E2 Elimination Anti

The term stereoselective is often confused with the term stereospecific, and the literature abounds with views as to the most satisfactory definition. To offer some clarification, it is perhaps timely to recall a frequently used term, introduced a decade or so ago, namely the stereoelectronic requirements of a reaction. All concerted reactions (i.e. those taking place in a synchronised process of bond breaking and bond forming) are considered to have precise spatial requirements with regard to the orientation of the reactant and reagent. Common examples are SN2 displacement reactions (e.g. Section 5.10.4, p. 659), E2 anti) elimination reactions of alkyl halides (e.g. Section 5.2.1, p.488), syn (pyrolytic) elimination reactions (Section 5.2.1, p.489), trans and cis additions to alkenes (e.g. Section 5.4.5, p. 547), and many rearrangement reactions. In the case of chiral or geometric reactants, the stereoisomeric nature of the product is entirely dependent on the unique stereoelectronic requirement of the reaction such reactions are stereospecific. 

Addition of water to the cation occurs from the outside—but, in fact, this is unimportant as that stereogenic centre is about to be lost anyway. Treatment with TsCl causes an E2 anti elimination. The only proton anti to the OTs group is away from the ring junction, so this is where the new double bond goes. 

As we have just seen in the formation of 53 or 54, regioselectivity is important is the formation of double bonds in elimination reactions. In Chapter 2 both E2 (anti) elimination (sec. 2.9.A) and syn elimination (sec. 2.9.C) processes were discussed. Regiocontrol in the elimination was achieved by binding the base to the... 

E2 anti elimination on A can lead to only one alkene B via removal of the circled hydrogen. Conversion to the iodo-derivative via Sn2 inversion allows cither of the two circled hydrogens to be removed in anti elimination, leading to a mixture of alkenes B and C. 

The first example of a stereo electronic effect in this text concerned anti elimination in E2 reactions of alkyl halides (Section 5 16)... 

Quaternary ammonium hydroxides un dergo elimination on being heated It is an anti elimination of the E2 type The regioselectivity of the Hofmann elimina tion IS opposite to that of the Zaitsev rule and leads to the less highly substi tuted alkene... 

In most cases, E2 elimination proceeds via a transition state involving the anti arrangement. Nevertheless, syn elimination is possible, and, when special structural features retard anti elimination, syn elimination becomes the dominant mode. 

For example, c/s-4-t-butyleyelohexyl bromide undergoes E2 elimination at a rate about 500 times greater than the tram isomer because only the cis isomer permits anti elimination from the favored chair conformation. ... 

We can conclude that anti elimination is generally favored in the E2 mechanism, but that steric (inability to form the anti-periplanar transition state), conformational, ion pairing, and other factors cause syn elimination to intervene (and even predominate) in some cases. 

However, the E2C mechanism has been criticized, and it has been contended that all the experimental results can be explained by the normal E2 mechanism. McLennan suggested that the transition state is that shown as 18. An ion-pair mechanism has also been proposed. Although the actual mechanisms involved may be a matter of controversy, there is no doubt that a class of elimination reactions exists that is characterized by second-order attack by weak bases. " These reactions also have the following general characteristics (1) they are favored by good leaving groups (2) they are favored by polar aprotic solvents (3) the reactivity order is tertiary > secondary > primary, the opposite of the normal E2 order (p. 1319) (4) the elimination is always anti (syn elimination is not found), but in cyclohexyl systems, a diequatorial anti elimination is about as favorable as a diaxial anti elimination (unlike the normal E2 reaction, p. 1302) (5) they follow Zaitsev s rule (see below), where this does not conflict with the requirement for anti elimination. 

The relative lack of stereoselectivity with cyclopentyl compounds is reflected in the behaviour of the trans- and cis-isomerides, (31) and (32). Each, if it eliminates by E2 at all, will be converted into the same alkene (33)—(31) via SYN elimination, and (32) via ANTI elimination ... 

An ab initio study of elimination and substitution has been done for the gas-phase reaction of F with chlorocyclopropane. Among various findings it emerged that at the MP2/6-31(- -)G //HF/6-31(- -)G level, the 5 n2 pathway has a lower activation barrier by 7.3kcal moF compared with the E2 anti) pathway. 

Nitrile-forming anti eliminations from the (Z)-oximes (31) and (32) have also been found to proceed by the E2 mechanism the symmetrical transition state is little... 

The treatment of the deuteriated cis oxirane 32 by EDA in HMPA yields exclusively the nondeuteriated alcohol 33. Indeed, complexation of the lithium cation by HMPA prevents the formation of the six-center transition state. The isomerization thus follows a more common E2 process, i.e. anti -elimination. 

Problem 7.34 Assuming that anti elimination is favored, illustrate the stereospecificity of the E2 dehydro-halogenation by predicting the products formed from (a) tneso- and (b) either of the enantiomers of 2,3-dibromobutane. Use the wedge-sawhorse and Newman projections. -4... 

In order for an E2 mechanism to take place a base must approach the proton marked. In C this proton is shielded on both sides by R and R. In D the shielding is on only one side. Therefore, when anti elimination does take place in such systems, it should give more cis product than trans. Also, when the normal anti elimination pathway is hindered sufficiently to allow the syn pathway to compete, the anti — trans route should be diminished more than the anti — cis route. When syn elimination begins to appear, it seems clear that E, which is less eclipsed than F, should be the favored pathway and syn elimination should generally give the trans isomer. In general, deviations from the syn-anti dichotomy are greater on the trans side than on the cis. Thus, trans olefins are formed partly or mainly by syn elimination, but cis olefins are formed entirely by anti elimination. Predominant syn... 

The catalysts which operate by means of an E2 mechanism give a high proportion of reaction products which are formed by the anti-elimination. This fact has been discussed in Sect. 2.1 and only few remarks need to be added here. Quantum chemical calculations [73] on the transition state of the dehydrochlorination of chloroethane, initiated by an attack of a basic species, confirmed the preference of the anti-elimination over the syn-mode. On the contrary, calculations on the transition state for non-catalytic (homogeneous) thermal elimination [201,202] confirmed the syn-elimination path. 

Because of the greater acidity of a vinylic than an alkyl proton, vinyl halides, RHC=CRX, are more likely than alkyl halides to undergo EjcB elimination. However, when the proton is not rendered even more acidic by a vicinal electron-withdrawing group, and when the basic catalyst is not too strong, E2 reaction obtains. Then anti elimination is much the preferred pathway. 

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