Anti-periplanar

Effects that arise because one spatial arrangement of electrons (or orbitals or bonds) IS more stable than another are called stereoelectronic effects There is a stereoelec tromc preference for the anti coplanar arrangement of proton and leaving group in E2 reactions Although coplanarity of the p orbitals is the best geometry for the E2 process modest deviations from this ideal can be tolerated In such cases the terms used are syn periplanar and anti periplanar... 

Anti periplanar geometry (staggered, lower energy)... 

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. 

Anti periplanar geometry for E2 reactions is particularly important in cyclohexane rings, where chair geometry forces a rigid relationship between the substituents on neighboring carbon atoms (Section 4.8). As pointed out by Derek Barton in a landmark 1950 paper, much of the chemical reactivity of substituted cyclohexanes is controlled by their conformation. Let s look at the E2 dehydro-halogenation of chlorocyclohexanes to see an example. 

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. 

Periplanar (Section 11.8) A conformation in which bonds to neighboring atoms have a parallel arrangement. In an eclipsed conformation, the neighboring bonds are syn periplanar in a staggered conformation, the bonds are anti periplanar. 

Molar absorptivity. 502 Molecular ion (M+), 410 Molecular mechanics. 130 Molecular model, dopamine, 930 acetaminophen, 29 acetylene, 18 adenine, 67 adrenaline, 323 alanine, 28, 1016 alanylserine, 1028 rr helix, 1039 p-aminobenzoic acid, 25 anti periplanar geometry, 387 a recoline, 79 aspartame, 29 aspirin. 17 ball-and-stick, 61 /3-pleated sheet, 1039 p-bromoacetophenone, 449 bromocyclohexane, 121 butane, 80... 

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. 

The mechanism of these reactions is often El. However, in at least some cases, an E2 mechanism operates.It has been shown that stereoisomers of cyclic y-amino halides and tosylates in which the two leaving groups can assume an anti-periplanar conformation react by the E2 mechanism, while those isomers in which the groups cannot assume such a conformation either fragment by the El mechanism or do not undergo fragmentation at all, but in either case give rise to side products characteristic of carbocations. " ... 

El reactions are not stereospecific. That is, they do not require anti-periplanarity in order for the reaction to occur. Nevertheless, El reactions are stereoselective. In other... 

The first step of the structure refinement is the appHcation of distance geometry (DG) calculations which do not use an energy function but only experimentally derived distances and restraints which follow directly from the constitution, the so-caUed holonomic constraints. Those constraints are, for example, distances between geminal protons, which normally are in the range between 1.7 and 1.8 A, or the distance between vicinal protons, which can not exceed 3.1 A when protons are in anti-periplanar orientation. 

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 Newman projection formula shows that the P-hydrogen and the chlorine are anti periplanar when they are both axial. 

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