BOND SHIFT REARRANGEMENT OF CHLOROBULLVALENE, BROMOBULLVALENE, AND IODOBULLVALENE IN THE SOLID-STATE AND IN SOLUTION - A C-13 AND PROTON MMR STUDY

The mechanisms of the Cope rearrangement in chloro-, bromo-, and iodob ullvalene 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 numbe rs refer to the substituted carbons in the bullvalene moiety). The kin etics of the rearrangement processes as studied by H-1 and C-13 NMR in volve three dominant bond shift rearrangements: interconversion betwee n isomers 2 and 3, degenerate rearrangement of isomer 2, and a pseudod egenerate rearrangement of isomer 3, with isomer 1 serving as an inter mediate. The solid state proper-ties of these compounds were studied b y carbon-13 MAS NMR and the bromo and iodo derivatives also by X-ray c rystallography, Bromo- and iodobullvalene crystallize entirely as isom er 2 in the orthorhombic Fdd2 space group. The molecules in the crysta ls are orientationally disordered, and the carbon-13 results show that this disorder is dynamic on the NMR time scale. Rotor-synchronized tw o-dimension exchange spectroscopy, magnetization transfer experiments, and analysis of dynamic MAS spectra show that the mechanism of the dy namic disorder involves a degenerate rearrangement of isomer 2 which r esults in an effective pi-fIip of the molecule in the crystal. The Arr henius 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 dagge r = 58.5 kJ/mol, A = 1.8 x 10(13) s(-1) for iodobullvalene. Chlorobull valene is Liquid at room temperature (mp 14 degrees C). Upon cooling o f this compound in the MAS probe to well below 0 degrees C, signals du e 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 t ype of crystals consisting of a stoichiometric mixture of both isomers . Rotor-synchronized two-dimensional exchange measurements show that t he chlorobullvalene isomers in this solid undergo Cope rearrangement. However, the bond shift processes involve only a degenerate rearrangem ent of isomer 2 and a pseudodegenerate rearrangement of isomer 3. No c ross-peaks corresponding to interconversion between the two isomers ar e observed.