This post, the fifth in the series, comes full circle. I started off by speculating how to invert the stereochemical outcome of an electrocyclic reaction by inverting a bond polarity. This led to finding transition states for BOTH outcomes with suitable substitution, and then seeking other examples. Migration in homotropylium cation was one such, with the “allowed/retention” transition state proving a (little) lower in activation energy than the “forbidden/inversion” path. Here, I show that with two electrons less, the stereochemical route indeed inverts. First, a [1,4] alkyl shift with inversion at the migrating carbon (ωB97XD/6-311G(d,p)/SCRF=chloroform); as a four-electron process, this is the “allowed” route. The “forbidden” route corresponds to retention of configuration at the migrating carbon. The barriers for each process can be seen below from the IRCs. That for inversion is ~4.5 kcal/mol lower than retention. This nicely transposes the values for the six-electron homologue shown in the previous post. There is one more nugget of insight that can be extracted. The start/end-point for the six-electron process (homotropylium cation) was, as the name implies, homoaromatic. Now, with a four-electron system we also have an inverse. Nominally, we should now end with homo-antiaromaticity (but see ). But antiaromaticity is avoided whenever possible, and so the homoaromatic bond observed in homotropylium is not formed. It resolutely remains a σ-bond (1.48Å) thus sequestering two electrons, and the remaining two electrons simply form a delocalised allyl cation. With the six-electron homotropylium, reactant/product were stabilised by that additional (homo)aromaticity, thus inducing a relatively high barrier. With the four-electron system here, no such reactant/product stabilisation occurs, and hence the reaction barriers are now significantly lower. A rather neat pedagogic example.
Henry S. Rzepa., "Gaussian Job Archive for C8H11(1+)", 2014. http://dx.doi.org/10.6084/m9.figshare.1142175
Henry S. Rzepa., "Gaussian Job Archive for C8H11(1+)", 2014. http://dx.doi.org/10.6084/m9.figshare.1142174
C.S.M. Allan, and H.S. Rzepa, "Chiral Aromaticities. A Topological Exploration of Möbius Homoaromaticity", Journal of Chemical Theory and Computation, vol. 4, pp. 1841-1848, 2008. http://dx.doi.org/10.1021/ct8001915