Cyclopropanation: the mechanism of the Simmons–Smith reaction.

These posts contain the computed potential energy surfaces for a fair few “text-book” reactions. Here I chart the course of the cyclopropanation of alkenes using the Simmons-Smith reagent,[1] as prepared from di-iodomethane using zinc metal insertion into a C-I bond.

Two reactions it can be compared with are the epoxidation of ethene using a peracid and dichlorocyclopropanation. The latter is a four-electron pericyclic process, which is thermally forbidden. The outcome there is that the two new bonds form very asynchronously to avoid transition state antiaromaticity. The former is a more complex reaction, best described as a six-electron process which allows both C…O bonds to form at the same rate. So which of these two does the Simmons–Smith mechanism correspond to?

The calculation as undertaken at a ωB97XD/Def2-TZVPD-PP (solvent=dichloromethane) level[2] shows the two C…C bonds forming at more or less the same rate. The reaction therefore resembles epoxidation rather than dichlorocyclopropanation.

The intrinsic reaction coordinate (IRC)[3] is shown below, revealing a concerted and almost synchronous reaction. The synchronicity is all the more surprising given the diversity of bonds forming/breaking.

There are slight and tantalizing hints that the alkene and the I-Zn-CH₂-I components initially form a weak π-complex, which then rearranges into the TS, and then again a weak complex at the end. The NCI surfaces for both are shown below, and show clear signs of dispersion-like stabilisations for both of them! The strange torus around the Zn-I bond is due to the need for a lower density threshold to filter out the covalent interactions.

References

1. H.E. Simmons, and R.D. Smith, "A NEW SYNTHESIS OF CYCLOPROPANES FROM OLEFINS", Journal of the American Chemical Society, vol. 80, pp. 5323-5324, 1958. http://dx.doi.org/10.1021/ja01552a080
2. Henry S Rzepa., "C 3 H 6 I 2 Zn 1", 2014. http://dx.doi.org/10.14469/ch/147617
3. Henry S. Rzepa., "Gaussian Job Archive for C3H6I2Zn", 2014. http://dx.doi.org/10.6084/m9.figshare.1270441