Frontier orbitals effects

An extensive review with many examples125 shows that the reactivity-selectivity principle cannot be used to predict the selectivity of a reaction except in unique systems where one reaction is close to or diffusion controlled. The relative importance of the Hammond effect and the frontier-orbital effects determines the reactivity-selectivity relationship that will be found in a particular system. The review also concludes that the Hammond-Leffler a-value cannot be used as an indicator of transition-state structure. 

There is also a frontier orbital effect that assists conjugate addition over the aldol reaction, You will reeall that the carbonyl carbon is a relatively hard centre, whereas the 3 carbon of an enone is soft, As the nucleophilic enolate becomes more stabilized with extra electron-withdrawing groups, it beeomes increasingly soft and hence more likely to attack the [J carbon,... 

Always bearthis in mind bond strength is only a guide to selectivity in radical reactions. As we shall see shortly, it s not the only factor involved. Indeed, you ve already seen steric effects in action when the Br radical added to the less hindered end of the alkene in the first radical reaction of this chapter, and you will later see how frontier orbital effects can operate too. 

This observation is related to the problem mentioned earlier (on p. 47), of what happens when charge and orbital terms are either both low or both high. In this case, we have a thousandfold increase in nucleophilicity towards a carbonyl group, accompanied by a tenthousandfold decrease in basicity. It serves to remind us that carbonyl groups, although very responsive to basicity (charge control), are not unresponsive to frontier orbital effects. 

The whole truth, however, is not nearly as simple as this. The experimental evidence has not been collected systematically, and it is not known, in most cases, whether singlet or triplet states are involved. In the last ten years, at least sixty papers have been published in which reasonably reliable structures have been assigned to the cyclobutane dimers produced by irradiation of a great variety of unsymmetrical olefins in solution. (The stereo- and regioselectivity in solid-state photodimerization reactions have also received a lot of attention, but they are determined by the alignment of the monomers in the crystal lattice, not by frontier orbital effects.) For irradiation in solution, head-to-head dimers are described as major or sole products in forty of these papers, and head-to-tail dimers are described in twenty of them. In addition, almost all photodimerizations of 9-substituted anthracenes (479) give the head-to-head dimer (480),385 whereas the... 

The most obvious factors which will lead molecules to adopt the HT course are dipole-dipole repulsions and steric effects.118 That both of these factors are overridden more often than not is striking evidence for the importance of frontier orbital effects. 

Frontier orbital effects are evidently much more dominant in cross-coupling photoreactions than they were in photodimerizations. The regioselectivity observed is almost always that predicted by frontier orbital theory. Thus there is no difficulty in predicting the orientation when a Z-substituted olefin couples with a C-substituted olefin whether we look at the n orbital of the one component interacting with the LUMO of the other (Fig. 6-5a), or at the n orbital... 

In the third place, we know that there are many factors of which frontier orbital theory takes little or no account. They were mentioned specifically on p. 32. In particular, arguments based on steric effects are, quite rightly, common in organic chemistry. One of the lessons to be learned from so many of the examples in this book is just how often quite powerful steric effects are overridden by frontier orbital effects. 

As before, we must not forget how much solvent effects may be the dominant influence in the regioselec-tivity. There is ample evidence that both the alkylation and the acylation of enolate ions in the gas phase take place, more often than not, on oxygen,301 the site that solvents protect. However, frontier orbital effects have been used to explain those gas phase reactions, and there are several, in which the enolate ion is attacked on carbon.302... 

The oligomers/trimers of polyaniline as shown in Fig. 2 have also proved to be effective in corrosion control. Their frontier orbital energies and measured oxidation potential indicate large positive oxidation potentials. Further analysis of frontier orbital effects suggests a similar mode of corrosion inhibition for the oligomers and full polymer. Parallel interactions occur for the oligomers relative to... 

---