E2 mechanism, elimination

Fig. Relative geometry of the atoms involved in the E2 elimination mechanism.

Until now, discussions have focused only on how carbanions and carbocations behave under conditions favorable for nucleophilic substitutions. However, these species may undergo other types of reactions in which unsaturation is introduced into the molecule. Such reactions are called elimination reactions and should be considered whenever charged species are of importance to the mechanistic progression of a molecular transformation. In previous chapters, SN1 and SN2 reactions were discussed. In this chapter, the corresponding El and E2 elimination mechanisms are presented. 

This product forms via an E2 elimination mechanism. Consequently, the elimination reaction is only favored if a frarcs-periplanar relationship exists between the acidic proton and the bromide. In the case of the starting material used in the fast reaction, this is the case. However, looking at the starting material used in the slow reaction, no trarcs-periplanar relationship exists between the acidic proton and the bromide. 

Figure 1.11 shows the second example. Triterpene alcohol 14 with an axial hydroxy group can be dehydrated smoothly by treatment with phosphorus oxychloride in pyridine to give 16 through conventional E2 elimination mechanism. However, dehydration of the equatorial alcohol 15 leads to a rearrangement product 17 through the mechanism as shown in Figure 1.11. This type of simple stereochemical knowledge is very useful in synthetic planning.

Although TsiSiMe2Cl is not susceptible to direct substitution by sodium methoxide in MeOH, it does nevertheless react, but the major product is (Me3Si)2CHSiMe2OMe148. The most likely mechanism for this reaction involves an intermediate sila-alkene, as shown in Scheme 5. It is interesting to note that this mechanism is directly analogous to the E2 elimination mechanism at carbon. A similar reaction has also been observed for TsiSi(CH=CH2)2Cl with sodium methoxide131. 

The base, as proton acceptor, deprotonates alkyl halide causing the removal of a halide anion. The reaction proceeds via the transition state in which both C-H and C-Br bonds break simultaneously. Such reactions are called concerted. The reaction is bimolecular because its rate depends on the concentrations of two molecular species, alkyl-bromide and the ethoxy anion. This mechanism is called the E2 elimination mechanism. 

In order to promote an Sn2 mechanism at a nonallylic 2° carbon, a different nucleophile is needed, such as acetate (AcO ). Since acetate is resonance stabilized, it is a weaker base than hydroxide, and the E2 elimination mechanism is minimized. 

You shoiald refer to the detailed discussions in Experiments [9] and [10], which describe in detail the chemistry associated with the El and E2 elimination mechanisms. The Elcb mechanism, like the El mechanism, involves a two-step process, but in this case the order of charge development is the reverse of the El mechanism. Here proton abstraction precedes loss of the leaving group. A generalized scheme is shown here ... 

The sulfene formation may occur by either a stepwise ElcB or a bimolecular E2 elimination mechanism depending on the particular reaction. 

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