Aufschlu? von Zinn(IV)-oxid

Ohne Zusammenfassung

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Decomposition of tin(IV) oxide

     

Auxiliary controlled singlet-oxygen ene reactions of cyclohexenes

The photooxygenation of homochiral cyclohexene ketals, which are easily available from 2-cyclohexenone and l-tartrates, affords hydroperoxides and after reduction the corresponding allylic alcohols in good yields and high regioselectivities. This can be rationalized by electronic repulsions in a perepoxide intermediate and provides evidence for unfavorable 1,3 diaxial interactions with a dioxolane oxygen atom. Only low stereoselectivities were observed, due to the flexibility of the cyclohexene ring. However, the diastereomers could be separated and after cleavage of the auxiliary, 4-hydroxy-2-cyclohexen-1-one was isolated in enantiomerically pure form, which can serve as a building block for natural product synthesis.     

Threshold behavior of the formation of nanometer islands in a Ge/Si(100) system in the presence of Sb

Atomic-force microscopy is used to study the behavior of an array of Ge islands formed by molecular-beam epitaxy on an Si (100) surface in the presence of an antimony flux incident on the surface. It is shown that, as the Sb flux increases to a certain critical level, the surface density of the islands increases; however, if this critical level is exceeded, nucleation of the islands is suppressed and mesoscopic small-height clusters are observed on the surface. This effect is explained qualitatively in the context of a kinetic model of the islands’ formation in heteroepitaxial systems mismatched with respect to their lattice parameters.