Anti-periplanarity and

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. 

If we managed to slow down the rapid interconversions in butane (by cooling to very low temperature, for example), we would be able to isolate the three stable conformations—the anti-periplanar and the two synclinal conformations. These different stable conformations of butane are some sort of isomers. They are called con/b rmational isomers or conformers for short. 

The vicinal (3/) coupling constant between the two black Hs is 11 Hz. This is rather large and can be explained by a predominant conformation shown in the Newman projection, with the two large groups (PhCO and Ph) as far from each other as possible, the two medium groups (Br) as distant as possible, and the two black Hs in the places which are left. The dihedral angle between the black Hs is then 180° (they are anti-periplanar) and a large/is reasonable. 

Accompanying experimental work [8], we report on DFT calculations of the l,2-(o X. -C->silyl migration of pentacarbonyltungsten phosphinidene complexes la (anti-periplanar) and lb (syn-periplanar) to the corresponding phosphaalkene complexes 2a and 2b. The... 

The results of all these studies indicate that, in solution, the MPA amides exist in a conformational equilibrium composed of two main conformers ap-Z (MeO and carbonyl, anti-periplanar) and sp-Z (MeO and carbonyl, syn-periplanar carbonyl and C(F)H bond, syn-periplanar). In both cases, the methoxy group, the carbonyl group, the N-H bond, and the Ca-H bond are in the same plane and the phenyl ring is almost coplanar to the Ca-H bond (Figure 106). 

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