Stabilization of p-block organoelement terminal hydroxides, thiols, and selenols requires newer synthetic strategies

Metal hydroxides represent a very interesting and highly useful class of compounds that have been known to chemists for a very long time. While alkali and alkaline earth metal hydroxides (s‐block) are commonplace chemicals in terms of their abundance and their use in a chemical laboratory as bases, the interest in Brønsted acidic molecular terminal hydroxides of p‐block elements, such as aluminum and silicon, has been of recent origin, with respect to the variety of applications these compounds can offer both in materials science and catalysis. Moreover, these systems are environmentally friendly, relative to the metal halides, owing to their ‐OH functionality (resembling that of water). Design and conceptualization of the corresponding terminal thiols, selenols, and tellurols (M? SH, M? SeH, and M? TeH) offer even more challenging problems to synthetic inorganic chemists. This concept summarizes some of the recent strategies developed to stabilize these otherwise very unstable species. The successful preparation of a number of silicon trihydroxides a few years back resulted in the generation of several model compounds for metal–silicates. The recent synthesis of unusual aluminum compounds such as RAl(OH)₂, RAl(SH)₂, and RAl(SeH)₂ with terminal EH (E=O, Se, or Se) groups is likely to change the ways in which some of the well‐known catalytic conversions are being carried out. The need for very flexible and innovative synthetic strategies to achieve these unusual compounds is emphasized in this concept.     

Recent applications of the ring-oven technique : A brief review

The ring-oven technique is a special type of spot analysis. In this review, papers published since 1976 are discussed. The aspects dealt with include developments in the technique and its use, the identification and determination of inorganic ions, organic substances, enzymatic analysis, applications to air and water samples, and combinations with other techniques.     

Darstellung und Kristallstruktur von Cs2Sb4S7

Preparation and Crystal Structure of Cs₂Sb₄S₇ The first thioantimonite of Cesium has been synthesized and its crystal structure determined. The substance crystallizes monoclinic with spacegroup P2₁/_c. The lattice constants are a = 1111.2(5) pm, b = 1227.1(5) pm, c = 1163.7(5) pm and ß = 97.60(5)°. There are four formula units in the unit cell. Sb–S chains are formed by trigonal SbS₃ pyramids and ψ-trigonal SbS₄ bipyramids.     

Fused lactones as a route to functionalized 1-substituted indoles

Indole lactones 8 and 14 were synthesized as a means of functionalizing the 1-isopropyl substituent of the indole nucleus. Lactone 8 , upon reduction with diisobutylaluminum hydride, produces lactol 9 which behaves like a masked hydroxy aldehyde and undergoes a Horner-Emmons reaction with triethyl phosphonoacteate to give α,β-unsaturated ester 3 which possesses a hydroxyl group on the 1-isopropyl moiety.     

Fractional revivals in the rovibrational motion of I2

Motivated by pump-probe experiments of I(2) in a room-temperature sample, the detection of fractional revivals is investigated using full-dimensional quantum wave packet calculations. It is shown that the structures observed in the pump-probe signal depend sensitively on the probe parameters employed and that the observed signal reflects a particular phase effect between fractional revivals.     

Chiroptical detection during liquid chromatography,part 5: On-line measurement of circular dichroism spectra Δε(λ) during stops of chromatographic flow

Summary A procedure is described which serves to measure circular dichrograms () on line during stops of flow in liquid chromatography. Since the concentration of substrate in the spectrometer cell during the stop is not known, the differential absorption coefficients are calculated from the experimental differential absorbances A by means of UV absorption (i. e. photomultiplier voltage) data. Verifications of the procedure are obtained by its application to three substrates (Table 1), the () spectra of which were known. The present on-line technique is compared with a corresponding off-line method.The N,N-dimethylthiobenzamides1 and2 as well as the 9,10-phenanthrenequinone7 consist of interconvertible enantiomers because their planar states are destabilized by steric overcrowding of groups. The unknown dichrograms () of1, 2 and7 are obtained (Figs. 2 and 4) and discussed with reference to the helicities of these molecules.In memory of the late Professor Dr. Dr. h. c. Günther Snatzke.     

Solid-phase development of chiral phosphoramidite ligands for enantioselective conjugate addition reactions

The development of a method for the optimization of chiral ligands for the steric steering of enantioselective Cu-catalyzed conjugate additions of Znalkyls to enones is described. The method is based on combinatorial principles and solid-phase techniques. It includes the combinatorial synthesis of chiral bispidine-derived ligands embodying a phosphoramidite group on the solid phase and their investigation in immobilized form in the conjugate addition of ZnEt2 to cyclohexenone as test reaction. The best identified ligands were also synthesized separately and investigated in its soluble form. The results obtained for the polymer-bound ligands correctly mirrored the performance of the soluble ligands. The library embodied members giving ee values varying between 3 and 67%. The "positional scanning" approach proved to be invalid for the study of the ligand system, indicating that this approach in general should be applied with care. Taken together, the method allowed for rapid and efficient optimization of the ligands and led to the development of the first enantioselective, Cu-catalyzed conjugate addition reaction with a polymer-bound ligand.     

The preparation of a cabbage reference material for environmental monitoring and food analysis

 Cabbage is frequently used in environmental monitoring and food control, and, hence, cabbage reference materials (RMs) are required for ensuring quality assurance. A cabbage RM was prepared in view of certification of specific elements from the "black list" of high toxicological interest and nutritive importance. All tasks of the RM production (production of the plant material, cutting and freeze-drying, determination of the residual water content, preparation of the RM, homogeneity testing, stability testing, certification measurements) are described in detail. Received: 12 November 1999 / Accepted: 29 January 2000     

The dynamics of rotational isomerism in crystals as studied by vibrational spectroscopy

Vibrational spectroscopy is uniquely capable of determining the structure and dynamics arising from the rotational degrees of freedom in molecular solids. Vibrational spectroscopy is sensitive to phenomena occurring on a time scale between the slow scale of magnetic, resonance methods and fast scale of diffraction methods; a time scale appropriate for both internal and overall rotation. Rotational motion of molecules in crystals provide examples of very simple reactions. Our understanding of the spectra of reacting molecules can thus be tested on these systems, and we conclude that Redfield equations can describe such spectra.A rich variety of motional effects are described: (1) The libration of the water of hydration in sodium perchlorate which illustrates a simple reacting system. (2) The libration of the adamantane molecule in both its ordered and disordered crystal phases which illustrates intermolecular interactions in organic crystals and the consequences of disorder. (3) The libration of the ammonium ion in crystals of ammonium salts which illustrated both change of orientational position by tunneling and the subtle orientating effects of isotopic substitution. (4) The internal rotation in n-alkane crystals which illustrates the ability to determine conformers and the relationship between the occurrence of disordered conformers and the occurrence of phase transitions.     

Totalsynthese von (+)-D-Homoöstron-3-methyläther

Total Synthesis of (+)-D-Homoestrone 3-methyl ether A novel total synthesis of (+)-D-homoestrone 3-methyl ether ( 21 ) is described starting from (S)-8a-methyl-3,4,8,8a-tetrahydro-2H, 7H-naphthalene-1,6-dione ( 1 ) as a chiral synthon for the rings C and D. The key step involves alkylation of the derived 3 with m-methoxyphenacyl bromide ( 4 ) as an AB-building block to give the dioxo-secosteroid 5 . Hydrogenation of 5 affords the trans-decalone 11 . As by-products the epimeric cis-decalones 12 and 13 were characterized. Cyclization of 11 leads under kinetic control predominantly to the Δ⁹⁽¹¹⁾-tetraene 14 . Catalytic hydrogenation of 14 and subsequent modification in ring D give the title compound 21 . It was found that 14 and also the derived Δ⁸-isomer 15a add hydrogen from the α-face of the molecule to an extent of about 80%. The 8α-D-homoestrone derivatives 20a and 23 as well as the 9β-isomers 19a and 22 were characterized.