Anti-periplanar elimination

Draw (lSP2S)-l,2-clibromo-l,2-diphenylethane so that you can see its stereochemistry and so that the —H and —Br groups Lo be eliminated are anti periplanar. Then carry out the elimination while keeping ail substituents in approximately their same positions, and see what alkene results. 

Anti periplanar elimination of HBr gives (Z)-l-bromo-l,2-diphenylethylene. 

The anti periplanar requirement for E2 reactions overrides Zaitsev s rule and can be met in cyclohexanes only if the hydrogen and the leaving group are trans diaxial (Figure 11.19). If either the leaving group or the hydrogen is equatorial, E2 elimination can t occur. 

A final piece of evidence involves the stereochemistry of elimination. (Jnlike the E2 reaction, where anti periplanar geometry is required, there is no geometric requirement on the El reaction because the halide and the hydrogen are lost in separate steps. We might therefore expect to obtain the more stable (Zaitsev s rule) product from El reaction, which is just what w e find. To return to a familiar example, menthyl chloride loses HC1 under El conditions in a polar solvent to give a mixture of alkenes in w hich the Zaitsev product, 3-menthene, predominates (Figure 11.22). 

Figure 11.22 Elimination reactions of menthyl chloride. E2 conditions (strong base in 100% ethanol) lead to 2-menthene through an anti periplanar elimination, whereas El conditions (dilute base in 80% aqueous ethanol) lead to a mixture of 2-menthene and 3-menthene.

Although anti periplanar geometry is preferred for E2 reactions, it isn t absolutely necessary. The deuterated bromo compound shown here reacts with strong base to yield an undeuterated alkene. Clearly, a svn elimination has occurred. Make a molecular model of the reactant, and explain the result. 

In open-chain compounds, the molecule can usually adopt that conformation in which H and X are anti periplanar. However, in cyclic systems this is not always the case. There are nine stereoisomers of 1,2,3,4,5,6-hexachlorocy-clohexane seven meso forms and a dl pair (see p. 161). Four of the meso compounds and the dl pair (all that were then known) were subjected to elimination of HCl. Only one of these (1) has no Cl trans to an H. Of the other isomers, the fastest elimination rate was about three times as fast as the... 

Some examples of syn elimination have been found in molecules where H and X could not achieve an anti-periplanar 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. 

With acylic molecules elimination could be envisaged as taking place from one or other of two limiting conformations—the anti-periplanar (24a) or the syn-periplanar (24b) ... 

ANTI elimination [(32) — (33)] was found to proceed only 14 times faster than SYN elimination [(31)— (33)] reflecting the fact that the energy needed to distort the ring, so that (32) can assume an approximately anti-periplanar conformation, almost outweighs the normal energetic advantage of the staggered conformation over the, syn-periplanar, eclipsed one, i.e. (31). 

A is called anti-periplanar, and this type of elimination, in which H and X depart in opposite directions, is called anti elimination. Conformation B is syn-periplanar, and this type of elimination, with H and X leaving in the same direction, is called syn elimination. Many examples of both kinds have been discovered. In the absence of special effects (discussed below) anti elimination is usually greatly favored over syn elimination, probably because A is a staggered conformation (p. 139) and the molecule requires less energy to reach this transition state than it does to reach the eclipsed transition state B. A few of the many known examples of predominant or exclusive anti elimination follow. 

The relative instability of the cis-2-oxopyrido[l,2-u]pyrimidines (158 R1 = Ph) was explained by the steric conditions of the molecule the anti-periplanar arrangement of bonds a and "b" facilitates a concerted olefinforming elimination reaction leading to the amide (159 R1 = Ph). The reaction is also aided by the unfavorable interactions arising among the cis-4-phenyl groups and between the 4-phenyl group and the 6-H atom. 

In conformationally mobile systems, both syn and anti eliminations are theoretically possible. The anti elimination should be favored electronically over the syn elimination because the electron pair of the C-H bond is anti-periplanar to the leaving group. It has also been suggested (84, 85) that the syn elimination might require a configurational inversion at the C-H bond, so the electron pair of that bond becomes antiperi planar to the C-X bond (297-298). 

The anti periplanar relationship of halide and proton can be achieved only when the chlorine is axial this corresponds to the most stable conformation of neomenthyl chloride. Menthyl chloride, on the other hand, must undergo appreciable distortion of its ring to achieve an anti periplanar Cl—C—C—H geometry. Strain increases substantially in going to the transition state for E2 elimination in menthyl chloride but not in neomenthyl chloride. Neomenthyl chloride undergoes E2 elimination at the faster rate. 

E2 elimination proceeds via anti-periplanar arrangement of the OTs and the adjacent H. 

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