Bimolecular 1,2-elimination reactions

There is a tendency in some elementary organic textbooks to indicate elimination reactions by encircling the two groups that are closest to each other, and then showing the final eliminated product. This type of lasso chemistry predicts that both groups are eliminated from the same side of the starting material, i.e. that there has been -elimination. In practice, this is found to be rather rare. Instead, what is observed is that the two groups that are eliminated come from opposite sides of the molecule, i.e. anti-elimination. We will now examine why this is the case. 

Write the equation for the E2 reaction between a general base, B, and the substrate, R2CH-CXR2. 

This reaction has resulted in HX being eliminated. Compare this elimination reaction with the SN2 

The species X- is called the nucleofuge. In each case, the attacking species, namely the nucleophile and the Bronsted-Lowry base, are fundamentally very similar. With respect to the electronic characteristics of the attacking species and the species that is being attacked, suggest what are the similarities that are shared by, and the differences that distinguish, these two reactions. 

In the case of the substitution reaction, the attacking species is acting like a nucleophile, which means that it is acting as a soft base, and so it attacks the softer centre, which in this case is the 8+ carbon. However, in the case of the elimination reaction, the attacking species is acting as a hard base, and thus attacks the harder centre, which is the a-hydrogen. The a bond electrons in the carbon/hydrogen bond then act in a similar manner to an internal substitution 

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E2 reaction (Section 11.8) A bimolecular elimination reaction in which both the hydrogen and the leaving group are lost in the same step. 

Notice that there is only one mechanistic step (no intermediates are formed), and that step involves both the substrate and the base. Because that step involves two chemical entities, it is said to be bimolecular. Bimolecular elimination reactions are called E2 reactions, where the 2 stands for bimolecular. ... 

J. F. Bunnett, The Mechanism of Bimolecular -Elimination Reactions, Angew. Chem. Int. Ed. Engl. 1962,1, 225-235. 

Tertiary alkyl chlorides are easily dehydrochlorinated by base (via the E2, or bimolecular elimination reaction mechanism), but the environment of the degrading resin is not basic. Loss of hydrogen chloride to yield an olefin can occur principally by the El, or monomolecular elimination reaction. This is a slow reaction because, in the rate-determining step, the C—Cl bond is broken to form two separated oppositely charged particles. The reaction rate is not assisted by the acid present. 

E2 reaction or bimolecular elimination reaction (Section 9.2) An elimination reaction that follows a concerted mechanism in which a base removes a proton simultaneously with the departure of the leaving group. 

This is an example of the first step of an E2 (bimolecular elimination) reaction mechanism. Note the base-mediated deprotonation of the diester converting the ferf-butoxide anion to ferf-butanol. For clarity, the anion was repositioned and the bond was lengthened. Arrow pushing is illustrated below ... 

Munk and Kim22 described a new approach to the synthesis of stereoisomeric enamines of known configuration which takes advantage of the well-documented facility and stereospecificity of the bimolecular -elimination reaction. 

Mixtures of Z and E stereoisomers have been obtained in most syntheses. This undoubtedly holds in the presence of an acid catalyst. The tendency of the E-enamine to isomerize to the Z-enamine was readily observed on numerous occasions under unexpectedly mild conditions. Munk and Kim357 summarized the requirements for the stereospecific synthesis of enamines. First, if the introduction of the double bond is to be the final step of the synthesis, it must be stereospecific in character. Second, once the enamine is formed it must retain its stereochemical integrity under the conditions employed in the double-bond-forming step. The base-induced bimolecular -elimination reaction fulfills both these requirements. Indeed, treatment of the mesitoate esters of ( )-threo- (99) and ( )-erythro- (100) l-(4-morpholino)-l,2-diphenylethanol with... 

The bimolecular elimination reaction (E2) also requires a specific arrangement (B) of four atomic centers. In B, the groups X and Y are coplanar with the two carbon atoms and antiparallel (anti-trans, or true... 

Sunderwirth, S. G., Wood, J. K. Stereochemistry and orientation in bimolecular elimination reactions. Trans. Kans. Acad. Sci. 1967, 70, 17-32. 

Sulfones and sulfoxides containing at least one b-hydrogen atom undergo a bimolecular elimination reaction (E2) an treatment with alkoxides, leading preferentially to the formation of the least substituted alkene (Hofmann s rule) (Scheme 37). The reaction resembles the analogous eliminations of quaternary ammonium and sulfonium salts (see Chapter 6, p. 83). 

We will now look at the special factors that are involved in the bimolecular -elimination reaction. 

Now that we have studied bimolecular elimination reaction in some depth, we will turn our attention to the unimolecular pathway. 

Are there uni- and bimolecular elimination reaction mechanisms, like there were for substitution reactions ... 

Pincock [Pi 64] characterized solvents on the basis of the solvent dependence of the ionic decomposition of t-butyl peroxyformate. However, not only the rate, but also the mechanism of decomposition of this compound is solvent-dependent. For example, in chlorobenzene it decomposes in a slow unimolecular reaction in which the peroxide bond is split in n-butyl ether the decomposition proceeds via radical attack on the peroxide oxygen atoms and in the presence of pyridine a bimolecular elimination reaction occurs, with the formation of t-butanol and carbon dioxide. Pincock used the solvent dependence of the rate of this latter reaction to characterize the solvent. 

This conclusion would be correct only if fcs and are both much greater than k and k2, however. In general, we would expect the rate of rotation about a carbon-carbon single bond to be much faster than the rate of a bimolecular elimination reaction, so equation 6.61 is almost certainly invalid. 

This assumption is adequate for our purposes, but it is not always rigorously correct. See, for example, reference 117a, p. 146, for a discussion of Ah/Ao values in studies of bimolecular elimination reactions. 

It is useful to discuss the stereochemistry of bimolecular elimination reactions in terms of the H—C-C—L dihedral angle (Figure 10.18). Inan anti-periplanar conformation, the dihedral angle H-C 3-Cq,-L is near 180°, while it is near 0° in a syn-periplanar conformation. If dihedral angle is exactly 180°, the conformation is anti-coplanar, while it is syn-coplanar if the dihedral is exactly 0°. An anti-clinal conformation has a dihedral angle of approximately 120°, while a syn-clinal conformation has a dihedral of about 60°. 

Among other factors to be considered in predicting the products of bimolecular elimination reactions is the role of aggregation of the base, which can be particularly significant for f-BuOK in t-BuOH solution. A variety of different models have been proposed to account for the interaction of aggregated bases with substrates in E2 reactions. For a discussion, see reference 21. 

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