E2 elimination vs ring contraction: anti-periplanarity in action.

The anti-periplanar principle permeates organic reactivity. Here I pick up on an example of the antiperiplanar E2 elimination (below, blue) by comparing it to a competing reaction involving a [1,2] antiperiplanar migration (red).

The relative rates of these two processes will depend on several factors such as the ability of Cl to donate electrons (red) vs the basicity of the chloride anion (blue) and of course solvent polarity. It is the balance between these two mechanisms that caught Barton’s eye and helped him formulate his ideas about conformational analysis. Calculations (ωB97XD/6-311G(d,p)/SCRF=water) help reveal the basic features of the competition; details can be found for the ring contraction and E2 elimination.

If you focus on the dashed bonds, you can easily identify the anti-periplanar components for each reaction. In this specific example, the E2 reaction wins out over the ring contraction/migration by ΔΔG₂₉₈ = 17.6 kcal/mol. (One of) the orbital interactions responsible for the antiperiplanar migration is shown below.

You might notice that via these posts, I am gradually building up a library of transition states for taught reactions. Still a few to go however!