R. Poupko et al., REACTION PATHWAYS IN SOLID-STATE PROCESSES .2. C-13 NMR AND X-RAY CRYSTALLOGRAPHY OF CYANOBULLVALENE AND BULLVALENECARBOXYLIC ACID, Journal of the American Chemical Society, 118(34), 1996, pp. 8015-8023
Carbon-13 MAS-NMR measurements and the X-ray crystallographic structur
e of cyanobullvalene (I) and bullvalenecarboxylic acid (II) are report
ed. The two compounds' crystallize as isomer 3 in the triclinic space
group <P(1)over bar>. Two-dimensional solid-state NMR exchange experim
ents indicate the occurrence of bond shift (Cope) rearrangement in bot
h cases. Analysis of the results shows that the process involves isome
r 1 as a transient intermediate. The Arrhenius kinetic parameters were
determined from the line broadening in the 1D MAS spectra and magneti
zation transfer experiments as function of temperature, yielding, A(C)
= 1.0 x 10(12) s(-1), 2.2 x 10(12) s(-1) and E(C) = 13.9 kcal mol(-1)
, 14.1 kcal mol(-1) for compounds I and II, respectively. A rearrangem
ent process involving isomer 2 as an intermediate may also occur, but
it does not result in permutation of atoms and therefore is NMR-invisi
ble. NMR exchange measurements on solid cyclooctatetraene dimer (III)
were also carried out; however, on the time scale of up to 1 min no dy
namic effects are observed.