In Summary—Applying the Rules

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. 

Occasionally, though, you will run across a more exotic pericyclic process, and will want to decide if it is allowed. In a complex case, a reaction that is not a simple electrocyclic ringopening or cycloaddition, often the basic orbital symmetry rules or FMO analyses are not easily applied. In contrast, aromatic transition state theory and the generalized orbital symmetry rule are easy to apply to any reaction. With aromatic transition state theory, we simply draw the cyclic array of orbitals, establish whether we have a Mobius or Hiickel topology, and then count electrons. Also, the generalized orbital symmetry rule is easy to apply. We simply break the reaction into two or more components and analyze the number of electrons and the ability of the components to react in a suprafacial or antarafacial manner. 

The best way to develop a feel for which model is best suited to any given reaction is practice. The Exercises give you several opportunities to do so. Also, extensive books and reviews, cited at the end of the chapter, provide many examples of both simple and complex pericyclic reactions to analyze. 

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