addition of PPh₃ affords the monomeric species [Rh(SR)(PPh₃)-(COD)] (7), (8) and (9). Reactions of the dimeric tetracarbonyl derivatives with PPh₃ vary with the nature of R; [Rh(-SR)(PPh₃)(CO)]₂ is obtained when R=p-C₆H₄F (10) and CF₃ (11) but monomeric [Rh(SR)-(PPh₃)(CO)₂] (12) is produced when R=p-C₆HF₄. The latter mononuclear compounds, with R=p-C₆H₄F (13) and CF₃ (14), are also formed by reaction of [Rh(SR)-(PPh₃)(COD)] with CO.     

Elucidation of the structure of quindolinone,a minor alkaloid of cryptolepis sanguinolenta: Submilligram 1H-13c and 1H-15N heteronuclear shift correlation experiments using micro inverse-detection

Elucidation of minor natural product structures has been significantly augmented by inverse-detection; further improvement has been afforded by the development of micro inverse-detection probes. We report here the elucidation of the structure of a new alkaloid, quindolinone (5H, 10H-indolo[3,2-b]quinolin-11-one), from the West African plant Cryptolepis sanguinolenta. All nmr data for this minor, preparative hplc-isolated alkaloid, including ¹H-¹⁵N one? bond heteronuclear shift correlation (HMQC) data, were recorded on an 800 μg sample of the alkaloid dissolved in 140 μl of 100% d₆-DMSO using a 400 MHz spectrometer.     

INAA determination of holmium in submilligram samples of cosmochemical and geochemical interest and the second-order activation interference

Second-order neutron activation becomes a serious interference in NAA determinations of REE in submilligram samples of meteoritic and terrestrial materials when neutron fluences reach levels of 10²⁰ n·cm^–2 or more. The Dy-interference to Ho determination and the Eu-interference to Gd determination are quite, large. We discuss the importance of second-order activation interferences in REE determinations, and present an experimental method to correct the Dy-interference to Ho determination by INAA. Ho concentrations in splits of less than 0.2 mg of BCR-1 and SP can be accurately determined by INAA with a neutron fluence of 2·10²⁰ n·cm^–2 in spite of large Dy interferences of up to 200%. It is necessary for NAA determinations of REE with high neutron fluences that each REE standard be separated from its neighboring REE, in order to correct the second-order activation interferences experimentally.     

Asymmetric synthesis of α-hydroxy carboxylic acids: direct oxidation of chiral amide enolates using 2-sulfonyloxaziridines

The direct oxidation of chiral amide enolates to optically active mandelic acid using 2-sulfonyloxaziridine $\text{1}$ is described. The diastereoselectivity is counterion dependent.     

Unsymmetrical selenides from selenocyanates and methyl grignard

The addition of methyl Grignard to diethyl acetamido(cyanoselenobenzyl)malonates 3 and 4 at ?78° followed by hydrolysis yields the 3-(4-and 3-methylselenophenyl)alanines 1 and 2.     

Methyl scanning: total synthesis of demethylasterriquinone B1 and derivatives for identification of sites of interaction with and isolation of its receptor(s)

The principle of methyl scanning is proposed for determination of the sites of interaction between biologically active small molecules and their macromolecular target(s). It involves the systematic preparation of a family of methylated derivatives of a compound and their biological testing. As a functional assay, the method can identify the regions of a molecule that are important (and unimportant) for biological activity against even unknown targets, and thus provides an excellent complement to structural biology. Methyl scanning was applied to demethylasterriquinone B1, a small-molecule mimetic of insulin. A new, optimal total synthesis of this natural product was developed that enables the family of methyl scan derivatives to be concisely prepared for evaluation in a cellular assay. The results of this experiment were used to design a biotin-demethylasterriquinone conjugate for use as an affinity reagent. This compound was prepared in tens of milligram quantities in a four-step synthesis.     

The chromatographic properties of a mixed-bed stationary phase combining reversed-phase and strong anion exchange properties

Summary The feasibility of using columns prepared by mixing together ion exchange and reversed-phase packing materials, for the simultaneous separation of ionised and unionised molecules, has been examined using a range of test solutes. Columns prepared in this way showed chromatographic properties which were intermediate between those of the individual phases. The dual nature of the retention mechanism allowed the retention of ionisable molecules to be adjusted using pH whilst that of uncharged compounds was unaffected. The simultaneous chromatography of model compounds and their glucuronide, sulphate and glycine conjugates, under a variety of conditions, was demonstrated.     

The carbonyl frequency of cyclopentanone as a function of its distortion,a vibrational model for 2- and 7-norbornanone

Using normal coordinate analysis, the infrared spectrum of cyclopentanone has been computed as the geometry is modified from the planar form to those in which the geometry about the 5-membered ring looks like 2-norbornanone and also like 7-norbornanone. The computed v _co of cyclopentanone is 1739.9 cm^–1 compared to the experimental value of 1740 cm^–1.As the cyclopentanone ring is deformed to resemble either the 2-norbornanone or the 7-norbornanone ring system, the computed v _co is too low using the force fields that give good agreement with experimental value for cyclopentanone. The carbonyl force constant had to be increased from 9.7 to 10.0 mdynes/Å for the 2-norbornanone-like model and to 10.4 mdynes/Å for the 7-model system to give v _co values in agreement with the experimental values.

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Zusammenfassung Mit Hilfe der Normalkoordinaten-Analyse wird das Infrarotspektrum des Zyklopentanons berechnet, wobei die Geometrie von der planaren Form bis zu der Form, bei der die Geometrie am Fünfring derjenigen des 2-Norbornanons oder auch des 7-Norbornanons ähnlich wird, variiert wird. Der berechnete Wert von v _co des Zyklopentanons ist 1739,9 cm^–1, während der experimentelle Wert 1740 cm^–1 beträgt. Bei den angegebenen Deformationen des Zyklopentanonrings wird der berechnete Wert von v _co zu niedrig, wenn man die Kraftfelder zugrunde legt, die für das Zyklopentanon gute Übereinstimmung mit dem Experiment liefern. Die Carbonylkraftkonstanten mußten von 9,7 auf 10,0 m dyn/Å für das 2-Norbornanon-Modell sowie auf 10,4 m dyn/Å für das 7-Norbornanon erhöht werden, um den Wert von v _co in Übereinstimmung mit dem Experiment zu bringen.

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Résumé Le spectre infra-rouge de la cyclopentanone a été calculé en employant l'analyse en coordonnées normales lorsque l'on modifie la géométrie depuis la forme plane jusqu'aux formes o u les configurations sur le noyau pentagonal ressemblent à celles de la 2-norbornanone et de la 7-norbornanone. La fréquence _co calculée pour la cyclopentanone est 1739.8 cm^–1 comparée à la valeur expérimentale de 1740 cm^–1. Lorsque l'on déforme le cycle de la cyclopentanone comme indiqué ci-dessus, la valeur calculée de _co est trop faible si l'on emploie les champs de force qui donnent un bon accord avec la valeur expérimentale pour la cyclopentanone. La constante de force du carbonyle doit etre augmentée de 9,7 à 10,0 mdynes/Å pour le modèle analogue à la 2-norbornanone et de 9,7 à 10,4 mdynes/Å pour le modèle analogue à la 7-norbornanone.

     

The effect of platinum oxide films on reaction kinetics at platinum electrodes

It has been known for some time that the pre-treatment of platinum electrodes often effects subsequent electrochemical reactions. Part of the effect of pre-treatment is due to the fact that anodized or chemically oxidized platinum electrodes become coated to some degree with a film of platinum oxide. This work was concerned with quantitative measurement of kinetic parameters as a function of the extent of oxide film formation. Whenever possible, variations in reaction mechanisms are proposed.

Most of the experimental evidence has been taken from current-potential curves but the techniques of chronopotentiometry and microscopy were also used.

The reduction of vanadium^V chromium^VI, arsenic^V, iodate and oxygen were investigated as well as the oxidation of vanadium^IV, arsenic^III, oxalic acid, and formic acid. The presence of the platinum oxide film effected the reactions studied in a variety of ways but in every case some variation in the kinetic parameters of the reactions studied was recorded. For a number of cases, a modified oxygen bridge theory was found useful.     

Forces in molecular recognition: Comparison of experimental data and molecular mechanics calculations

Summary NMR studies of the rotation barrier of the disaccharide of the glycopeptide antibiotic vancomycin have been used to test the performance of computer simulation techniques using molecular mechanics. In the absence of any solvated water, no correlation could be found between experiment and calculation. By introducing solvent water molecules into the binding region of the antibiotic, the NMR results could be simulated both qualitatively and quantitatively within experimental error without using massive computational resources.