Limitations and Exceptions of Frontier Orbital Theory

Although the applicability of the frontier orbital theory is very broad indeed, it is nevertheless necessary to be aware of the fact that the nature of this approach is still only approximate so that in certain cases some exceptions cannot be ruled out. The origin of these eventual failures was thoroughly discussed by Dewar in the study [51], where it was demonstrated that the greatest potential weakness of the approach consists in the very assumption attributing the decisive role only to interactions between the HOMO and LUMO orbitals of the individual components. It appears, namely, that this assumption need not be satisfied in all cases, and if this happens, the predictions of frontier orbital theory may fail. The typical example in this respect is, e.g., the addition of electron rich alkenes to ketenes, which is not, as demonstrates the formation of cyclobutanone instead of the expected a-methyleneoxetane, 

Another class of reactions exceeding the scope of the straightforward applicability of frontier orbital theory are the processes governed by the so-called paradoxial orbital interactions [17,52]. The typical feature of these reactions, represented by the addition of the singlet oxygen to ethene, is that their course is not governed by the 

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