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E1cB-elimination reaction

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E1cB-elimination reaction

An example of the E1cB reaction mechanism in the degradation of a hemiacetal under basic conditions.

The E1cB elimination reaction is a type of elimination reaction which occurs under basic conditions, where a particularly poor leaving group (such as -OH or -OR) and an acidic hydrogen eliminate to form an additional bond. E1cB is a two-step process. First, a base abstracts the most acidic proton to generate a stabilized anion. The lone pair of electrons on the anion then moves to the neighboring atom, thus expelling the leaving group and forming double or triple bond.[1] The name of the mechanism - E1cB - stands for Elimination Unimolecular conjugate Base. Elimination refers to the fact that the mechanism is an elimination reaction and will lose two substituents. Unimolecular refers to how this reaction only involves one molecular entity. Finally, conjugate base refers to the formation of the carbanion intermediate, which is the conjugate base of the starting material.

Mechanism

α and β assignments in a molecule with leaving group, LG.

Alpha carbon is with respect to leaving group. There are two main requirements to have a reaction proceed down an E1cB mechanistic pathway. The compound must have an fluoroalkanes. It has also been suggested that the E1cB mechanism is more common among alkenes eliminating to alkynes than from an alkane to alkene.[2] One possible explanation for this is that the sp2 hybridization creates slightly more acidic protons. Although it should be noted that this mechanism is not limited to carbon-based eliminations. It has been observed with other heteroatoms, such as nitrogen in the elimination of a phenol derivative from ethiofencarb.[3]

Degradation of ethiofencarb illustrating the presence of a stable anion due to resonance between the amide functional group and the carbonyl group.

Distinguishing E1cB-elimination reactions from E1 and E2-elimination reactions

All elimination reactions involve the removal of two substituents from a pair of adjacent atoms in a compound. Alkene, alkynes, or similar heteroatom variations (such as carbonyl and cyano) will form. The E1cB mechanism is just one of three types of elimination reaction. The other two elimination reactions are E1 and E2 reactions. Although the mechanisms are similar, they vary in the timing of the deprotonation of the α-carbon and the loss of the leaving group. E1 stands for unimolecular elimination, and E2 stands for bimolecular elimination. In an E1 mechanism, the molecule contains a good leaving group that departs before deprotonation of the α-carbon. This results in the formation of a carbocation intermediate. The carbocation is then deprotonated resulting in the formation of a new pi bond. The molecule involved must also have a very good leaving group such as bromine or chlorine, and it should have a relatively less acidic α-carbon.

Example of the preferential elimination of fluorine in an E1cB-elimination reaction.

In an E2-elimination reaction, both the deprotonation of the α-carbon and the loss of the leaving group occur simultaneously in one concerted step. Molecules that undergo E2-elimination mechanisms have more acidic α-carbons that undergo E1 mechanisms, but their α-carbons are not as acidic as those of molecules that undergo E1cB mechanisms. The key difference between the E2 vs E1cb pathways is a distinct carbanion intermediate as opposed to one concerted mechanism. Studies have been shown that the pathways differ by using different halogen leaving groups. One example uses chlorine as a better stabilizing halogen for the anion than fluorine,[4] which makes fluorine the leaving group even though chlorine is a much better leaving group.[5] This provides evidence that the carbanion is formed because the products are not possible through the most stable concerted E2 mechanism.

The following table summarizes the key differences between the three elimination reactions; however, the best way to identify which mechanism is playing a key role in a particular reaction involves the application of chemical kinetics.
E1 E2 E1cB
Stepwise reaction Concerted reaction Stepwise reaction
Carbocation intermediate Simultaneous removal of proton, formation of double bond, and loss of leaving group Carbanion intermediate
no kind of conclusion No preference no kind of conclusion
Good leaving groups Leaving group Poor leaving groups
Less acidic α-carbon Acidic α-carbon More acidic α-carbon

Chemical kinetics of E1cB-elimination mechanisms

When trying to determine whether or not a reaction follows the E1cB mechanism, chemical kinetics are essential. The best way to identify the E1cB mechanism involves the use of rate laws and the kinetic isotope effect. These techniques can also help further differentiate between E1cB, E1, and E2-elimination reactions.

Rate Law

When trying to experimentally determine whether or not a reaction follows the E1cB mechanism, chemical kinetics are essential. The best ways to identify the E1cB mechanism involves the use of rate laws and the kinetic isotope effect.

The rate law that governs E1cB mechanisms is relatively simple to determine. Consider the following reaction scheme.

An example of an E1cB-elimination mechanism with a generic leaving group, LG, and ethoxide as the base.

Assuming that there is a steady-state carbanion concentration in the mechanism, the rate law for an E1cB mechanism.

The rate law and observed rate for the E1cB-elimination mechanism in the previous example where the base is ethoxide.

From this equation, it is clear the second order kinetics will be exhibited.[6] E1cB mechanisms kinetics can vary slightly based on the rate of each step. As a result, the E1cB mechanism can be broken down into three categories:[7]

  1. E1cBanion is when the anion is stable resulting in a rapid first step, followed by the slow formation of products (k1>>k2).
  2. E1cBrev is when the first step is reversible but the formation of product is slower than reforming the starting material, this again results from a slow second step (k-1>>k2).
  3. E1cBirr is when the first step is slow but once formed the product quickly follows (k2>>k1,k-1). This leads to an irreversible first step.

Kinetic Isotope Effect

Deuterium

The kinetic isotope effect can help distinguish between E1cBrev, E1cBanion, and E1cBirr. If deuterium is present in the base in place of hydrogen, then the exchange of protons can be monitored. If the reaction occurs in deuterated base and starting material is recovered that contains deuterium, then the reaction is most likely undergoing an E1cBrev type mechanism. Recall, in this mechanism k-1 is faster than the k2. This means after the carbanion is formed, it will quickly remove a proton from the base to form the starting material. So if the starting material ends up with deuterium in place of its original hydrogen, then the compound was deprotonated, and then took a deuterium off of the base.
An additional kinetic isotope experiment would be to replace the solvent with deuterated solvent. So if the reaction is run in water, it can be run in deuterium oxide. If the reaction takes place faster in the deuterium oxide than water, then the proton transfer step is not rate determining. Since deuterium is larger than hydrogen, the proton transfer step should be slower if the E1cBrev was dictating the reaction. A faster reaction would suggest that the reaction is either E1cBanion or E1cBirr.

Fluorine-19 and Carbon-11

Another way that the kinetic isotope effect can help distinguish E1cB mechanisms involves the use of 19F. Fluorine is a relatively poor leaving group, and it is often employed in E1cB mechanisms. Fluorine kinetic isotope effects are also applied in the labeling of Radiopharmaceuticals and other compounds in medical research. This experiment is very useful in determining whether or not the loss of the leaving group is the rate-determining step in the mechanism and can help distinguish between E1cBirr and E2 mechanisms. 11C can also be used to probe the nature of the transition state structure. The use of 11C can be used to study the formation of the carbanion as well as study its lifetime which can not only show that the reaction is a two-step E1cB mechanism (as opposed to the concerted E2 mechanism), but it can also address the lifetime and stability of the transition state structure which can further distinguish between the three different types of E1cB mechanisms.[8]

Aldol reactions

The most well known reaction that undergoes E1cB elimination is the [9]

An aldol condensation reaction is one of the most common examples of an E1cB mechanism.

Photo-induced E1cB

A photochemical version of E1cB has been reported by Lukeman et al.[10] In this report, a photochemically induced decarboxylation reaction generates a carbanion intermediate, which subsequently eliminates the leaving group. The reaction is unique from other forms of E1cB since it does not require a base to generate the carbanion. The carbanion formation step is irreversible, and should thus be classified as E1cBirr.

E1cB reaction mechanism through photo-induced decarboxylation.

See also

Elimination reaction
Reaction mechanism
Carbocation
Carbanion

References

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