The E1cB elimination reaction is a special type of elimination reaction in organic chemistry. This reaction mechanism explains the formation of alkenes from (mostly) alkyl halides through a carbanion intermediate given specified reaction conditions and specified substrates. The abbreviation stands for Elimination Unimolecular conjugate Base. The reaction takes place around a sp3 - sp3 carbon to carbon covalent bond with an α-acidic hydrogen atom substituent and a β-leaving group. This leaving group can be a halide or a sulfonic acid ester such as a tosyl group. A strong base abstracts the α proton generating a carbanion. The electron pair then expels the leaving group and the double bond is formed. When the first step to the carbanion is slow and the second step fast the reaction is irreversible and named (E1cB)i. When the first step is fast and the leaving group is expelled reversibly then the reaction mechanism is (E1cB)r. In the (E1cB)anion variation the carbanion is especially stable with a rapid first step and a slow second step.
Simply, the nucleophilicity of the leaving group and the stability of the carbanion determine whether or not the reaction is reversible.
A named reaction displaying E1cB elimination mechanism is the Boord olefin synthesis.
Being able to distinguish between different elimination reaction mechanisms (E2, E1, E1cB) is often difficult and involves the study of chemical kinetics, kinetic isotope effects, substituent effects and computational chemistry. The reaction of 1-methyl-2-(2-fluoroethyl)pyridinium iodide with the hydroxide ion in water is an example of a 2-step E1cB reaction because the carbanion is stabilized by the resonance effect with the enamine. Computational chemistry predicts that the carbanion in this example is stabilized by only 2.8 kcal/mol (11.7 kJ/mol) and only when solvation of all the chemical species with water is taken into account. The same fluoride substrate but as a free pyridine does not have this stabilization and the reaction proceeds by a concerted one-step E2 reaction mechanism. When the leaving group in the methylated pyridine compound is chlorine instead of fluoride the reaction mechanism is also E2 because now the carbon to chloride bond is too weak.
A photochemical version of E1cB has been reported by Lukeman et al. In this report, a photochemically induced decarboxylation reaction generates a carbanion intermediate, which subsequently eliminates a beta 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 E1cBI.
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