Elimination steric considerations

Now let s consider the effect of the substrate on the rate of an E2 process. Recall from the previous chapter that Sn2 reactions generally do not occur with tertiary substrates, because of steric considerations. But E2 reactions are different than Sn2 reactions, and in fact, tertiary substrates often undergo E2 reactions quite rapidly. To explain why tertiary substrates will undergo E2 but not Sn2 reactions, we must recognize that the key difference between substitution and elimination is the role played by the reagent. In a substitution reaction, the reagent functions as a nucleophile and attacks an electrophilic position. In an elimination reaction, the reagent functions as a base and removes a proton, which is easily achieved even with a tertiary substrate. In fact, tertiary substrates react even more rapidly than primary substrates. 

Nucleophilic attack by this species at the a-carbon atom will also be governed by the same steric considerations as in thermal decomposition and hence the inverse relationship of thermal stability and resistance to nucleophilic attack, and anti-wear activity and ease of nucleophilic attack. Further thermal processes involve olefin elimination from alkyl groups and lead to the formation of phosphorus acids. Nucleophilic substitutions of one phosphorus species by another leads to P-O-P structures and zinc mercaptide Zn(SR)2 as a reaction intermediate. Reaction of this mercaptide with dithiophosphate leads to trithiophosphates and eventually tetrathiophosphates. Finally, an oil-insoluble deposit is formed of a mixture of zinc thiophosphate and zinc pyro- and polypyrothiophosphates. 

The ease of elimination of the RX molecule of Eq. (d) depends on steric considerations and on the facility of RN and BX bond cleavage. The choice of R combinations and X combinations in the starting materials can direct the syntheses to the desired products. Reactions of primary amines with BCI3 have been most widely exploited owing to the ease of elimination of HCl. The addition of tertiary amine to these reactions facilitates sterically hindered eliminations. 

Elimination will also be favored over substitution when the alkyl portion of the alkyl halide can have a fairly stable carbocation (i.e. tertiary alkyl halides), and if the alkyl portion is a larger group (because of steric considerations, elimination will predominate over substitution), With this in mind, we may predict the major products of reactions (a) through (d). 

As in the El-E2-ElcB mechanistic spectrum, Bredt s rule applies and if a double bond is present, a conjugated system will be preferred, if sterically possible. Apart from these considerations, the following statements can be made for Ei eliminations ... 

These studies on steric assistance (12) led to a detailed consideration of the possible role of steric effects in a wide variety of systems, such as steric effects in displacement reactions 10, 14), steric effects in elimination reactions 15,16), steric effects in alicyclic systems (77), and steric effects in... 

Yet another possibility is illustrated by the propene (or ethylene) dimerization catalyzed by 7r-l,l,3,3-tetraphenylallylnickel bromide (26) activated with ethylaluminum dichloride the isolation of considerable amounts of 1,1,3,3-tetraphenylpropene (27) from the reaction mixture suggests that a hydrogen atom has been transferred from the substrate olefin to the sterically hindered 1,1,3,3-tetraphenylallyl system under formation of 3 [Eq. (7)] (81). The subsequent formation of the HNiY species from 3 can then take place by insertion of a second propene molecule and /3-hydrogen elimination, as discussed above. 

The process for isomerization of EB requires that some fraction of the feed be maintained in a saturated state, as described in Sections 14.4.1.1 and 14.4.1.2. The ability to isomerize the EB is affected by the naphthene concentration and constrained by the equilibrium ratio of EB/xylenes at the reaction temperature. Use of a pore-restricted molecular sieve can be used to eliminate more sterically demanding species from the reaction network and can effectively remove these from consideration. In this maimer, one can achieve higher levels of a desired species, such as PX from EB, than could ordinarily be obtained by isomerization over a larger-pore zeolite. 

In the two systems so far discussed it is impossible to obtain a quantitative idea of the relative importance of the inductive and resonance effects because it is impossible to achieve the operation of one of the effects without the other. When nitrogen is the basic centre, this becomes possible by steric fixation of the nitrogen lone pair orbital in the plane of the benzene ring, which virtually eliminates its overlap with the 7r-electron orbital of the ring carbon and hence also the mesomerism. So the enhanced acidity of the anilinium ion (pAT = 4-62) as compared with methylammonium (pAfg = 10-67) has been shown (Wepster, 1952) to be half inductive and half mesomeric in origin by a consideration of the following systems ([10]-[12]) ... 

As with other enaminones, aqueous KOH effects addition of OH- to C-4 and subsequent elimination of ammonia involving conversion to 3-acyl tetramic acids (14) (Section III). The reaction pathway, according to this mechanism, depends on the steric requirements of the substituents at C-5. The reaction proceeds rapidly and to completion in the case of R2 = R3 = Me and R2—R3 = —(CH2)4—. Slightly more sterically hindered substituents like R2 = R3 = Et considerably hinder the reaction (87TH2 89MI1). (See Fig. 6.)... 

Further support for the intermolecular route is provided by the isolation of the substituted trans-cinnamic acid, the normal Perkin product. This arises through elimination of carboxylic acid from the fully acylated species (393) rather than cyclization. For example, a considerable amount of 2-acetoxy-3-methoxycinnamic acid is formed from 3-methoxysalicylaldehyde, perhaps as a result of steric interference with cyclization so allowing the intermolecular process to predominate (39JPR(152)23). It should be noted that trans- 2-hydroxycinnamic acids do not cyclize under normal Perkin conditions, though the cis isomer does so quite readily. Isomerization of the trans acid has been effected by treatment with a trace of iodine in acetic anhydride or by UV irradiation. 

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