Four-electron destabilization

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. 

The four-electron destabilization rationale The rotation barrier of ethane is sometimes explained in terms of the mnemonic energy-level-splitting diagram shown in Fig. 3.58. The figure purports to depict how two filled MOs of ethane ( and 4>+) evolve perturbatively from two... 

Figure 3.58 The mnemonic energy-level-splitting diagram for the four-electron destabilizing interaction of two occupied non-orthogonal orbitals.

However, application of the four-electron destabilizing rationale to the ethane barrier is misleading for three reasons. 

If the doubly occupied MO s, i and 0j, are degenerate the four electron destabilization energy is given by the equation ... 

It is clear that when overlap is neglected the four electron destabilizing interaction becomes zero, i.e. AE = 0. 

Accordingly, the four electron destabilization energy will vary in the following order cis > trans > gauche. 

We now focus on the four electron destabilizing interactions. A comparison of the destabilizing interaction w—0j and is very simple. The destabilizing... 

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. 

On the other hand, the four electron destabilizing interaction, <(>i —i, is larger for cis union than trans because 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... 

Unusual M-H H-M interaction is rationalized in the following terms The weak H H contact is a four-electron destabilizing interaction between the a M-H bonding levels, where mixing of the a M-H levels produces a weakly bonding character. 

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... 

Fig. 9. A schematic representation of the interaction between the ns orbital (SHOMO) of the s-m-butadiene unit in 37 and the rr-orbital (HOMO) of the dienophilic component of 38, assuming the formation of syn-adducts [82, 84, 85]. The four center/four electron destabilization of exo-face approach (exo-2) is characterized by a larger net antibonding energy [87] than endo-face approach (endo-2) which is consequently favored. The orbitals show the effects of the disrotatory tilts which they are believed to experience. Note that in the terms endo-2 and exo-2, the 2 refers to the diene unit. Subsequently, the terms endo-1 and exo-1 are also employed where the 1 refers to the ene portions of the dienophilic unit...

The four-electron destabilization energy AE, for the interaction of orbitals k and / is given (248) by... 

If we consider a two-electron interaction between doubly occupied Xi and empty Xz (1-22), the two electrons are stabilized by 2AE . The stahlization associated with a two-electron interaction between two orbitals of different energy is therefore proportional to the square of the overlap and inversely proportional to the energy difference between the two orbitals, that is, proportional to S /Ae. However, a four-electron interaction is destabU-izing, since AE is larger than AE" " (1-23). It can be shovm that this four-electron destabilization is proportional to the square of the overlap,. ... 

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