Elimination reactions ElcB processes

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. 

Abstract This chapter emphasises on the important aspects of steric and stereo-electronic effects and their control on the conformational and reactivity profiles. The conformational effects in ethane, butane, cyclohexane, variously substituted cyclohexanes, and cis- and tra/ ,v-decalin systems allow a thorough understanding. Application of these effects to E2 and ElcB reactions followed by anomeric effect and mutarotation is discussed. The conformational effects in acetal-forming processes and their reactivity profile, carbonyl oxygen exchange in esters, and hydrolysis of orthoesters have been discussed. The application of anomeric effect in 1,4-elimination reactions, including the preservation of the geometry of the newly created double bond, is elaborated. Finally, a brief discussion on the conformational profile of thioacetals and azaacetals is presented. 

The (3-elimination reaction of carbonyl compounds having a leaving group at the p-position is promoted by superbases via an ElcB process. Allin et al. described a synthesis of deplacheine (107) [32], in which the p-methanesulfonyloxycarbonyl compound 105 obtained from the aldol reaction of 104 and acetaldehyde was selectively converted into the desired -isomer 106 by the use of DBN in THE This enone 106 was successfully led to the natural product 107 (Scheme 7.22). 

Two basic mechanisms can account, in general, for the interpretation of /3-elimination reactions. The E2 type pathway is a one-step process occurring in a concerted fashion, whereas in a two-step El mechanism a carbocationic intermediate is involved. A third mechanistic possibility, the ElcB elimination, may occur when a molecule possesses a poor leaving group and a highly acidic... 

The mechanism for an aldol condensation has two parts (Mechanism 22.6). The first part is just an aldol addition reaction, which has three mechanistic steps. The second part has two steps that accomplish the elimination of water. Normally, alcohols do not undergo dehydration in the presence of a strong base, but here, the presence of the carbonyl group enables the dehydration reaction to occur. The a position is first deprotonated to form an enolate ion, followed by expulsion of a hydroxide ion to produce a,p unsaturation. This two-step process, which is different from the elimination reactions we saw in Chapter 8, is called an Elcb mechanism. In an Elcb mechanism, the leaving group only leaves after deprotonation occurs. 

Elcb mechanism (Sect. 22.3) An elimination reaction in which the leaving group only leaves after deprotonation occurs. This process occurs at the end of an Aldol condensation. 

The Ef,l reaction involves the same intermediate carbonium ion as an Sjyl replacement of Y and its rate is governed by similar considerations. Likewise, the ElcB process involves an initial step analogous to a prototropic reaction (Y replacing hydrogen). Both reactions are of EOg type and the effect of structure on their rates can be predicted in the same way as the rates of S,yl reactions (p. 237) or of deprotonations by base (p. 243). Thus the Effl reaction will be favored by -I, E, and —E substituents a to X and the lcB reaction by -H/, E, or -f substituents a to Y. Indeed, elimination reactions involving a proton a to a powerful -f group such as acyl always take place by the ElcB mechanism, as in the conversion of )S-chloroethyl ketones to vinyl ketones. 

STEP 3 OF FIGURE 20.5 FUMARATE ELIMINATION The third step in the urea cycle, conversion of argininosuccinate to arginine plus fumarate, is an elimination reaction catalyzed hy argininosuccinate lyase. The process occurs hy an ElcB mechanism (Section 17.7), with a histidine residue on the enzyme... 

In contrast with the previously commented one-step E2 reactions, ElcB eliminations are stepwise processes, consisting in the initial loss of a proton by the action of the base, followed by the departure of the leaving group (Scheme 12.3). The relative magnitudes of the rate constants involved k, k., k2) determine which is the slow step of the reaction. Hence, different types of ElcB mechanisms could be considered. ... 

These relations seem to be valid for the dehydration of primary alcohols, but secondary and tertiary alcohols may need other combinations of acidic and basic sites. It has been observed that the dehydration of tert-butanol was more sensitive to the presence of strongly acidic sites than the reaction of methanol, but both processes required basic sites [8]. All this is in accordance with the dynamic model of elimination mechanisms presented in Sect. 2.1, which allows transition from El to E2 or further to ElcB according to the structure of the reactant and the nature of the catalyst. 

The nature of nitrooleflns has little influence on the efficiency of this one-pot process, and other functionalities snch as cyano, ether, and chlorine can be preserved. The role of the nitro gronp is crncial in the reported process since its electron-withdrawing effect firstly helps the nncleophilic attack of the amino or hydroxyl fnnctionality to the alkene, then it allows a stabilized carbanion with the consequent formation of an intramolecnlar C-C bond (nitroaldol reaction) and, finally, favors the elimination of water (ElcB). Moreover, the nse of solvent-free conditions (SFC) in combination with heterogeneons catalyst represents one of the more powerful green chemical technology procednres. 

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