Pericyclic reactions aromatization method

Goldstein and Hoffmann have extended their arguments to a number of possible geometries of interaction in addition to the simple single ring.15 Dewar has also presented a perturbation approach to aromaticity that is historically antecedent to that of Goldstein and Hoffmann. We shall return to the topic of aromaticity, and describe Dewar s method, when we discuss pericyclic reactions in Chapter 11. 

Although Otto Diels and Kurt Alder won the 1950 Nobel Prize in Chemistry for the Diels-Alder reaction, almost 20 years later R. Hoffmann and R. B. Woodward gave the explanation of this reaction. They published a classical textbook, The Conservation of Orbital Symmetry. K. Fukui (the co-recipient with R. Hoffmann of the 1981 Nobel Prize in Chemistry) gave the Frontier molecular orbital (FMO) theory, which also explains pericyclic reactions. Both theories allow us to predict the conditions under which a pericyclic reaction will occur and what the stereochemical outcome will be. Between these two fundamental approaches to pericyclic reactions, the FMO approach is simpler because it is based on a pictorial approach. Another method similar to the FMO approach of analyzing pericyclic reactions is the transition state aromaticity approach. 

Application of this method to pericyclic reactions led to the generalization that thermal reactions take place via aromatic or stable transition states whereas photochemical reactions proceed via antiaromatic or unstable transition states. This is the case because a controlling factor in photochemical processes is conversion of excited state reactants into ground state products. Thus, the photochemical reactions convert the reactants into the antiaromatic transition states that correspond to forbidden thermal pericyclic reactions and so lead to corresponding products. 

We have introduced several different ways to analyze thermal pericyclic reactions. The highly formalized orbital symmetry analyses produce explicit rules, the conclusions of which are summarized in several tables. Other methods, such as FMO and aromatic transition state theories can be applied on a case-by-case basis, although they can be generalized in the same way. The realities are, in day-to-day chemistry, most pericyclic reactions will be familiar types or straightforward extensions of reactions we have covered here. 

Several methods, alternative to the treatments outlined in the previous chapter, have been put forward for the rationalization of pericyclic reactions since the original publications of Woodward and Hoffmann first appeared. We will concentrate here on only two of these methods because they have achieved a greater currency than the others they feature the concept of aromaticity. 

In summary, the PMO method when extended to photochemical reactions of the G-type predicts a reversal of aromatic properties from ground state to excited state. This justifies the set of rules for photochemical pericyclic reactions given above. Dougherty s analysis also reveals dangers in the use of orbital and state correlation diagrams for assessing photo-pericyclic reactions because the breakdown in the BO approximation leads to a destruction of symmetry. 

---