Molecular reactions Sigmatropic

Sigmatropic shifts are reversible reactions in which a o-bonded group (usually H) migrates from one point to another within a molecular system. Sigmatropic shifts may occur either thermally or photochemi-cally, but 1,3- and 1,7-shifts are the most commonly encountered for the photochemical reactions. 

In this primer, Ian Fleming leads you in a more or less continuous narrative from the simple characteristics of pericyclic reactions to a reasonably full appreciation of their stereochemical idiosyncrasies. He introduces pericyclic reactions and divides them into their four classes in Chapter 1. In Chapter 2 he covers the main features of the most important class, cycloadditions—their scope, reactivity, and stereochemistry. In the heart of the book, in Chapter 3, he explains these features, using molecular orbital theory, but without the mathematics. He also introduces there the two Woodward-Hoffmann rules that will enable you to predict the stereochemical outcome for any pericyclic reaction, one rule for thermal reactions and its opposite for photochemical reactions. The remaining chapters use this theoretical framework to show how the rules work with the other three classes—electrocyclic reactions, sigmatropic rearrangements and group transfer reactions. By the end of the book, you will be able to recognize any pericyclic reaction, and predict with confidence whether it is allowed and with what stereochemistry. 

For the analysis of these sigmatropic reactions, correlation diagrams are not relevant since it is only the transition state and not the reactants or products which may possess molecular symmetry elements. 

The second reaction is an isomerization. That is, the molecular formula of 7-11 is the same as that of 7-10. Because xylene is an unreactive aromatic hydrocarbon, a reasonable assumption is that it functions only as the solvent. Therefore, the first thing to look for is a concerted reaction. A [3,3] sigmatropic reaction is possible. 

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