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.
Ring contraction
E2 Elimination
Ring contraction. Click for 3D.
E2 elimination. Click for 3D.
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.
The orbital overlap in the app migration. Click for 3D
The Intrinsic reaction coordinates for the E2 elimination and migration are shown below, oriented to demonstrate their app nature.
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!
The so-called E2 elimination mechanism is another one of those mainstays of organic chemistry. It is important because it introduces the principle that anti-periplanarity of the reacting atoms is favoured over other orientations such as the syn-periplanar form; Barton used this principle to great effect in developing the theory of…
The previous post explored why E2 elimination reactions occur with an antiperiplanar geometry for the transition state. Here I have tweaked the initial reactant to make the overall reaction exothermic rather than endothermic as it was before. The change is startling. The exothermicity is of course due to the aromatisation…
Here I offer another spin-off from writing a lecture course on conformational analysis. This is the famous example of why 1,2-difluoroethane adopts a gauche rather than antiperiplanar conformation. One major contribution to the greater stability of the gauche is the stereoelectronic interactions, and this is best probed using the NBO…
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One Response to “E2 elimination vs ring contraction: anti-periplanarity in action.”
[…] induced rearrangement which might be described as elimination/migration (in a sense similar to this ring contraction migration/elimination), in this case implicating a nitrene intermediate. Wikipedia is normally very much on the ball with […]
[…] induced rearrangement which might be described as elimination/migration (in a sense similar to this ring contraction migration/elimination), in this case implicating a nitrene intermediate. Wikipedia is normally very much on the ball with […]