Two-electron stabilization

Cations are by no means the only species where the effects of hyperconjugative delocalization reveal themselves in such a striking manner. Similar effects exist in neutral systems or in anions. For instance, the normal propyl anion should tend to be eclipsed (E) since in this manner the molecule would optimize the 4-electron interactions between the ethyl group t orbital and the p orbital which carries the electron pair. In the bisected conformation, where ttchs and ttchs have both been raised in energy, the four-electron, destabilizing (see Section 1.7, rule 2) p ->7r interaction is stronger than in the eclipsed conformation. At the same time the two-electron, stabilizing p ->ir interaction is weaker than in the eclipsed conformation. Both effects favor the eclipsed conformation. 

Figure 1.3 The two-electron stabilizing interaction between a filled donor orbital (pi<(]) and an unfilled acceptor orbital corresponding to perturbation...

The general Lewis-acid-base reaction (3.95) exemplifies the two-electron stabilizing donor-acceptor interaction of Fig. 1.3 (namely the nN->-nB interaction for (3.94)), which may be distinguished from the complementary bi-directional donor-acceptor interactions of covalent-bond formation (Section 3.2.1). However, this leaves open the question of whether (or how) the equilibrium bond reflects the formal difference between heterolytic (3.95) and homolytic (3.96) bond formation. 

The interaction of a singly occupied MO, 0j, with a singly occupied degenerate MO, 0j, leads to two electron stabilization given by the following equation ... 

The two electron stabilization energy will increase as the cis conformer is distorted from planarity since the antibonding Cjpz—C4pz overlap will decrease and eventually become bonding at 0 = 45°... 

B + C) union will be most unfavorable in gauche 1,3,5-hexatriene because four electron destabilization is maximized and two electron stabilization is minimized. 

The overlap integral SnA is greater for the cis isomer because the normalization factor of the nA group MO, given by the expression (2—2 Sff)- 2, is greater for the cis isomer. We conclude that the cis isomer of 1,2-difluoroethylene will be more stable than the trans isomer due to a greater two electron stabilization as well as a... 

From the interaction diagram of Fig. 8, we immediately see that the two electron stabilizing interaction, 2 —2 will be greater for cis union than for tram since the overlap integral, is larger in the former case ... 

However, the two electron stabilizing interaction dominates the four electron destabilizing interaction. Thus, cis union will be more favorable than trans union. In other words, here we have an application of the regiochemical rule stating that the union of two open shell fragments having a total of 4N + 2 pi-electrons will be predisposed to be cis. 

In the previous sections, we saw that, in most cases, a nonbonded attractive or repulsive interaction is enforced by both four electron destabilization and two electron stabilization. Hence, in order to simplify subsequent discussions, we shall adopt the OEMO model with neglect of overlap. Consequently, in the remainder of this work... 

In addition, the question arises as to whether there is any way of testing whether the two electron stabilizing interaction will increase or decrease as the FCF angle shrinks. The answer is affirmative and in this case the information is conveyed by the variation in the bond order of the 2pz fluorine AO s. We distinguish two cases ... 

An interesting problem arises when we examine the relative stabilization of the Css and Cee conformations of the model systems dimethyl ether and isobutene. In the former case, there is only one dominant two electron stabilizing interaction which favors the C conformation. On the other hand, in the case of isobutene there are two key two electron stabilizing interactions, one favoring the Css and the other favoring the Cee conformation. These considerations can be best understood by reference to Fig. 29. 

The dissection of this cation, illustrated for the staggered conformation, is shown above and the interaction diagrams for the staggered and eclipsed conformations are shown in Fig. 31. It is concluded that the eclipsed conformation will be favored over the staggered conformation since the two electron stabilizing i interaction is greater for the former case. Furthermore, change transfer from to pi results... 

The interaction diagrams for the above conformations are identical with that of methyl vinyl ether (Fig. 30) except that the oxygen lone pair AO is replaced by an unoccupied carbon 2p AO. With this in mind we conclude that the transoid conformation of the cation, Ts, will be more stable than the cisoid conformation, Cs, since the <(>j—Pz two electron stabilizing interaction is greater for the Ts conformation. 

Analysis of the two electron stabilizing interactions is complicated by the fact... 

If the antibonding Walsh MO is involved in the two-electron stabilizing interactions (bicyclo[2.1. Ojpentene and bicyclo[3.1. Ojhexenyl cation, Figure 11), then charge transfer to 7t(LUMO) of the polyene will lead to... 

Cieplak, A. S. Stereochemistry of nucleophilic addition to cyclohexanone. The importance of two-electron stabilizing interactions. J. Am. Chem. Soc. 1981, 103, 4540-4552. 

The strongest interaction between the C3H5 and CH2+ fragments will occur between the p orbital and the cyclopropane HOMO (Figure 7.4). Only one of the two degenerate cyclopropane HOMOs has a finite coefficient at the substituted carbon. This two-electron stabilizing interaction increases with the overlap, so 14 is the most stable configuration. This prediction is fully vindicated by experiment.8... 

The problem of the anion is slightly more complicated (Figure 7.6). The p orbital is doubly occupied, so four interactions must be considered the two-electron stabilizing (2) and (4) and the four-electron destabilizing interactions (1) and (3). Fortunately, their effects are complementary. 7t Mi. and 7i Me lie at lower energy than nMe and 7i Me, so... 

These orbitals have the correct symmetry for mixing with VP2 and 3. Figure 7.9 shows the interaction of 3 with dx2 y2 in CBFe(CO)3. This two-electron (stabilizing) interaction removes the thermodynamic instability. Furthermore, the single electrons in the parent fragments are paired in the complex, so the kinetic instability is also eliminated. 

The controlling interactions are likely to be the two-orbital, two-electron stabilizing interactions 0 and . Depending on the relative... 

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