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  • Artikel  (2)
  • Catalysis  (2)
  • 1995-1999  (1)
  • 1990-1994  (1)
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  • Artikel  (2)
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  • 1995-1999  (1)
  • 1990-1994  (1)
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  • 1
    ISSN: 1434-193X
    Schlagwort(e): Alkenes ; Epoxidation ; Manganese ; Porphyrins ; Catalysis ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A series of MnIII porphyrins progressively halogenated in the β-pyrrolic positions was employed to catalyse the epoxidation of cis-stilbene by iodosylbenzene, and to study the role of the electronic effects on the stereoselectivity of this process. A gradual improvement in the stereoselectivity on increasing the number of β-halogen atoms was observed. The role of steric effects upon the epoxidation was also investigated by placing ortho-substituents in the meso-phenyl rings, and it was found that steric effects are more important than electronic effects toward the stereoselectivity of this process. These results can be rationalised by proposing a competition between a nonstereoselective electrophilic pathway of addition and a stereospecific pathway of oxygen insertion, the former being disfavoured by electron-withdrawing substituents. Alternatively, the formation of an open intermediate between the MnV oxene and the substrate could be suggested, where the stereoselectivity ought to be determined by the competition between closure of the epoxide ring and rotation around the C-C bond. In this case, the enhanced stereoselectivity given by our polyhalogenated porphyrins might be attributed to an acceleration of the epoxide ring closure caused by the electron-withdrawing effect of the halogen substituents.
    Zusätzliches Material: 1 Ill.
    Materialart: Digitale Medien
    Standort Signatur Erwartet Verfügbarkeit
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  • 2
    ISSN: 0749-1581
    Schlagwort(e): meso-Tetra(α, β, α, β-o-camphanylamidophenyl)porphyrin ; Rhodium complex ; Catalysis ; 1H and 13C NMR ; Chemistry ; Analytical Chemistry and Spectroscopy
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The synthesis of a new chiral porphyrin, meso-tetra(α, β, α, β-o-camphanylamidophenyl)porphyrin, and its Mn(III) and Rh(III) derivatives is reported. Complete resonance assignments of both the 1H and 13C spectra of the ligand has been achieved via homo- and hetero-nuclear 2D NMR. Insertion of metal ions into the porphyrin ring was performed both in order to test the catalytic properties of the resulting complexes and to characterize the effect of of metal coordination on the spatial distribution of the bulky substituents. The Mn complex was then tested in catalytic chiral epoxidation reactions, giving low enantiomeric excesses (20%). The 2D NMR spectra of the Rh derivative indicate the existence of four molecular species generated by the interaction of the metal atom with the peripheral substituents of the porhyrin ring. In fact, a complex combination of axial coordination, together with the chirality of the camphanyl substituents, removed the symmetry of the porphyrin plane.
    Zusätzliches Material: 8 Ill.
    Materialart: Digitale Medien
    Standort Signatur Erwartet Verfügbarkeit
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