El Eliminations

Primary alkyl halide slowest rate of El elimination... 

The best examples of El eliminations are those carried out m the absence of added base In the example cited m Eigure 5 12 the base that abstracts the proton from the car bocation intermediate is a very weak one it is a molecule of the solvent ethyl alcohol At even modest concentrations of strong base elimination by the E2 mechanism is much faster than El elimination... 

Dehydration of alcohols (Sections 5 9-5 13) Dehydra tion requires an acid catalyst the order of reactivity of alcohols IS tertiary > secondary > primary Elimi nation is regioselective and proceeds in the direction that produces the most highly substituted double bond When stereoisomeric alkenes are possible the more stable one is formed in greater amounts An El (elimination unimolecular) mechanism via a carbo cation intermediate is followed with secondary and tertiary alcohols Primary alcohols react by an E2 (elimination bimolecular) mechanism Sometimes elimination is accompanied by rearrangement... 

El eliminations begin with the same uni molecular dissociation we saw in the Sfsjl reaction, but the dissociation is followed by loss of H+ from the adjacent carbon rather than by substitution. In fact, the El and SN1 reactions normally occur together whenever an alkyl halide is treated in a protic solvent with a non-basic nucleophile. Thus, the best El substrates are also the best SN1 substrates, and mixtures of substitution and elimination products are usually obtained. For example, when 2-chloro-2-methylpropane is warmed to 65 °C in 80% aqueous ethanol, a 64 36 mixture of 2-methyl-2-propanol (Sjql) and 2-methylpropene (El) results. 

I Tertiary7 alkyl halides E2 elimination occurs when a base is used, but SN1 substitution and El elimination occur together under neutral conditions, such as in pure ethanol or water. ElcB elimination takes place if the leaving group is two carbons away from a carbonyl group. 

Carboxylic acids can be converted into olefins, when there is a leaving group such as H (Eq. 27), SiMcj, SPh or COjH in the p-position. The olefin is formed, when the carbocation, that is generated by decarboxylation, undergoes a subsequent El-elimination. Some examples are summarized in Table 11 (Nos. 1-10). 

For El eliminations, if there is a free carbocation (25), it is free to rotate, and no matter what the geometry of the original compound, the more stable situation is the one where the larger of the D-E pair is opposite the smaller of the A-B pair and the corresponding alkene should form. If the carbocation is not completely free, then to that extent, E2 type products are formed. Similar considerations apply in ElcB eliminations. ... 

The factors that promote unimolecular, as opposed to bimolecular (E2), elimination are very much the same as those that promote SN1 with respect to Sw2, namely (a) an alkyl group in the substrate that can give rise to a relatively stable carbocation, and (b) a good ionising, ion-solvating medium. Thus (a) is reflected in the fact that with halides, increasing El elimination occurs along the series,... 

Thus in the above case the elimination product is found to contain 82 % of (7). Unexpected alkenes may arise, however, from rearrangement of the initial carbocationic intermediate before loss of proton. El elimination reactions have been shown as involving a dissociated carbocation they may in fact often involve ion pairs, of varying degrees of intimacy depending on the nature of the solvent (cf. SN1, p. 90). 

A different P-hydrogen can be removed from the carbocation, so as to form a more highly substituted alkene than the initial alkene. This deprotonation step is the same as the usual completion of an El elimination. (This carbocation could experience other fates, such as further rearrangement before elimination or substitution by an S l process.)... 

Deprotonation of acidic C6 by DBU gives a carbanion, which undergoes a Michael reaction to CIO. The new carbanion at CIO can deprotonate C3 to give a new carbanion, and this can undergo an aldol reaction to C12. Now our two new C-C bonds have been formed. We still have to break C2-C6 and two C-0 bonds. The alkoxide at 013 can deprotonate MeOH, which can then add to C2. Fragmentation of the C2-C6 bond follows to give a C6 enolate. The C6 enolate then deprotonates 013, and intramolecular transesterification occurs to form the 013-C7 bond and to break the C7-09 bond. MeO then comes back and promotes El elimination across the C3-C12 bond to break the Cl2-013 bond and give the product. The intramolecular transesterification explains why C7 becomes an acid and C2 remains an ester in the product. 

The immediate precursor retains the 06-C3 bond and would have a C8-06 bond and a C8=C9 n bond. This calls for an SnI substitution at C8 to replace the C8-OMe bond with a C8-06 bond and an El elimination to make the C8=C9 k bond. The overall reaction is an orthoamide Claisen rearrangement. 

Finally, conversion of 10 to 11 involves addition of the very nucleophilic MeLi to the ketone workup gives the alcohol. Then El elimination promoted by the acid TsOH gives the alkene. 

Secondary amides 38 based on cumylamine or dialkyIhydrazines are particularly useful. The cumyl protecting group is removed by El elimination in strong acid to form 39 or by formation of a nitrile 40 (Scheme 20), while hydrazines can be cleaved oxidatively. 

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