L. Olivier et al., BOND SHIFT TAUTOMERISM OF BIBULLVALENYL IN SOLUTION AND IN THE SOLID-STATE - A C-13 NMR-STUDY, Journal of physical chemistry, 100(45), 1996, pp. 17995-18003
High-resolution carbon-13 NMR of bibullvalenyl solutions and magic ang
le spinning (MAS) of solid bibullvalenyl are reported over a wide temp
erature range and interpreted in terms of the isomeric distribution an
d the kinetics of the bond shift (Cope) rearrangements. In solution, b
ibullvalenyl exists predominantly as a mixture of the 3-3, 3-2, and 2-
2 isomers, in which the bullvalenyl radicals are linked at the olefini
c carbons. Their relative concentrations in the temperature range -50
to -20 degrees C are 0.67, 0.28, and 0.05, respectively. Above -30 deg
rees C bond shift rearrangement results in broadening of the NMR signa
ls. Detailed analysis of these dynamic spectra indicates the occurence
of at least three main rearrangement processes: (i) direct interconve
rsion of the isomeric pairs 3-X reversible arrow 2-X (X = 2 or 3), (ii
) degenerate rearrangement of the 2-X isomers, 2-X reversible arrow 2
-X (where the asterisk indicates a rearranged bullvalenyl radical), an
d (iii) a pseudodegenerate rearrangement of the 3-X isomers via the in
termediate 1-X, 3-X --> [1-X] --> 3-X, and possibly also via the inte
rmediate 4-X. In the solid state, bibullvalenyl crystallizes entirely
as isomer 3-3, with the two bullvalenyl radicals most probably in crys
tallographically unrelated sites. Above room temperature the MAS spect
ra exhibit selective line broadening due to the Cope rearrangement. De
tailed analysis shows that the pathway for the reaction involves the i
somer 1-3, which serves as a transient intermediate in the reaction, I
t is likely that at higher temperatures additional pathways involving
the isomers 4-3 and/or 2-3 also contribute to the line broadening in t
he solid state.