Ei mechanism

Compounds that undergo elimination through cyclic transition states upon heating, with no other reagents present, are given the designation as Ei reactions.

For acyclic substrates, the Z-isomer is typically the minor product due to the destabilizing gauche interaction in the transition state, but the selectivity is not usually high.

[2] The pyrolysis of N,N-dimethyl-2-phenylcyclohexylamine-N-oxide shows how conformational effects and the stability of the transition state affect product composition for cyclic substrates.

When subjected to temperatures above 400 °C, esters containing β-hydrogens can eliminate a carboxylic acid through a 6-membered transition state, resulting in an alkene.

[9] β-hydroxy phenyl sulfoxides were found to undergo thermal elimination through a 5-membered cyclic transition state, yielding β-keto esters and methyl ketones after tautomerization and a sulfenic acid.

[12] Initially, a sulfenate ester is formed followed by a [2,3]-sigmatropic rearrangement to afford an allylic sulfoxide which undergoes thermal syn elimination to yield the 1,3-diene.

Selenoxides are preferred for this type of transformation over sulfoxides due to their increased reactivity toward β-elimination, in some cases allowing the elimination to take place at room temperature.

[18] The one-pot dehydration of a primary alcohol to give an alkene through an o-nitrophenyl selenoxide intermediate is called the Grieco elimination.

The selenide is then treated with excess hydrogen peroxide leading to the selenoxide which eliminates the β-hydrogen through a 5-member cyclic transition state, yielding an alkene.

Cyclic amine oxides (5, 7-10-membered nitrogen containing rings) can also undergo internal syn elimination to yield acyclic hydroxylamines containing terminal alkenes.

[24][25] Secondary and tertiary alkyl iodides with strongly electron-withdrawing groups at the α-carbon were found to undergo a pericyclic syn elimination when exposed to m-chloroperbenzoic acid (mCPBA).