Syn-periplanar conformation

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 ... 

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). 

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. 

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 ... 

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. 

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°. 

For Ei eliminations, the starting materials adopt a syn-periplanar conformation (Section 6.2.1)... 

The syn-periplanar conformation is the least stable (it is higher in energy by 19kJmol ) as the two methyl groups are forced extremely close to one another (0° separation). 

In this case, the product is the Z isomer. The E isomer is not obtained, because that would require elimination from a syn-periplanar conformation, which is eclipsed and too high in energy. 

The microwave spectrum was assigned to the syn-periplanar conformer (the anti-periplanar rotamer is expected to be 10-14 kJ/mol less stable) [03Sam],... 

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