ElcB Mechanism Reaction

In the ElcB reaction, C-H bond-breaking occurs first. A base abstracts a proton to give an anion, followed by loss of the leaving group from the adjacent carbon in a second step. The reaction is favored when the leaving group is two carbons removed from a carbonyl, which stabilizes the intermediate anion by resonance. Biological elimination reactions typically occur by this ElcB mechanism. 

Saunders (1997) used quantum-chemical methods in the most recent attempt to establish the mechanistic importance of negative ion hyperconjugation. Earlier, Saunders (1976) had proposed that one could distinguish between an E2 and an irreversible Elcb(irr) mechanism (Lowry and Richardson, 1987) by measuring the leaving group KIE for the reaction. The... 

There was no evidence of a second-order term in amine, nor did amine self-association account for the non-linear behaviour. Hammett p values (for variation of RNHSO2) determined for formation of the complex [S.amine] (p = 1.64) and for expulsion of the anion ( ONp) (pacyi = -1-78) are consistent with an E cB process and uncomplicated by any steric effects of bound amine in the complex. The value of Pacyi is identical with that reported previously for ElcB reaction of the same esters in 50% acetonitrile-water and much greater than for their 2-type reactions in chloroform. Consequently, an ElcB mechanism involving extensive S-O bond cleavage with the formation of a A(-sulfonylamine, ArN=S02, is supported. 

As indicated in Scheme 3.7, the first step of an ElcB mechanism can be reversible and therefore deprotonation at the 3-carbon does not always lead to product formation. By applying a steady-state approximation to the carbanion concentration, the following rate law is obtained for an ElcB reaction ... 

Fluorines present in the /i-position (/f-fluorination) can also influence the outcome of elimination reactions of fluorocarbons as earbanion intermediates are involved in the ElcB elimination mechanism.83 The regio- and stereoselectivity of the elimination step is governed by CH acidities,84 which, as has been discussed, arc influenced by /f-fluorination. 

The mechanistic borderline between E2 and ElcB mechanisms has been studied under various conditions.1,2 The mechanism of the elimination reaction of 2-(2-fluoroethyl)-1-methylpyridinium has been explored explored by Car-Parrinello molecular dynamics in aqueous solution.3 The results indicated that the reaction mechanism effectively evolves through the potential energy region of the carbanion the carbon-fluoride bond breaks only after the carbon-hydrogen bond. 

It is not always easy to distinguish an elimination reaction that is following the Elcb mechanism from one that follows the E2 pathway because the Elcb reaction usually exhibits second-order kinetics also. However, because the Elcb reaction is not concerted, there are no strict requirements concerning the stereochemistry of the reaction. In contrast to the preferred anti elimination that occurs in the E2 mechanism, Elcb reactions often produce a mixture of stereoisomers, as illustrated in the following equation ... 

The first looks El (stabilized cation) the second E2—but in fact both are ElcB reactions. The most reliable way to spot a likely ElcB elimination is to see whether the product is a conjugated carbonyl group. If it is, the mechanism is probably ElcB. 

None of these intermediates is detected or isolated in practice—simple treatment of the ketone with acid gives the enone in good yield. A base-catalysed reaction gives the same product via the aldol-ElcB elimination mechanism. 

The find stage is dehydration of the aldol and an ElcB reaction that involves the carbonyi group sa in a standard aldol reaction (Chapter 27). Another enolate must form In the same position as the last, the Robinson anneletlon mechanism—stage 3t the ElcB dehydration... 

ElcB)i mechanism was the observation of a change in the rate-determining step in the elimination reaction of Al-(2-cyanoethyl)pyridinium... 

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