Z. Luz et al., BOND SHIFT REARRANGEMENT OF CHLOROBULLVALENE, BROMOBULLVALENE, AND IODOBULLVALENE IN THE SOLID-STATE AND IN SOLUTION - A C-13 AND PROTON MMR STUDY, Journal of the American Chemical Society, 120(22), 1998, pp. 5526-5538
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.