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In organic chemistry, the Ei mechanism (Elimination Internal/Intramolecular), also known as a thermal syn elimination or a pericyclic syn elimination, is a special type of elimination reaction in which two vicinal (adjacent) substituents on an alkane framework leave simultaneously via a cyclic transition state to form an alkene in a syn elimination. This type of elimination is unique because it is thermally activated and does not require additional reagents, unlike regular eliminations, which require an acid or base, or would in many cases involve charged intermediates. This reaction mechanism is often found in pyrolysis.
General features
Compounds that undergo elimination through cyclic transition states upon heating, with no other reagents present, are given the designation as Ei reactions. Depending on the compound, elimination takes place through a four, five, or six-membered transition state.
The elimination must be syn and the atoms coplanar for four and five-membered transition states, but coplanarity is not required for six-membered transition states.
There is a substantial amount of evidence to support the existence of the Ei mechanism such as: 1) the kinetics of the reactions were found to be first order, 2) the use of free-radical inhibitors did not affect the rate of the reactions, indicating no free-radical mechanisms are involved <ref name=ref4>Barton, D.H.R.; Head, A.J.; Williams, R.J. (1953). "Stereospecificity in Thermal Elimination Reactions. Part III. The Pyrolysis of (−)-Menthyl Benzoate". J. Chem Soc. 453 1715- .</ref> 3) isotope studies for the Cope elimination indicate the C-H and C-N bonds are partially broken in the transition state, this is also supported by computations that show bond lengthening in the transition state and 4) without the intervention of other mechanisms, the Ei mechanism gives exclusively syn elimination products.
There are many factors that affect the product composition of Ei reactions, but typically they follow Hofmann’s rule and lose a β-hydrogen from the least substituted position, giving the alkene that is less substituted (the opposite of Zaitsev's rule). Some factors affecting product composition include steric effects, conjugation, and stability of the forming alkene.
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.
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.
In the trans isomer, there are two cis-β-hydrogens that can eliminate. The major product is the alkene that is in conjugation with the phenyl ring, presumably due to the stabilizing effect on the transition state. In the cis isomer, there is only one cis-B-hydrogen that can eliminate, giving the nonconjugated regioisomer as the major product.
Ester (acetate) pyrolysis
The pyrolytic decomposition of esters is an example of a thermal syn elimination. 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.
Isotopic labeling was used to confirm that syn elimination occurs during ester pyrolysis in the formation of stilbene.
Sulfur-based
Sulfoxide elimination
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.
Allylic alcohols can be formed from β-hydroxy phenyl sulfoxides that contain a β’-hydrogen through an Ei mechanism, tending to give the β,γ-unsaturation.
1,3-Dienes were found to be formed upon the treatment of an allylic alcohol with an aryl sulfide in the presence of triethylamine. 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.