R. Poupko et al., DYNAMIC NMR INVESTIGATION OF THE COPE REARRANGEMENT IN SOLUTIONS OF MONOSUBSTITUTED BULLVALENES, Journal of the American Chemical Society, 118(34), 1996, pp. 7995-8005
High resolution carbon-13, fluorine-19, and proton NMR measurements in
solutions of the monosubstituted bullvalenes, C(10)H(9)X with X = F,
CN, and COOH, as function of temperature are reported. The spectra at
low temperatures exhibit signals due to more than one isomer (three fo
r X = F and two for X = CN, COOH). On heating the peaks broaden due to
bond shift (Cope) rearrangement involving the various isomers. Detail
ed analysis of the line shapes shows that in all cases interconversion
between the four possible isomers must be assumed, even though the co
ncentration of some of them is too weak to be observed. For fluorobull
valene a complete analysis of the interconversion kinetics and equilib
ria is presented. For cyanobullvalene and bullvalenecarboxylic acid on
ly a semiquantitative analysis of the results was made.