Unimolecular Rearrangements and Eliminations

Many of the reactions to be discussed are concerted processes and may be treated mechanistically in light of the concepts of orbital-symmetry control introduced in Part A, Chapter 10. Some others are stoichiometrically similar, but on mechanistic scrutiny have been found to proceed through discrete, short-lived intermediates. 

The reactions to be discussed in this section result in the formation of a new ring from two reacting molecules. A truly concerted mechanism requires that a single transition state, and therefore no intermediate, lie on the reaction path between 

A firm mechanistic understanding of concerted cycloadditions had to await the formulation of the reaction mechanism within the framework of molecular orbital theory. Consideration of the molecular orbitals of reactants and products revealed that in some cases a smooth transformation of orbitals of the reactant to those of the product is possible. In other cases, reactions that appear feasible, if no consideration is given to the symmetry and spatial orientation of the orbitals, are found to require high-energy transition states when the orbital properties are considered in detail. Those considerations have permitted description of many types of cycloadditions as allowed or forbidden. As has been discussed (Part A, Chapter 10), the application of orbital-symmetry relationships permits a conclusion as to whether a given concerted reaction is or is not energetically feasible. In this chapter, the synthetic application of these reactions will be emphasized. The same orbital-symmetry relationships that are informative as to the feasibility of a given reaction often are predictive of features of the stereochemistry of cycloaddition products. This predictable stereochemistry is an attractive feature for synthetic purposes. 

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CHAPTER 7 CYCLO ADDITIONS AND UNIMOLECULAR REARRANGEMENTS AND ELIMINATIONS... 

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