BOND SHIFT TAUTOMERISM OF BIBULLVALENYL IN SOLUTION AND IN THE SOLID-STATE - A C-13 NMR-STUDY

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.