Elimination reactions regiochemistry

The nature of the transition state in elimination reactions is of great importance, since it controls the regiochemistry of p elimination in compounds in which the double bond can be introduced in one of several positions. These effects are discussed in the next section. 

Elimination reactions are more complex than substitution reactions for several reasons. There is, for example, the problem of regiochemistry. What... 

The regiochemistry of this elimination reaction resembles that observed by Davis et al. (see Scheme 9) [23]. The special nature of the bonds in three-mem-bered rings is probably responsible for this exclusive regiochemistry. It is of interest to note that 3,3-dimethylaziridine-2-carboxylic ester indeed leads to the corresponding 3H-azirine ester upon Swern oxidation here there is, of course, no choice. 

Here is where we get back to mechanisms. Whether we are talking about Zaitsev vs. Hoffman elimination reactions or about Markovnikov vs. anti-Markovnikov addition reactions, the explanation of the regiochemistry for every reaction is contained within the mechanism. If we completely understand the mechanism, then we will understand why the regiochemistry had to be the way it turned out. By understanding the mechanism, we eliminate the need to memorize the regiochemistry for every reaction. With every reaction you encounter, you should consider the regiochemistry of the reaction and look at the mechanism for an explanation of the regiochemistry. 

We have mentioned many times that you need to think about the regiochemistry and stereochemistry of every reaction. We will now consider those issues for elimination reactions, beginning with regiochemistry. 

Bottom line How should you study addition reactions For every addition reaction that you encounter, you must draw the mechanism first. Once you completely understand it, then you can look for the stereochemistry and regiochemistry and try to justify them based on the mechanism. Then you will be in a position to understand any of the factors that your textbook mentions about that reaction. Those factors will often help you determine when and how quickly the reaction occurs. There will usually be fewer factors than we saw in substitution and elimination reactions. Usually only one or two factors will be covered on any reaction (if even that). You should then turn to the end of Chapter 8 and summarize this information for each reaction. You will record the mechanism and the key information regarding stereochemistry and regiochemistry. If you repeat this process for every reaction that you learn (not just addition reactions, but all reactions), then you will be in really good shape. 

Chapter 8 discussed the stereochemistry of substitution reactions—that is, what happened to the stereochemistry when the reaction occurred at a carbon chirality center. This section discusses the regiochemistry of the elimination reaction—that is, what happens when a reaction can produce two or more structural isomers. The structural isomers that can often be produced in elimination reactions have the double bond in different positions. As shown in Figure 9.5, elimination of hydrogen chloride from neomenthyl chloride produces two structural isomers but in unequal amounts. 

Another problem that occurs with eliminations is the regiochemistry of the reaction. As we saw in Chapter 9, most eliminations follow Zaitsev s rule and produce the more highly substituted alkene as the major product. However, a significant amount of the less highly substituted product is also formed. In addition, mixtures of ds and trans isomers are produced when possible, further complicating the product mixture. Because separating a mixture of such isomers is usually a difficult task, elimination reactions are often not the best way to prepare alkenes. (Other methods will be described in subsequent chapters.) However, if only one product can be formed, or if one is expected to greatly predominate in the reaction mixture, then these elimination reactions can be quite useful. 

Follow the steps listed in the preceding Visual Summary of Key Reactions section. Identify the leaving group, the electrophilic carbon, and the nucleophile (or base). Then determine which mechanism is favored (see Section 9.7). Watch out for stereochemistry where important, regiochemistry in elimination reactions, and carbocation rearrangements when the mechanism is SN1 or El. 

In the case presented in Scheme SI the di erences in behavior of the p-hydroxyalkyl selenoxides of different origin may be due to different diastereoisomeric ratios the equilibration of which is hampered by steric crowding. Therefore, there is a pronounced stereochemical effect on the regiochemistry of sele-noxide syn elimination reactions. This elimination reaction leads to allyl alcohols where the a,3-disub-stituted double bond possesses exclusively the ( )-stereochemistry when reasonably bulky groups are present (Scheme 52 heme 105, c Scheme 174, Those substituted with stnaller groups... 

The regiochemistry of the elimination depends on the type of elimination process that occurs. The El process favors the formation of the more substituted alkene because reversible protonation of the double bond occurs and creates an equilibrium mixture that favors the more stable product. The E2 regiochemistry is controlled by the need to minimize steric interactions in the transition state the size of the base is important because one proton may be more accessible than another, as in Figure 4.21. The ElcB regiochemistry is determined by the loss of the most acidic proton. Elimination reactions can produce different stereoisomers, for example, cis and trans alkenes. Since the trans isomer is usually of lower energy because of steric reasons, it usually predominates over the cis isomer in the product mixture. 

Chapter 5 considers the relationship between mechanism and regio- and stereoselectivity. The reactivity patterns of electrophiles such as protic acids, halogens, sulfur and selenium electrophiles, mercuric ion, and borane and its derivatives are explored and compared. These reactions differ in the extent to which they proceed through discrete carbocations or bridged intermediates and this distinction can explain variations in regio- and stereochemistry. This chapter also describes the El, E2, and Elcb mechanisms for elimination and the idea that these represent specific cases within a continuum of mechanisms. The concept of the variable mechanism can explain trends in reactivity and regiochemistry in elimination reactions. Chapter 6 focuses on the fundamental properties and reactivity of carbon nucleophiles, including... 

---