Anti-clinal conformation

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°. 

Similarly, the rate constant for dehydrochlorination of 9, which can react by syn-coplanar elimination, was found to be about eight times greater than the rate constant for 10, which can only react from an anti-clinal conformation (Figure 10.20). ... 

The variation ranges of all the torsion angles are very wide from 145 ° for T5-T8, through 172°-180° for T3, T4 up to 225 °-228 ° for T2 and Tl. This means that individual enamine fragments may adopt, in respect of particular torsion angles, any conformation from syn-c ina through sjw-periplanar to anti-clinal, obviously, some of these conformations are observed more frequently than others. 

In the rigid 2,3-dihalogenonorbornane system, the only possible eliminating conformations in the substrate are planar syn and anti-clinal. Rate studies on the dibromo and dichloro substrates , the use of deuterium tracers "... 

The importance of coplanarity of the eliminating fragments is emphasised by the large difference in the Arrhenius activation energies for elimination from cw-4-r-butylcyclohexanol (E (cis) = 21 kcal.mole" ) and rra/iJ-4-r-butylcyclo-hexanol (Epitrans) = 38 kcal.mole ) . In the trans isomer, elimination must occur from a diequatorial syn-clinal conformation, or a diaxial conformation with the bulky r-butyl substituent in an unfavourable axial conformation, or an initial epimerisation to the cis isomer, or via a boat conformation, all possibilities which involve much higher energy intermediates than the diaxial elimination from the cis isomer. The products of elimination from the four stereoisomers of 1-decalol are also consistent with anti stereospecificity ... 

Representing the C -L and C -H bonds as shown in Figure 10.19 makes it apparent that both the anti-coplanar and syn-coplanar conformations allow the development of n bonding without the necessity for rotation about the carbon-carbon double bond as the elimination occurs. With the anti-clinal... 

A completely different rationale for the stereochemical outcome of aldol additions relies on open-transition-state models. These involve anti-periplanar orientation of enolate and carbonyl group, in contrast with their syn-clinal conformation assumed in the six-membered cyclic transition states. Open-transition-state structures have been proposed to offer a rationale for those aldol additions that give predominantly syn products, irrespective of enolate geometry [90]. This outcome has been observed in aldol reactions of tin and zirconium enoiates and of naked enoiates generated from enolsilanes by treatment with tris(diethylamino)sulfonium difluoro-methylsiliconate [70]. As shown in Scheme 1.12, the driving force for the... 

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