Transition states for E2 reactions

From the earlier discussion on the nature of the transition state for E2 reactions, two salient factors affecting reactivity can be recognised, these being polar and steric effects. The polar effect can be divided into inductive and conjugative or electromeric components . The influence of a substituent will depend principally on the nature of the transition state, which to a large extent is determined by the leaving group and the base and solvent. A reaction... 

Although El reactions show some stereo- and regioselectivity, the level of selectivity in E2 reactions can be much higher because of the more stringent demands on the transition state for E2 elimination. We will come back to the most useful ways of controlling the geometry of double bonds in Chapter 31. 

Geometric features of transition states for addition reactions of olefins (Chapter 3), bimolecular substitutions (Si f3 reactions p. 36), bimolecular eliminations (E2 reactions p. loi) and many other reactions have been defined from studies of a variety of steroid systems (e.g. Fig. la). Reactions proceeding through carboniumions (e.g. Fig.Tb seep. 228) are sensitive to... 

Although El reactions show some stereo and regioselectivity, the level of selectivity in E2 reactions can be much higher because of the more stringent demands on the transition state for E2 elimination. In an E2 elimination, the new n bond is formed by overlap of the C—H o bond with the C—X a antibonding orbital. The two orbitals have to lie in the same plane for best overlap, and now there are two conformations that allow this. One has H and X syn-periplanar, the other anti-periplanar. The anti-periplanar conformation is more stable because it is staggered (the syn-periplanar conformation is eclipsed) but, more importantly, only in the anti-periplanar conformation are the bonds (and therefore the orbitals) truly parallel. 

Draw the E2 transition state for the reaction of bromocyclopen-tane (29) with KOH. 

Figure 11.18 The transition state for the E2 reaction of an alkyl halide with base. Overlap of the developing p orbitals in the transition state requires periplanar geometry of the reactant.

One way to determine just where a given reaction stands on the El-E2-ElcB spectrum is to study isotope effects, which ought to tell something about the behavior of bonds in the transition state. For example, CH3CH2NMe3 showed a nitrogen isotope effect of 1.017, while PhCH2CH2NMe3" gave a corres-... 

Transition state for an E2 reaction The carbon-carbon bond has some of the character of a double bond. 

The E2 reaction is a stereospecific reaction, i.e. a particular stereoisomer reacts to give one specific stereoisomer. It is stereospecific, since it prefers the anfi-coplanar transition state for elimination. The (R,R) diastereomer gives a cis-alkene, and the (S,R) diastereomer gives a trans-alkene. 

The catalysts which operate by means of an E2 mechanism give a high proportion of reaction products which are formed by the anti-elimination. This fact has been discussed in Sect. 2.1 and only few remarks need to be added here. Quantum chemical calculations [73] on the transition state of the dehydrochlorination of chloroethane, initiated by an attack of a basic species, confirmed the preference of the anti-elimination over the syn-mode. On the contrary, calculations on the transition state for non-catalytic (homogeneous) thermal elimination [201,202] confirmed the syn-elimination path. 

The nature of the transition state for the E2 reaction of some 3-thiophenoxypro-panones in aprotic solvents has been investigated.5 The linear free energy correlation and the calculated activation parameters Aand Aof the reactions suggested an unequal and synchronous E2cB mechanism via base-catalysed amine-elimination reaction. 

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. 

Of these possible conformations, the anti-coplanar arrangement is most commonly seen in E2 reactions. The transition state for the anti-coplanar arrangement is a staggered conformation, with the base far away from the leaving group. In most cases, this transition state is lower in energy than that for the syn-coplanar elimination. 

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