Journal of the American Chemical Society, Vol.120, No.22, 5526-5538, 1998
Bond shift rearrangement of chloro-, bromo-, and iodobullvalene in the solid state and in solution. A carbon-13 and proton MMR study
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 H-1 and C-13 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 proper-ties 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 pi-fIip of the molecule in the crystal. The Arrhenius activation parameters for this process are Delta E dagger = 57.1 kJ/mol, A = 5.2 x 10(12) s(-1) for bromobullvalene and Delta E dagger = 58.5 kJ/mol, A = 1.8 x 10(13) s(-1) for iodobullvalene. Chlorobullvalene is Liquid at room temperature (mp 14 degrees C). Upon cooling of this compound in the MAS probe to well below 0 degrees 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.