Elimination ElcB-like

Fig. 6.5. Representation of changes in transition-state character in the variable transition state E2 elimination reaetion, showing displacement of transition-state location as a result of substituent effects (a) substituent Z stabilizes catfaanion character of Elcb-like transition state (b) substituent R stabilizes carbocation character of El-like transitions state.

For E2 eliminations in 2-phenylethyl systems with several different leaving groups, both the primary isotope effect and Hammett p values for the reactions are known. Deduce from these data the relationship between the location on the E2 transition state spectrum and the nature of the leaving group i.e., deduce which system has the most El-like transition state and which has the most Elcb-like. Explain your reasoning. 

L is the hydrogen or deuterium atom that is not transferred in the elimination reaction and T is tritium that is present in tracer quantities. These substrates were chosen so that the reactions would have transition states ranging from very ElcB-like for [22], to central or intermediate for [23], to El-like for [24]. For practical reasons, the base/solvent system could not be kept constant as was originally intended. EtO /EtOH was used in the reaction with substrates [22] and [23] whereas Bu,0"/But0H was used with substrate [24]. Although the secondary tritium KIE (when L = H) for the reaction of [22] was... 

The effects of reactant structures on the ketene-forming elimination reactions of aryl esters of substituted phenylacetic acids (9) and (10) with secondary amines in acetonitrile (Scheme 2) have been studied in anticipation that the transition state might have ElcB-like E2 character.3 The reactions are second order for R2NH-MeCN and... 

According to the definition given above, E2 eliminations are one-step eliminations. Still, in an E2 transition state the C—H bond can be broken to a different extent than the C—Het bond. If the C—H bond is broken to a greater extent than the C—Het bond, we have an E2 elimination with an Elcb-like distortion of the transition state geometry. Such transition states exhibit characteristic partial charges. In the Elcb-like distorted E2 transition state, a... 

However, in some cases, HF is eliminated in preference to dehydrobromination, e.g. in the succinic acid series [5, 6] (Figure 6.4). In these less common processes, the transition state has significant carbanion (ElcB-like) character, and the products are probably governed by the relative stabilities of the possible carbanionic transition states, 6.5A and 6.5B (Figure 6.5), where a fluorine atom situated (3 to a developing carbanion centre, as in 6.5B, is more stabilising than when directly attached, as in 6.5A. This effect is also seen in eliminations from dihaloacenaphthenes [7]. 

Like El elimination. ElcB requires two steps. Unlike E1, though, the intermediate in El cB is a carbanion, not a carbocation. ElcB stands for Elimination, unimolecular, conjugate base. 

The second reaction is like an aldol condensation between the methyl group on the imidazvJr proving the enoT and benzaldehyde as the electrophile. The nitro group provides the mrir stabilization for the enoT but that would not be enough without the imidazole. The elimination a -e is ElcB-like, going through a similar enoT intermediate. 

Figure 5 More O Ferrall-Jencks diagram for the elimination reaction (Equation 17). A, El-like E2 mechanism B, synchronous E2 mechanism C, Elcb-like E2 mechanism. The vector (a.) corresponds to a Hammond (parallel) effect and (b) to a Thornton (perpendicular) effect...

Two possible mechanisms both Elcb-like 127,138,147,148 been proposed for HAL and PAL to help define the role of the electrophilic MIO cofactor (49) in catalyzing a-elimination of the amino groups and subsequent stereo-specific /3-proton abstractions from their substrates. For mechanism A " (Figure 14(a), shown only for PAL), the... 

As the mechanism becomes more ElcB-like, the regiochemistry can start to change. With an E2 reaction that has a lot of carbanion character, the deprotonation of the more acidic hydrogen will dictate the elimination regiochemistry. In ElcB reactions, the regiochemistry is completely dictated by the relative acidity of the protons that can be removed. The double bond will form oriented to the carbon with the most acidic proton. 

FIGURE 7.92 In the ElcB-like transition state for Hofmann elimination, a partial primary carbanion (less substituted, more stable) is formed. In the ElcB-like transition state for Saytzeff elimination, it is a partial secondary carbanion (more substimted, less stable). 

Nitrile-forming eliminations from (E)-2,4-dinitrobenzaldehyde 0-aryloximes promoted by tertiary amines in acetonittile have been studied kinetically. The results have been interpreted in terms of a highly ElcB-like TS with extensive C -H bond cleavage... 

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