I have in the past (around 2012 to be specific) taken an interest in a particular type of [3,3]sigmatropic pericyclic reaction called the semibullvalene rearrangement.[1],[2],[3]. This system can apparently exhibit very long C-C bonds in the region of 2.1Å, in which form it would be called a “frozen” transition state, also referred to a bis-homoaromatic molecule, which in this form is NOT regarded as a [3,3]sigmatropic pericyclic transition state. Time for an update I thought, starting with a crystal structure search (Figure 1) for these types of species.
References
- H. Rzepa, "Predicted properties of a candidate for a frozen semibullvalene.", 2012. https://doi.org/10.59350/cq2a3-g2g35
- H. Rzepa, "Frozen Semibullvalene: a holy grail (and a bis-homoaromatic molecule).", 2012. https://doi.org/10.59350/585c3-dpy86
- H. Rzepa, "The ten-electron homologue of semibullvalene.", 2012. https://doi.org/10.59350/8nkna-ycy23