# Stereochemistry of eliminations similar to E2' elimination

In the E2 reaction, the proton and the leaving group are anti-periplanar typically, so as to attain best overlap of the C-H bonding orbital and C-LG anti-bonding orbital.

In a similar reaction, with a hydrogen alpha to a C=C and a leaving group alpha the other side, we have a syn-elimination, which can be explained by a tau-bond model. Eg:

Eliminates to give:

Demonstrating that the groups eliminate from the same side in a so called E2' reaction.

My question is what happens to the required stereo-chemistry as a we increased the number of alkenes units in between - does the stereo-chemistry flip-flop, or do we get a fixed stereo-chemistry requirement. Or is the stereochemistry requirment, as I would theorise, relaxed as we increased the number of alkenes?

• Unfortunately, allylic strain probably determines the product here. Alignment is less important since it would need to be mediated by a $\pi$ bond that can't overlap perfectly with the breaking $\sigma$ bonds anyway. – Zhe Jan 17 '18 at 22:47