Elimination anti-periplanar conformation

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

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). 

Mechanism of E2 elimination from syn-periplanar and anti-periplanar conformations. 

Upon E2 elimination. (I R.2R)-1 -bromo-1,2-diphenylpropane produces only (Z)-1,2-diphenylpropene. This result demonstrates that the reaction occurs entirely from the anti-periplanar conformation shown. The enantiomer of this compound, (IS,2S)-l-bromo-l,2-diphenylpropane, also produces only the (Z)-alkene when it undergoes an E2 elimination reaction. (To simplify viewing, the phenyl groups are shown as single blue atoms in the ball-and-stick models.)... 

The removal of a molecule of a hydrogen halide from an alkyl halide to yield an alkene is effected under strongly basic conditions, e.g. a concentrated alcoholic solution of sodium or potassium hydroxide or alkoxide. This overall reaction has been submitted to most rigorous mechanistic studies. Most of the factors (temperature, nature of base, structure of substrate, solvent, etc.) which control product composition have been evaluated. It thus appears that under the conditions noted above, an E2 process, in which the participating sites adopt an anti-periplanar conformation leading to an anti-elimination process, is generally favoured. 

In an E2 elimination, the new 7t bond is formed by overlap of the C-H a bond with the C-X a antibonding orbital. The two orbitals have to lie in the same plane for best overlap, and now there are two conformations that allow this. One has H and X syn-periplanar, the other anti-periplanar. The anti-periplanar conformation is more stable because it is staggered (the syn-periplanar conformation is eclipsed) but, more importantly, only in the anti-periplanar conformation are the bonds (and therefore the orbitals) truly parallel. 

E2 eliminations therefore take place from the anti-periplanar conformation. We shall see shortly how we know this to be the case, but first we consider an E2 elimination that gives mainly one of two possible stereoisomers. 2-Bromobutane has two conformations with H and Br anti-periplanar, but the one that is less hindered leads to more of the product, and the -alkene predominates. 

It is useful to discuss the stereochemistry of bimolecular elimination reactions in terms of the H—C-C—L dihedral angle (Figure 10.18). Inan anti-periplanar conformation, the dihedral angle H-C 3-Cq,-L is near 180°, while it is near 0° in a syn-periplanar conformation. If dihedral angle is exactly 180°, the conformation is anti-coplanar, while it is syn-coplanar if the dihedral is exactly 0°. An anti-clinal conformation has a dihedral angle of approximately 120°, while a syn-clinal conformation has a dihedral of about 60°. 

Experimental evidence indicates that the five atoms involved in the E2 elimination reaction must lie in the same plane the anti-periplanar conformation is preferred. This conformation is necessary for the orbital overlap that must occur for the TT bond to be generated in the alkene. The sp -hybridized atomic orbitals on carbon that comprise the C—H and C—a bonds broken in the reaction develop into the p orbitals comprising the -ir bond of the alkene formed ... 

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... 

Syn elimination and the syn-anti dichotomy have also been found in open-chain systems, though to a lesser extent than in medium-ring compounds. For example, in the conversion of 3-hexyl-4-d-trimethylammonium ion to 3-hexene with potassium ec-butoxide, 67% of the reaction followed the syn-anti dichotomy. In general syn elimination in open-chain systems is only important in cases where certain types of steric effect are present. One such type is compounds in which substituents are found on both the P and the y carbons (the unprimed letter refers to the branch in which the elimination takes place). The factors that cause these results are not completely understood, but the following conformational effects have been proposed as a partial explanation. The two anti- and two syn-periplanar conformations are, for a quaternary ammonium salt ... 

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) ... 

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. 

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