P- eliminations

Athene formation requires that X and Y be substituents on adjacent carbon atoms By mak mg X the reference atom and identifying the carbon attached to it as the a carbon we see that atom Y is a substituent on the p carbon Carbons succeedmgly more remote from the reference atom are designated 7 8 and so on Only p elimination reactions will be dis cussed m this chapter [Beta (p) elimination reactions are also known as i 2 eliminations ] You are already familiar with one type of p elimination having seen m Section 5 1 that ethylene and propene are prepared on an industrial scale by the high temperature dehydrogenation of ethane and propane Both reactions involve (3 elimination of H2... 

Except for the biochemical example just cited the stractures of all of the alcohols m Section 5 9 (including those m Problem 5 13) were such that each one could give only a single alkene by p elimination What about ehmmahon m alcohols such as 2 methyl 2 butanol m which dehydration can occur in two different directions to give alkenes that are conshtutional iso mers Here a double bond can be generated between C 1 and C 2 or between C 2 and C 3 Both processes occur but not nearly to the same extent Under the usual reachon con dihons 2 methyl 2 butene is the major product and 2 methyl 1 butene the minor one... 

In 1875 Alexander M Zaitsev of the University of Kazan (Russia) set forth a gen erahzation describing the regioselectivity of p eliminations Zaitsev s rule summarizes the results of numerous experiments m which alkene mixtures were produced by p elim matron In its original form Zaitsev s rule stated that the alkene formed in greatest amount is the one that corresponds to removal of the hydrogen from the f3 carbon hav mg the fewest hydrogens... 

Dehydrohalogenation is the loss of a hydrogen and a halogen from an alkyl halide It IS one of the most useful methods for preparing alkenes by p elimination... 

The regioselectivity of dehydrohalogenation of alkyl halides follows the Zaitsev rule p elimination predominates m the direction that leads to the more highly substi tuted alkene... 

Alkenes are prepared by P elimination of alcohols and alkyl halides These reactions are summarized with examples m Table 5 2 In both cases p elimination proceeds m the direction that yields the more highly substituted double bond (Zaitsev s rule)... 

Step 2 The oxidation step can be viewed as a p elimination Water acts as a base to remove a proton from carbon while the O—Cr bond breaks... 

When quaternary ammonium hydroxides are heated they undergo p elimination to form an aUcene and an amine... 

Mechanism of the initial reaction, known as alkaline peeling, is shown in equation 4. EnoHzations and tautomerizations take place easily because of the contiguous hydroxyl groups. The hydroxyl or substituted hydroxyl on the second, ie, P-carbon, from a carbonyl group is released from the molecule by P-elimination. 

The metallocycle [67719-69-1] (24) undergoes an apparent P-elimination to a carbene-like reagent, which adds regiospecrfically to terminal acetylenes... 

The P eliminations can be fiirther subdivided by closer examination of the mechanisms involved. Three distinct limiting mechanisms are outlined below. 

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. 

A large variety of methods is applicable to the formation of isolated double bonds. This permits selection of reagents compatible with other functionality present. Alcohol dehydration, ester elimination and other nonreductive p eliminations are the most common methods. Reductive elimination of halo-hydrins, vic-dihalides, etc., and of a variety of ketone derivatives has also been used. 

Dehydrogenation of alkanes is not a practical laboratory synthesis for the vast majority of alkenes. The principal methods by which alkenes are prepared in the laboratory are two other p eliminations the dehydration of alcohols and the dehydrohalo-genation of alkyl halides. A discussion of these two methods makes up the remainder of this chapter. 

When quaternary anmonium hydroxides are heated, they undergo p elimination to fonn an alkene and an amine. 

P Elimination (Section 5.8) Reaction in which a double or triple bond is formed by loss of atoms or groups from adjacent atoms. (See dehydration, dehydrogenation, dehydro-halogenation, and double dehydrohalogenation.)... 

The E2 P-elimination reaction is a base-catalyzed concerted mechanism ... 

The intermediacy of dipolar species such as 186 has been demonstrated by reaction of enamines with 2-hydroxy-1-aldehydes of the aromatic series (129). The enamine (113) reacts in benzene solution at room temperature with 2-hydroxy-1-naphthaldehyde to give the crystalline adduct (188) in 91 % yield. Oxidation with chromium trioxide-pyridine of 188 gave 189 with p elimination of the morpholine moiety. Palladium on charcoal dehydrogenation of 189 gave the known 1,2-benzoxanthone (129). 

Mesylates and tosylates may be used as variants of the 0-sulfate ester. For instance, 55% of aziridine 7 was obtained from base-mediated cyclization of amino mesylate 6. In comparison, the classic Wenker protocol only gave 3% of 7. In another instance, A-tosyl amino alcohol 8 was tosylated to give 9, which was transformed to aziridine 10 in 64% yield, along with 29% of the P-elimination product due to the presence of the ester moiety. Likewise, aziridine 12 was assembled from tosylate 11 in two steps and 60% yield. ... 

In the case of unsubstituted BFO 1 reacting with an enamine, the following mechanism is generally accepted in the literature. The first step is nucleophilic addition of an enamine 2 to electrophilic BFO 1 to form the intermediate 12. Ring closure occurs via condensation of the imino-oxide onto the iminium functionality to give 13. Finally, P-elimination of the dialkyl amine produces the quinoxaline-1,4-dioxide 4. 

Eleb.2-step, base-indueed P-elimination via earbanion... 

A significant effect of Lewis acids on such transamiular C-H insertion reactions has been demonstrated. Treatment of 5,6-epoxycydooctene (31) with s-BuLi/ (-)-sparteine gave allylic alcohol 32, formally the product of P-elimination, in good yield (and ee) (Scheme 5.9). In the presence of BF3-Et20, however, alcohol 33 was produced as a result of a-lithiation, in 75% yield and 71 % ee [16]. 

The first step in the nonreversible degradation reactions is the formation of a reactive a-dicarbonyl species through the p-elimination of a hydroxide ion. The subsequent reaction pathways to all degradation products can be described by just five reaction types, namely, p-elimination, benzilic acid rearrangement, a-dicarbonyl cleavage, aldol condensation, and retro-aldol condensation (see Fig. 7).31 Retro-aldol condensation and a-dicarbonyl cleavage involve C-C bond... 

These reactions can take place by either heterolytic or pericyclic mechanisms. Examples of the latter are shown on page 1322. Free-radical P eliminations are extremely rare. In heterolytic ehminations, W and X may or may not leave simultaneously and may or may not combine. 

This is an extrusion reaction (see Chapter 17). In another example of a process in category 2, aldehydes and ketones with a 7 hydrogen can cleave in still another way (a P elimination, see Chapter 17) ... 

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