Syn-coplanar conformation

Preliminary calculations indicate that the syn coplanar conformation between the allylic hydrogen and the C-N double bond is more stable by 1.6 keal. Consequently, the approach of the diazene takes place preferentially on the face of the diene opposite the chlorine in D in order to avoid the electronic repulsions between the carboxylatc group of the dicnophilc and the chlorine atom, affording the like isomer42. 

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... 

Newman projections of anti coplanar and syn-coplanar conformations. 

E2 reactions are stereospecific, because they generally occur via the anti-coplanar conformation, rather than the syn-coplanar conformation. Small deviations from coplanarity can be tolerated, and it is sufficient for the proton and the leaving group to be anti-periplanar. 

The M-C-C-H unit must be able to take up a roughly syn-coplanar conformation, which brings the (3 hydrogen close to the metal. 

Although both stereoisomers yield 4 tert butylcyclohexene as the only alkene they do so at quite different rates The cis isomer reacts over 500 times faster than the trans The difference in reaction rate results from different degrees of rr bond develop ment in the E2 transition state Since rr overlap of p orbitals requires their axes to be parallel rr bond formation is best achieved when the four atoms of the H—C—C—X unit he in the same plane at the transition state The two conformations that permit this are termed syn coplanar and anti coplanar... 

In the syn-periplanar conformation, the C—H and C—L sigma bonds are coplanar and on the same side of the C—C bond. As the bonds to the hydrogen and the leaving group start to break, the hybridization at each carbon begins to change to sp2, and the two sp3 orbitals, which have some pi-type overlap initially, change to the p orbitals of the pi bond. 

Of these possible conformations, the anti-coplanar arrangement is most commonly seen in E2 reactions. The transition state for the anti-coplanar arrangement is a staggered conformation, with the base far away from the leaving group. In most cases, this transition state is lower in energy than that for the syn-coplanar elimination. 

In E2, the two groups to be eliminated must be coplanar. In conformationally mobile systems like acyclic molecules, or in cyclohexanes, anti-coplanar is the preferred orientation where the H and leaving group are 180° apart. In rigid systems like norbomanes, however, SYN-coplanar (angle 0°) is the only possible orientation and E2 will occur, although at a slower rate than anti-coplanar. 

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 ... 

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). ... 

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