Push-pull ethylenes

Conversely, there are compounds in which nearly free rotation is possible around what are formally C=C double bonds. These compounds, called push-pull or captodative ethylenes, have two electron-withdrawing groups on one carbon and two electron-donating groups on the other (66). The contribution of di-ionic... 

Sandstrom, J., Static and Dynamic Stereochemistry of Push-Pull and Strained Ethylenes, 14, 83. Sargeson, A. M., see Buckingham, D. A., 6, 219. 

Pappalardo, R. R., Sanchez Marcos, E., Ruiz-Lopez, M. F., Rinaldi, D. and Rivail, J. L. Solvent effects on molecular geometries and isomerization processes A study of push-pull ethylenes in solution, JAm.Chem.Soc., 115 (1993), 3722-3730... 

In the ground state, aminomethylenemalonates possess an essentially planar geometry, which maximizes the electron delocalization in the molecules. In the heteropolar transition state, the plane of the groups R3 and NR R2 and the plane of the two carbonyl groups occupy orthogonal positions. More details of the dynamic and static stereochemistry of push-pull ethylenes, as in compounds 1 and 2, are discussed in two excellent reviews (73TS295 83TS83). 

B. Review of Conformational Properties of Acyclic Push-Pull Ethylenes. 87... 

IV. Strained Ethylenes without Push-Pull Effect. 160... 

Low torsional barriers in combination with strong steric interactions between donor and acceptor groups in push-pull ethylenes have in several cases been demonstrated to cause permanently twisted double bonds, in which a planar arrangement of substituents at the double bond may represent an energy maximum. 

Push-pull or capto-dative ethylenes can be represented by the general formula 1 where A and/or B represent electron-donating groups and X and/or Y represent electron-accepting groups. Other substituents may be aryl or alkyl groups. The conformational properties of such compounds have been the subject of much... 

In the following pages, the results of conformational studies of important classes of push-pull ethylenes will be reviewed, after which experimental and theoretical results bearing on the electronic structure of these compounds will be discussed. 

With push-pull ethylenes in which the donor part is a cyclic conjugated system with An + 2 ir electrons and/or the acceptor part is one with An tt electrons, the possibility exists for aromatic stabilization of the transition state to C=C rotation. Several such systems with both carbocylic and heterocyclic ring components have been studied. 

The change in potential energy accompanying torsion about die C,=C2 bond in a push-pull ethylene has an important component which depends on the overlap between the p2 orbitals on C] and C2, and which we may call E , which has maxima at torsion angles of 90° and 270°. In many of the molecules discussed... 

It is also clear that the delineation of these three cases is based on the lower limit to measurability of torsional barriers. In practice, it is difficult to go below 7 kcal/mol with push-pull ethylenes, since these rather polar compounds tend to aggregate at low temperatures and give very broad bands below -120 to - 130°C. As will be discussed later, several compounds that show a Case 1 type of NMR spectrum in solution are shown by X-ray crystallography to be twisted... 

Figure 4. Case 2 push-pull ethylene with prochira substituents.

The separation of formal charges in a polar limiting structure like 2b creates a dipole moment of ca. 20 D. Therefore, if such structures were of great importance, quite high dipole moments should be expected for push-pull ethylenes. Data for a reasonable number of mostly symmetrical and rather rigid compounds are known (Table 20). Several high dipole moments are observed, though not in the vicinity of those required for a complete transfer of the double-bond it... 

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