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  • 1
    ISSN: 1434-193X
    Keywords: Cope rearrangements ; Isotope effects ; Solvent effects ; Thermochromism ; Transition states ; Valence Isomerisation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The deuterium-labelled 2,4,6,8-substituted barbaralanes [D5]-1a and b, and the two model barbaralanes 15 und 19 for the estimation of 13C chemical shifts in the slow-exchange limit are synthesised from bicyclo[3.3.1]nona-3,7-diene-2,6-dione (5). - The extension of Saunders′ isotopic perturbation method bridges the gap between the limiting cases “perturbation of shift equivalence in delocalised systems” and “perturbation of degeneracy” by considering the simultaneous presence of delocalised species of higher symmetry and skewed equilibria between localised molecules that are degenerate in the absence of the isotopic perturbation. An equation (Equation 7) is derived for such multi-component systems which describes the temperature dependence of the relative isotopic splittings in 13C spectra with three parameters, viz. the isotopic perturbation parameters ΔHP and ΔSP of the skewed equilibria and the enthalpy difference ΔH0 between the delocalised and localised species. - Relative isotopic splittings ΔδP/Δδ are calculated from estimated chemical shifts in the slow-exchange limit (Δδ) and isotopic splittings (ΔδP) of signals in variable-temperature 151-MHz 13C NMR spectra recorded for solutions of [D5]-1a and b in [D8]toluene and N,N′-dimethylpropylene urea. The results obtained from [D5]-1a in both solvents and from [D5]-1b in the former are compatible with either a skewed equilibrium between localised valence tautomers alone or the simultaneous presence of localised and small amounts of delocalised valence tautomers. In striking contrast, the small isotopic splittings themselves and their small temperature dependence, observed for solutions of [D5]-1b in N,N′-dimethylpropylene urea, demonstrate that one half of the solvated compound exists in the delocalised state [D5]-1b*, which is more stable by 2 kJ mol-1 than the equilibrating localised species [D5]-1b ⇌ [D5]-1b′.
    Additional Material: 9 Ill.
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  • 2
    ISSN: 1434-193X
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---No Abstract.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1434-193X
    Keywords: C-C coupling ; Cycloadditions ; Dehydrogenations ; Electron deficiency ; Isomerisations ; Oxidations ; Zwitterionic intermediates ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oxidation with barium manganate of the enol of tetraacetylethane (10) affords tetraacetylethylene (7a) in good yield. Treatment of the 1,3-diketones 15a and b with iodosobenzene in the presence of boron trifluoride does not result in oxidative coupling yielding 10 and diacetyldibenzoylethane 17, respectively, as has been reported by Moriarty et al. Instead the known difluoroborylenolates 16 are formed. Oxidative coupling of the sodium enolate of 15b with iodine affords the meso-tetraketone meso-17 and diacetyldibenzoylethylene (E)-7b besides small amounts of rac-17 and the bisenol 19. In the presence of 1,4-diazabicyclo[2.2.2]octane, meso-17 and 19 tautomerise yielding identical equilibrium mixtures of meso-17, rac-17, and 19 (26:29:45). Dehydrogenation with 5,6-dichloro-2,3-dicyanobenzoquinone of meso-17 yields (E)-7b and an isomer (77:23) which was assigned structure (Z)-7b on the basis of spectroscopic evidence. - Tetraacetylethylene (7a) reacts with 1,2-dimethylenecyclohexane to afford two different [4+2]cycloadducts, viz. the spirocyclic dihydropyran 25 (30 %) and the tetraacetyloctalin 27. Whereas the latter is stable, the former isomerises to the latter on heating. Formation of both [4+2]cycloadducts and the isomerisation 25 → 27 are interpreted by invoking the zwitterion 26 as common intermediate. Monoolefines that cannot generate stable carbenium ions don't give [4+2]cycloadducts with 7a. In contrast, α-methylstyrene yields the oxa Diels-Alder product 30. The structures of (E)-7b, 19, 25, and 27 are established by X-ray diffraction analyses.
    Additional Material: 5 Ill.
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  • 4
    ISSN: 1434-193X
    Keywords: Asymmetric synthesis ; Automerisation ; Circular dichroism ; Conformation analysis ; Enantiomeric resolution ; Polycycles ; Solid-state structures ; Solvent effects ; Thermochromism ; Transition states ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Diphenylbicyclo[3.3.1]nonane-2,6-dione rac-3 is resolved in 57 % overall yield by chromatographic separation of the diastereomeric (R)-N-(1-phenylethyl)carbamates 9 which are obtained from (R)-(1-phenylethyl) isocyanate (8) and the 6-hydroxydiphenylbicyclo[3.3.1]nonan-2-ones endo- and exo-4. The enantiomers (1R)- (e.r. = 98:2) and (1S)-3 (e.r. = 97:3) are regenerated from 9 by reduction with lithium aluminium hydride followed by Swern oxidation of the resulting diols 5. The title compound (1S)-1 is synthesised in three steps from (1S)-3 in improved yield on the route that had led to rac-1. The absolute configurations are established by X-ray diffraction analyses of the carbamates endo-(1R)-9 and exo-(1S)-9. X-ray diffraction analyses were also performed of the camphanoate (1R)-7, the intermediates rac-endo-4 and (1S)-3, and the title compound (1S)-1. Hydroxy ketone rac-endo-4 adopts similar conformations in the solid state and in solution as shown by a comparison of vicinal 1H,1H coupling constants from proton spectra with those calculated from torsional angles in the crystal. The molecular structures of (1S)-1 and (1S)-3 closely resemble those of the corresponding racemates investigated previously. These results show (i) that intermolecular interactions in the solid state are of minor importance and (ii) that the unusually long C2-C8 distance of (1S)-1 and rac-1 (168 pm) is a molecular but not an averaged property due to a non-degenerate Cope rearrangement in the crystal. CD spectra are reported for (1R)- and (1S)-3, the unsaturated dicarbonitrile (1S)-13, and (1S)-1. The CD spectrum of (1S)-1 exhibits a weak positive band at 459 nm where rac-1 shows a temperature-dependent absorption which has been assigned to the higher, vibronic state represented by rac-1*. The intensity of the weak CD band depends on the temperature and the solvents in the same way as the UV/Vis absorption of rac-1. This supports the conclusion that both bands originate from the same source, viz. the transition state 1* of the degenerate Cope rearrangement 1 ⇄ 1′.
    Additional Material: 11 Ill.
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