EicB mechanism

As an example of the system, this is how an EIcb mechanism (p. 1308) would be represented ... 

For compounds in which hydrogen is sufficiently acidified by enough fluorines in 3 positions, the EIcB mechanism may be operating [II]... 

Note that in this and the following EicB mechanisms the rates are not really independent of the base concentration and therefore the 1 part of the classification may be misleading, but it is analogous to the Ssl reaction of Equations 5.24 and 5.25. 

In several preparations of deuterium labeled analogs, elimination of deuterium bromide was much slower than that of hydrogen bromide, suggesting an EicB mechanism for the elimination. Thus, in the preparation of cis- and rans -l,2-dimethyl-3-methylenecyclopropane (17), formation of the dideuteromethylene analogs took 7 days at room temperature, as opposed to 14 hours for the unlabeled compounds. In the case of endo- and exo-2-methyl-6-methylene-bicyclo[3.1.0]hexane (19), the dideutero analog also required heating to 45°C to complete the reaction. ... 

In both reactions, a proton is removed from a carbon atom (Cjs) by a base, a double bond is formed, and an electron pair shifts from a carbon atom (Ca) to a basic (electronegative) atom. Elimination reactions have been more finely subdivided 17) depending on whether the atom removed initially is the proton (carbanion or EICB mechanism), the base (carbonium ion or E-1 mechanism), or both (concerted or E-2 mechanism). The mode of activation of enzyme-catalyzed enolization and elimination reactions by metals will be classified according to the site on the substrate which is coordinated by the metal. 

The iPr2NEt produces the elimination of the benzyloxy group on p to the ketone, by means of an EicB mechanism, producing an a,p-unsaturated ketone. 

Potassium cm bonate acts as a base by producing the abstraction of a proton on an a position of the cyclopentanone carbonyl. The resulting anion evolves by expulsion of the alkoxide in the p position by an EicB mechanism. 

The triethylamine generates a carbanion on a to the ketone, which evolves by an EicB mechanism, expelling a carboxylate. 

The oxygen on the p position is eliminated through an EicB mechanism. 

The ammonium salt is expelled by mean of an EicB mechanism, causing the recovery of the alkene conjugated with the ester. 

Elimination of methanesulfinic acid is produced, by means of a EicB mechanism, producing again an enone. 

An anion on a to the remaining ester carbonyl is formed. This anion evolves by carboxylate elimination through an EicB mechanism. 

An unsaturated ester is formed by p-elimination of the silyloxy group by a EicB mechanism. 

A related type of covalent-ionic curve cro.ssing occurs in the S l and El mechanisms. In these cases, though, the positive ion R is an encumbered carbenium ion, much larger than the H, and does not allow a compact ion-paired structure with significant electrostatic stabilization. Here, the solvent has to be involved and to stabilize the ion-paired structure below the crossing point of the two covalent forms. A similar description applies to the EIcb mechanism with the exception that here the ionic form is... 

The two major mechanisms for /(-eliminations involving the removal of an H and an adjacent functional group are the E2 and El. Their features are compared and summarized in Table 7-3. A third mechanism Eicb, is occasionally observed. Table 7-4 compares E2 and SN2. 

Occasionally a chemical transformation is mediated by an intermediate which is electronically distinct from its Lewis-type precursor and successor reactants and products. Stepwise mechanisms in organic chemistry, such as S l, El, EIcb, and S V are common cases of this type. The intermediacies of pentacoordinated SiX species and of various + i radicals (e.g., M = P) are also of the same electronic reorganization type [4 12b, c]. In all of these cases, the use of a many-curve model is both necessary and illuminating. 

FIGURE 20.9 Mechanism ofthe conversion of asparagine and aspartate to oxaloacetate and fumarate. Initial PLP-dependent transamination is followed by an EicB elimination of ammonium ion. 

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