The ionization of a C-X bond (X=halogen) to form what we call a carbocation and which is known as the SN-1 reaction goes way back in the history of chemistry. Julius Steglitz was probably the first person to suggest such an ionization, back in 1899 (Steglitz, J.; Am. Chem.
Click on diagram to see model. The reaction above is ostensibly a very simple pericyclic ring opening of a cyclopropyl carbocation to an allyl cation, preceeded by a preparatory step involving SN-1 solvolysis. As a 2-electron thermal process, the second step proceeds with disrotation of the terminii. Can this stereochemistry be illustrated with a computed model for the transition state for this process?
Mauksch and Tsogoeva have recently published an article illustrating how a thermal electrocyclic reaction can proceed with distoratory ring closure, whilst simultaneously also exhibiting 4n electron Möbius-aromatic character. Why is this remarkable? Because the simple Woodward-Hoffmann rules state that a disrotatory thermal electrocyclic reaction should proceed via a Hückel-aromatic 4n+2 electron transition state.
Click on the static image to get an active model.
