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Abstract:
The mechanisms of the Cope rearrangement in chloro-, bromo-, and iodobullvalene in solution and in the solid state were investigated by NMR techniques. The dominant species in solution, for all three compounds, are isomers 2 and 3 with nearly equal concentrations (where the numbers refer to the substituted carbons in the bullvalene moiety). The kinetics of the rearrangement processes as studied by 1H and 13C NMR involve three dominant bond shift rearrangements: interconversion between isomers 2 and 3, degenerate rearrangement of isomer 2, and a pseudodegenerate rearrangement of isomer 3, with isomer 1 serving as an intermediate. The solid state properties of these compounds were studied by carbon-13 MAS NMR and the bromo and iodo derivatives also by X-ray crystallography. Bromo- and iodobullvalene crystallize entirely as isomer 2 in the orthorhombic Fdd2 space group. The molecules in the crystals are orientationally disordered, and the carbon-13 results show that this disorder is dynamic on the NMR time scale. Rotor-synchronized two-dimension exchange spectroscopy, magnetization transfer experiments, and analysis of dynamic MAS spectra show that the mechanism of the dynamic disorder involves a degenerate rearrangement of isomer 2 which results in an effective π-flip of the molecule in the crystal. The Arrhenius activation parameters for this process are ΔE† = 57.1 kJ/mol, A = 5.2 × 1012 s-1 for bromobullvalene and ΔE† = 58.5 kJ/mol, A = 1.8 × 1013 s-1 for iodobullvalene. Chlorobullvalene is liquid at room temperature (mp 14 °C). Upon cooling of this compound in the MAS probe to well below 0 °C, signals due to both isomer 2 and isomer 3 are observed in the solid state. It is not known whether the solid so obtained is a frozen glass, a mixture of crystals due to, respectively, isomer 2 and isomer 3, or a single type of crystals consisting of a stoichiometric mixture of both isomers. Rotor-synchronized two-dimensional exchange measurements show that the chlorobullvalene isomers in this solid undergo Cope rearrangement. However, the bond shift processes involve only a degenerate rearrangement of isomer 2 and a pseudodegenerate rearrangement of isomer 3. No cross-peaks corresponding to interconversion between the two isomers are observed.
