ElcB reaction carbanion intermediate

It is well known that base-induced elimination reactions can proceed either by a single, concerted step (E2), or by two steps, proton transfer and leaving group expulsion, with a carbanion intermediate (ElcB) to yield an alkene. " The... 

That is, the reaction took place faster in D2O than in H2O. This is compatible only with an ElcB mechanism in which the proton-transfer step is not entirely rate determining. The isotope effect arises from a partitioning of the carbanion intermediate 11. This intermediate either can go to product or it can revert to starting compound, which requires taking a proton from the solvent. In D2O, the latter process is slower (because the O-D bond of D2O cleaves less easily than the O-H bond of H2O), reducing the rate at which 11 remrns to starting compound. With the return reaction competing less effectively, the rate of conversion of 11 to product is increased. 

The intermediate of the ElcB mechanism is a carbanion, and thus any factors that stabilise such an ion should favour this mechanism. We have already noted above that on the face of it, elimination reactions are the reverse of addition reactions. However, we also noted that the actual mechanistic pathways involved in elimination reactions were more similar to substitution reactions than addition reactions. This is because normally elimination reactions proceed via a carbonium ion or in a single step that has certain similarities to an SN2 substitution reaction. However, there are also addition reactions that proceed via a carbanion intermediate, for example the Michael-type reaction, in which a carbanion adds to an a,(3-unsaturated carbonyl compound. Indicate the Michael-type addition between the anion formed from the diester of propandioic acid (or malonic acid) and 2-butenal. 

Figure 35 Mechanism of the ECH reaction, (a) Concerted mechanism that proceeds via a carbanionic transition state, (b) Elcb reaction proceeding via the formation of an enoiate intermediate.

The ideas embodied in the variable transition state theory of elimination reactions can be depicted in a two-dimensional potential energy diagram. If we consider the case of an ethyl halide, both stepwise reaction paths require the formation of high-energy intermediates. The El mechanism requires formation of a primary carbocation, whereas the Elcb proceeds via a carbanion intermediate. 

Similar observations of base catalysis have been used to invoke the ElcB mechanism for elimination from 4,4-dicyano-3-p-nitrophenyl-1 -phenylbutan-1 -one in neutral and acidic methanol (34) , and l,l,l,3-tetranitro-2-phenylpro-pane in methanol in the presence of hydrochloric acid and pyridine-pyridine hydrochloride buffers (36) . In the former reaction, an example of a reverse Michael addition, the carbanion intermediate with the electron pair alpha to the carbonyl rather than in the gamma position is favoured, as the methyl isomer (35) eliminates more rapidly than the parent compound. 

The apparent rate law for a particular Elcb reaction depends on the relative magnitudes of /ci, fc i, and and on the concentrations of B and BH. If ki is much greater than both k i and k2, and if the initial concentration of B is larger than the initial concentration of RL, then essentially all of RL is converted to the carbanion intermediate. Changes in the concentration of B therefore have no appreciable effect on the rate of the reaction, and the reaction appears to follow first-order kinetics. 

The anti-coplanar relationship is not possible with the (E) isomer, however, so an Elcb mechanism involving formation of a vinyl carbanion and subsequent elimination of the bromide ion was proposed (equation 10.29). The ratio of the rate constant for reaction of (Z)-p-nitro-j8-bromostyrene to that of the (E) isomer was an order of magnitude smaller than the ratio of rate constants for (Z)- and (E)-)8-bromostilbenes. This difference was attributed to the stabilization of the carbanion intermediate by the p-nitro group. With a less acidic H proton, the carbanion mechanism in equation 10.29 is slower, but the concerted mechanism in equation 10.28 is not as significantly affected. Therefore, the ratio of rate constants is much greater for elimination of HCl from isomeric chloroalkenes. 

To understand the interdependence of the extent of leaving group departure and deprotonation, let s examine what makes a reaction such as E2 concerted. Jencks has described concerted reactions such as E2 as "forced" to be concerted. This term is used to describe a situation when no barrier separates a possible intermediate from the product. If there is no barrier to reaction of an intermediate, it has no lifetime, and does not represent a stable structure on the potential energy surface. For example, if a carbenium ion is too unstable to exist, an El reaction will become an E2 reaction. Furthermore, if a carbanion is too unstable to exist, an ElcB reaction will convert to E2. Stated another way, once we create a reactant that can achieve a carbanion or carbenium ion structure that is stable enough to exist, an E2 mechanism will convert to ElcB or El, respectively. 

In contrast to the El reaction, which involves a carbocation intermediate, the ElcB reaction takes place through a carbanion intermediate. Base-induced abstraction of a proton in a slow, rate-limiting step gives an anion, which expels... 

ElcB reaction (Section 12.13) A unimolecular elimination reaction in which a proton is first removed to give a carbanion intermediate, which then expels the leaving group in a separate step. 

In Chapter 10 we looked at El reactions (two-step elimination reactions that form a car-bocation intermediate) and E2 reactions (concerted elimination reactions). The preceding base-catalyzed dehydration represents the third kind of elimination reaction— namely, an ElcB (elimination unimolecular conjugate base) reaction, a two-step elimination reaction that forms a carbanion intermediate. ElcB reactions occur only if the carbanion can be stabilized by electron delocalization. 

In the ElcB reaction, C H bond-breaking occurs first. A base abstracts a proton to give a carbanion, 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. 

Because of the potential for carbanion inversion in the intermediate in ElcB reactions, a mixture of stereoisomers is usually obtained. If the reaction of 10.17 had been strictly an E2 process, only the Z-isomer would have been obtained. 

We have already seen examples of carbanions involved as intermediates, e.g. (40), in elimination reactions, i.e. those that proceed by the ElcB pathway (p. 251), for example ... 

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