Anti-periplanar conformation

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

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. " ... 

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). 

High diastereofacial selectivities are observed in cycloadditions and Michael additions with ot,(3-unsaturated esters having chiral heterocyclic auxiliary at the p-position, as shown in Schemes 11.20, 11.21, and 11.25, and cannot be well-explained using Kozikowski s awfi-periplanar model (124,125) or Houk s inside alkoxy model (126,127). Both the anti-periplanar conformation and the syn-periplanar conformation of the acceptors participate in the transition structures, depending on nonbonding interactions in the dipole-chiral auxiliary pair (121). 

The three-membered ring system, although the most highly strained of all the ring sizes, is readily formed in the cyclisation step. This arises from the fact that carbon-carbon bond formation in this case is an irreversible intramolecular S 2 displacement of halogen reaction, which is facilitated by the thermodynamically favourable anti-periplanar conformation of the anion. 

Vic -dihalides exhibit a single 2e polarographic wave, while nonvicinal ( >3) dibromides normally are reduced in two separate 2e transfers. The reduction potential of the dihalides is more positive (A E +0,4 to +0,7 V) than that of the monohalides 3,6>321 3221. A plot of the half-wave potentials of 21 vicinal dibromides versus the dihedral angle,, between the two C-Br bonds exhibits maxima (most positiveE t j2) for y = 180 0 (anti-periplanar conformation) and i/) = 0°... 

In the anti-periplanar conformation, the C—H and C—L sigma bonds are coplanar but on opposite sides of the C—C bond. Again, as the bonds break, the carbons change hybridization, and the sp3 orbitals change to the parallel p orbitals of the pi bond. 

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

Closer examination reveals that the syn-periplanar conformation has all the bonds eclipsed, whereas the anti-periplanar conformation has these bonds staggered, as shown in the following Newman projections ... 

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. 

You now have a more thorough explanation of the zig-zag arrangement of carbon chains, first introduced in Chapter 2 when we showed you how to draw molecules realistically. This isthe shape you get if you allow all the C-C bonds to take up the anti-periplanar conformation, and will be the most stable conformation for any linear alkane. 

As in ethane, the eclipsed conformations are not stable since any rotation leads to a more stable conformation. The staggered conformations are stable since they each lie in a potential energy well. The anti-periplanar conformation, with the two methyl groups opposite each other, is the most stable of all. We can therefore think of a butane molecule as rapidly interconverting between synclinal and anti-periplanar conformations, passing quickly through the eclipsed conformations on the way. The eclipsed conformations are energy maxima, and therefore represent the transition states for in ter conversion between conformers. 

Butane can exist in an infinite number of conformations (we have chosen to show only the six most significant) but has only three conformers (potential energy minima)— the two synclinal (gauche) conformations and the anti-periplanar conformation. 

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. 

The second diastereoisomer forms the Z-alkene for the same reasons the two phenyl groups are now on the same side of the H—C-C—Br plane in the reactive anti-periplanar conformation (again, this is clear in the Newman projection) and so they end up cis in the product. Each diastereoisomer gives a different alkene geometry, and they do so at different rates. The first reaction... 

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