Bond shift tautomerism of bibullvalenyl in solution and in the solid state. A carbon-13 NMR study

High-resolution carbon-13 NMR of bibullvalenyl solutions and magic angle spinning (MAS) of solid bibullvalenyl are reported over a wide temperature range and interpreted in terms of the isomeric distribution and the kinetics of the bond shift (Cope) rearrangements. In solution, bibullvalenyl exists predominantly as a mixture of the 3−3, 3−2, and 2−2 isomers, in which the bullvalenyl radicals are linked at the olefinic carbons. Their relative concentrations in the temperature range −50 to −20 °C are 0.67, 0.28, and 0.05, respectively. Above −30 °C bond shift rearrangement results in broadening of the NMR signals. Detailed analysis of these dynamic spectra indicates the occurence of at least three main rearrangement processes:  (i) direct interconversion of the isomeric pairs 3−X ⇄ 2−X (X = 2 or 3), (ii) degenerate rearrangement of the 2−X isomers, 2−X ⇄ 2*−X (where the asterisk indicates a rearranged bullvalenyl radical), and (iii) a pseudodegenerate rearrangement of the 3−X isomers via the intermediate 1−...