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  • 1
    ISSN: 1434-193X
    Keywords: Cyclizations ; Hydrocarbons ; Isomerizations ; Kinetics ; Polycycles ; Rearrangements ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -Whereas 1,5-dimethylsemibullvalene (1b) equilibrates with 1,5-dimethylcyclooctatetraene (2b), the semibullvalene-2,6-dicarbonitriles 13 rearrange irreversibly to afford mixtures of the isomeric cyclooctatetraene-1,5-dicarbonitriles 14 and 15. Arrhenius and Eyring parameters of these thermal isomerisation reactions have been determined for the gas phase (1b → 2b) and [D6]benzene solutions (1b → 2b, 13 → 14 + 15). Furthermore, the activation parameters of the known rearrangement of octamethylcyclooctatetraene (3) to octamethylsemibullvalene (4) have been determined. - The data for these compounds, together with those for related compounds previously reported in the literature, show that substituents not only influence the relative stabilities of the semibullvalene and cyclooctatetraene systems but also the height of the energy barrier for their interconversion. Those substituents that lower the barrier toward the degenerate Cope rearrangement of semibullvalenes simultaneously accelerate their rearrangement to cyclooctatetraenes thus limiting the thermal stability of the former.
    Additional Material: 2 Ill.
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  • 2
    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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  • 3
    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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  • 4
    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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  • 5
    ISSN: 1434-193X
    Keywords: Automerization ; Carbocycles ; Fluxionality ; Hydrocarbons ; NMR spectroscopy ; Pericyclic reactions ; Polycycles ; Rearrangements ; Strained molecules ; Substituent effects ; Valence isomerisation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The activation parameters [ΔG# (200 K), ΔH#, and ΔS#] for the degenerate Cope rearrangements of barbaralane (1a) and several semibullvalenes (3a, 7a, 8a), and for those of a number of their 3,7- and 2,6-disubstituted derivatives, have been determined by variable-temperature carbon-13 NMR spectroscopy at 126 and 151 MHz. the disubstituted compounds studied include the barbaralanes 1c, d, e, h, j, k, and 2e, h, j, and the semibullvalenes 3d, e, h, 4e, h, i, l, m, and 5h-8h. Careful attention has been given to the measurement of temperatures. The data for these compound, together with those for related compounds previously reported in the literature, show that conjugating subsituents (e. g. aryl, CH, CO2R) in the 2,6 positions lower the barriers of the degenerate Cope rearrangement, whereas substituents that are electron-withdrawing through the inductive effect (e. g. Cl, CF3, SO2Ar) retard the reaction. Substituents in the 3,7-positons have little effect or are rate-retarding. The presence of 1,5-tri- and tetramethylene bridges in semibullvalenes accelerates the rearrangements, the effect being larger in the case of the former.
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