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... 

Values of k/k QL) = 1.045,1.044 and 1.049 0.011 and values of k/k = 1.051 0.004,1.036 0.005 and 1.036 0.004 have been obtained for R = / -MeO, H and p-C 1 compounds. They indicate that large changes in bonding both at C(a) and C(/J) take place in the rate-determining steps of these reactions consistent with an Elcb-like E2 mechanism. The observed relatively large C(a) isotope effects are inconsistent with an irreversible Elcb mechanism. The E2 transition state for 388, R = / -MeO is somewhat Elcb-like. As R becomes a better EWG, the transition state becomes more reactant-like (less double-bond character) and more carbanion-like. 

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]. 

It has been suggested that favourable hydrogen-bonding interactions between R and trifluoromethyl in an ElcB-like transition state, 6.14B, could account for the formation of the least thermodynamically stable isomer, 6.14C, from 6.14A 6.14C is converted to 6.14D on heating with caesium fluoride [18] (Figure 6.14). 

In particular, Schanze proposed that the mechanism of base-catalyzed fragmentation in 2-(4-X-phenylamino)-l,2-diphenylethanol radical cations (Scheme 61) changes with the oxidation potential of the substrate, going from an ElcB-like mechanism (as in Scheme 59, paths a and d) with the more stable radical cations (X = OMe and Me), to an E2-hke mechanism (as in Scheme 59, path e) when X = H, Cl, CN. 

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. 

The mechanism by which proton acids catalyze the dehydration of primary and secondary alcohols in water is not perfectly well understood (1). There is universal agreement that the dehydration of tertiary alcohols can be explained by an El mechanism (1,2) involving either a II complex ( ) or a symmetrically solvated carbonium ion (4) as the key reaction intermediate. Although an occasional text ( ) also describes the dehydration of primary alcohols by an El mechanism, authoritative reviews (1/4) conclude that a concerted E2 type mechanism is more probable. The dehydration behavior of secondary alcohols is presumed to be similar to primary alcohols (4). Discussions of the gas phase dehydration of alcohols by heterogeneous Lewis acid catalysts admit more possibilities. In their authoritative review Kut, et al. (1) consider E1-, E2-, and ElcB-like mechanisms, as well as the possible role of diethyl ether as a reaction intermediate, but they reach no conclusion concerning the relative importance of these mechanisms in the formation of olefins from alcohols. 

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... 

The Elcb like the El mechanism involves two steps but the order is reversed. Proton abstraction precedes expulsion of the leaving group.3... 

The variable transition state theory allows discussion of reactions proceeding through TSs of intermediate character in terms of the limiting mechanistic types. These are called Elcb-like and El-like, as illustrated in Eigure 5.11. The most important factors to be considered are (1) the nature of the leaving group, (2) electronic and steric effect of substituents in the reactant molecule, (3) the nature of the base, and (4) solvent effects. 

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