Modern separation techniques for the efficient workup in organic synthesis

The shift of paradigm in combinatorial chemistry, from large compound libraries (of mixtures) on a small scale towards defined compound libraries where each compound is prepared in an individual well, has stimulated the search for alternative separation approaches. The key to a rapid and efficient synthesis is not only the parallel arrangement of reactions, but simple work-up procedures so as to circumvent time-consuming and laborious purification steps. During the initial development stages of combinatorial synthesis it was believed that rational synthesis of individual compounds could only be achieved by solid-phase strategies. However, there are a number of problems in solid-phase chemistry: most notably there is the need for a suitable linker unit, the limitation of the reaction conditions to certain solvents and reagents, and the heterogeneous reaction conditions. Further disadvantages are: the moderate loading capacities of the polymeric support and the limited stability of the solid support. In the last few years several new separation techniques have been developed. Depending on the chemical problem or the class of compounds to be prepared, one can choose from a whole array of different approaches. Most of these modern separation approaches rely on solution-phase chemistry, even though some of them use solid-phase resins as tools (for example, as scavengers). Several of these separation techniques are based on liquid-liquid phase separation, including ionic liquids, fluorous phases, and supercritical solvents. Besides being benign with respect to their environmental aspects, they also show a number of advantages with respect to the work-up procedures of organic reactions as well as simplicity in the isolation of products. Another set of separation strategies involves polymeric supports (for example, as scavengers or for cyclative cleavage), either as solid phases or as soluble polymeric supports. In contrast to solid-phase resins, soluble polymeric supports allow reactions to be performed under homogeneous conditions, which can be an important factor in catalysis. At the same time, a whole set of techniques has been developed for the separation of these soluble polymeric supports from small target molecules. Finally, miscellaneous separation techniques, such as phase-switchable tags for precipitation by chemical modification or magnetic beads, can accelerate the separation of compounds in a parallel format.     

Nucleosides. Part L.. Structure of lumazine N1-(2′-deoxy-D-ribonucleosides) ( = 1-(2′-deoxy-D-ribofuranosyl)pteridine-2,4(1H,3H)-diones): A revision of the anomeric configuration

The anomeric configuration of the glycosidic bond in lumazine N¹-(2′-deoxy-D -ribonucleosides) 1–6 was investigated by NOE difference spectroscopy. The former configurational assignment of the α - and β -D -anomers 1 and 2, 3 and 4 , and 5 and 6 , respectively, has to be reversed to be in agreement with the physical data. Additional proof is presented by X-ray analysis of 3 and 6 . Chemical interconversions of 1-(2′-deoxy-β-D -ribofuranosyl)-6,7-diphenyllumazine ( 6 ) into 2,3′ -anhydrolumazine 2′-deoxyribonucleosides 16 and 17 are also in agreement with the revised anomeric configuration.     

Isotopic metrology of carbon dioxide. II. Effects of ion source materials,conductance, emission,and accelerating voltage on dual-inlet cross contamination

We report high-precision isotopic carbon dioxide measurements, made before and after ion source modification to gas isotope ratio mass spectrometry (IRMS) instruments. Measurement protocols were designed to explore the effects of ion source material substitution, source conductance, inlet pressure, electron emission, acceleration potential, and inlet changeover equilibration time. After modification of the IRMS instruments at the National Institute of Standards and Technology (NIST) and the Max-Planck-Institute for Chemistry (MPI-Mainz), immediate changes were observed. At NIST, measurements were no longer sensitive to inlet equilibration times greater than 15 s, and different settings of ion source conductance resulted in delta(13)C shifts of about 0.04 per thousand per 10 per thousand measurement difference between sample and reference, a five-fold improvement. No significant changes in machine performance were observed after a month of use. After a year, performance had degraded slightly, but was controlled by ion source cleaning and the use of low-energy ion acceleration to minimize sputtering. At MPI-Mainz, results were very similar. We report cross-contamination coefficients measured since 1996, and discuss the role of adsorption, ion implantation, and sputtering on cross contamination in mass spectrometry systems. We recommend that users of high-precision IRMS instruments test for and minimize the effects described.     

Zur Spurenanalyse sehr reinen Siliciums

Zusammenfassung Die zur Erzielung reproduzierbarer Ergebnisse bei Spurenanalysen höchster Empfindlichkeit zu beachtenden Bedingungen werden diskutiert. Hierzu gehören die Art der Zerkleinerung des Probematerials, die Vermeidung anorganischer Reagenzien und das Arbeiten im geschlossenen System. Eine Apparatur zum Ausschluß der Laboratoriumsatmosphäre wird beschrieben und eine Arbeitsvorschrift zur polarographischen Bestimmung der Elemente Blei, Cadmium, Eisen, Indium, Kupfer, Nickel, Thallium, Wismut und Zink bis 10^–6% in reinstem Silicium angegeben. Eisen und Thallium werden von den anderen Elementen durch Mikroextraktion im Spitzröhrchen abgetrennt. Der Zeitbedarf für eine Analyse in mehreren Parallelproben beträgt 7 bis 8 Stunden. Die Fehler der Einzelbestimmungen liegen auch bei den niedrigsten Konzentrationen unterhalb ± 20%.

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Summary The conditions under which reproducible results can be obtained in trace analyses of highest sensitivity are discussed. Among these factors are: the way in which the sample is pulverized, the avoidance of inorganic reagents, and working in a closed system. An apparatus for excluding the laboratory atmosphere is described and a procedure is given for the polarographic determination of lead, cadmium, iron, indium, copper, nickel, thallium, bismuth, and zinc down to 10^–6% in purest silicon. Iron and thallium are separated from the other elements by micro extraction in conical tubes. The time consumed for an analysis in several parallel samples is 7 to 8 hours. The errors in the individual determinations even at the lowest concentrations are less than ± 20%.

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Résumé Les résultats d'analyse très sensible pour des traces, ont été considérés dans le but de les rendre reproductibles, dans les conditions considérées. Rentrent dans cette catégorie, le procédé de division de l'échantillon à analyser, la précaution d'éviter les réactifs inorganiques et le travail en vase clos. On décrit un appareil pour se protéger de l'atmosphère du laboratoire et l'on donne un mode opératoire pour le dosage polarographique des éléments plomb, cadmium, fer, indium, cuivre, nickel, thallium, bismuth et zinc jusqu'à 10^–6% dans le silicium très pur. Le fer et le thallium sont séparés des autres éléments par micro-extraction dans le tube effilé. La durée nécessaire pour une analyse chez plusieurs essais simultanés demande 7 à 8 heures. Les erreurs des dosages individuels sont inférieures, pour les faibles concentrations, à ± 20%.

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Gewidmet zur 100. Wiederkehr des Geburtstages vonFriedrich Emich.

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Für die sorgfältige Ausführung von Versuchen danken wir frauI. Stumpe     

Controlled metalation of self-assembled porphyrin nanoarrays in two dimensions.

We report a bottom-up approach for the fabrication of metallo-porphyrin compounds and nanoarchitectures in two dimensions. Scanning tunneling microscopy and tunneling spectroscopy observations elucidate the interaction of highly regular porphyrin layers self-assembled on a Ag(111) surface with iron monomers supplied by an atomic beam. The Fe is shown to be incorporated selectively in the porphyrin macrocycle whereby the template structure is strictly preserved. The immobilization of the molecular reactants allows the identification of single metalation events in a novel reaction scheme. Because the template layers provide extended arrays of reaction sites, superlattices of coordinatively unsaturated and magnetically active metal centers are obtained. This approach offers novel pathways to realize metallo-porphyrin compounds, low-dimensional metal-organic architectures and patterned surfaces which cannot be achieved by conventional means.     

Absorption of ultrashort laser pulses in strongly overdense targets

Absorption measurements on solid conducting targets have been performed in s and p polarization with ultrashort, high-contrast Ti:sapphire laser pulses at intensities up to 5x10{16}W/cm{2} and pulse duration of 8 fs. The particular relevance of the reported absorption measurements lies in the fact that the extremely short laser pulse interacts with matter close to solid density during the entire pulse duration. A pronounced increase of absorption for p polarization at increasing angles is observed reaching 77% for an incidence angle of 80 degrees . Simulations performed using a 2D particle in cell code show a very good agreement with the experimental data for a plasma profile of L/lambda approximately 0.01.