Untersuchungen an Nickeloxid-Mischkatalysatoren. I. Strukturelle Eigenschaften von NiO/SiO2-Katalysatoren

Studies on Nickel Oxide Mixed Catalysts. I. Structural Properties of NiO/SiO₂ Catalysts Structural properties of NiO/SiO₂ catalysts prepared by precipitation-deposition and impregnation have been investigated. A nickel-layer-silicate like structure is formed only in the precipitated catalysts showed infrared spectroscopic measurements. After thermal treatment at 723 K by means of magnetic measurements and reflectance spectroscopy besides Ni²⁺ ions in octahedral environment tetrahedral coordinated Ni²⁺ions were found. The part of tetrahedral coordinated Ni²⁺ ions is independent of the NiO-content up to 40 mole % NiO. Nickel oxide is formed above a content of 40 mole %. In the case of the impregnated catalysts nickel oxide cluster are formed on the surface after annealing at 723 K.     

Electron Excited L X-Ray Spectra of the Elements 24 ≤ Z ≤ 33

A reinvestigation was undertaken in order to obtain reliable data of the relative intensities of the L spectra for the elements 24 ≤ Z ≤ 33. A TAP crystal with a periodicity of 25.757 ? was used as the dispersing element. With this crystal one is able to resolve the lines/bands Ll = L₃M₁, Lη = L₂M₁, Lα_1,2 = L₃M_4,5, Lβ₁ = L₂M₄, and Lβ_3,4 = L₁M_2,3. Among the investigated elements ³³As is the only one for which the energy of the lines Lα_1,2 and Lβ₁ is below the L₃ absorption edge. For all the other elements the lines Ll, Lη, and Lα_1,2 are below the L₃ edge, whereas Lβ₁ and Lβ_3,4 are above this edge. This difference leads to effects of differential absorption, where the absorption is stronger for decreasing line energy. For the net peak height ratio β₁/α we obtained results which are of the same order of magnitude as those given by White and Johnson (W&J) in their popular tables. But for l/α and β_3,4/β₁ our results show an atomic number dependence which is completely different from those given by W&J.     

Total synthesis of (-)-bafilomycin A(1)

A highly stereoselective total synthesis of (-)-bafilomycin A(1), the naturally occurring enantiomer of this potent vacuolar ATPase inhibitor, is described. The synthesis features the highly stereoselective aldol reaction of methyl ketone 8b and aldehyde 60c and a Suzuki cross-coupling reaction of the highly functionalized advanced intermediates 12 and 39. Vinyl iodide 12 was synthesized by a 14-step sequence starting from the readily available beta-alkoxy aldehyde 14, while the vinylboronic acid component 39 was synthesized by a nine-step sequence from beta-hydroxy-alpha-methyl butyrate 44 via a sequence involving the alpha-methoxypropargylation of chiral aldehyde 49 with the alpha-methoxypropargylstannane reagent 54. Syntheses of fragments 12 and 39 also feature diastereoselective double asymmetric crotylboration reactions to set several of the critical stereocenters. The Suzuki cross-coupling of 12 and 39 provided seco ester 40, which following conversion to the seco acid underwent smooth macrolactonization to give 41. The success of the macrocyclization required that C(7)-OH be unprotected. The Mukaiyama aldol reaction between aldehyde 60c and the TMS enol ether generated from 8b provided aldol 65 with high diastereoselectivity. Finally, all silicon protecting groups were removed by treatment of the penultimate intermediate 65 with TAS-F (tris(dimethylamino)sulfonium difluorotrimethylsilicate), thereby completing the total synthesis of (-)-bafilomycin A(1).     

Steric effects in hula-twist photoisomerization. 1,4-dimethyl- and 2,3-dimethyl-1,4-diphenylbutadienes

1,4-Dimethyl-1,4-diphenylbutadiene was shown to be able to execute regiospecific HT-1 photoisomerization around a methyl group when irradiated in a low-temperature organic glass, albeit at reduced efficiency. 2,3-Dimethyl-1,4-diphenylbutadiene exhibited a different type of steric effect, causing the E,E-isomer to undergo regiospecific HT-1 photoisomerization.