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? Well, starting with a naked cation, its actually quite tricky to find such a transition state. In reality such cations are always solvated in real reactions, and a gas phase model is actually somewhat artificial. A much better bet is to also include in the model the SN-1 step that is presumed to preceed the actual pericyclic ring opening. Here we have included a simple protonated water molecule as the leaving group, rather than the tosyl group shown in the diagram above. Now when attempts are made to locate the ring opening transition state, it turns out that the SN-1 reaction and this ring opening are strongly coupled together. The ring opening helps to evict the water, or alternatively, one can look on it as the departing water helping to open the ring.
Such an intimate coupling of one mechanistic type (a pericyclic reaction) with another (SN-1 solvolysis) is a relatively unrecognized aspect of reaction mechanisms (although of course it is one of the characteristics of enzyme-catalysed reactions). I will cover several other examples of such synchrony in mechanisms in future entries in the blog.