Nucleophilic Rearrangements and Tautomerizations

On this basis, structural relationships have been established for several types of reaction mechanisms, which allow one to predict whether a rearrangement can occur in an intramolecular way and to assess the ease with which a reaction may develop depending on the initial structure of the compound being rearranged. Let us now consider two important examples, which manifest a deep connection between these relationships and the conclusions drawn from a theoretical analysis of the stereochemistry of organic reactions. 

The rearrangements of Eq. (7.2) belong to the transformations of this type. By their mechanism, they are classified under the reactions of nucleophilic substitution at the central atom of a migrating group 

The topology of the reactive site (the parameters (p, Ixy) imposes certain constraints upon the length and configuration of the chain that links X and Y. It is not difficult to lay down the corresponding rules of selection . Thus, in the case of conjugated chains, intramolecular l,j-shifts of the migrants of type II can occur in the systems of type IV-VI  

In the systems of type VII the reactive centers X and Y are located at a distance too large for the l,5-(j-bond shift. By contrast, in the system VIII, in spite of a fairly close promixity of the reactive sites, the reaction is inhibited due to the unfavorable angular parameter (p. 

The simplest chain Z, which provides conditions for migration of the type III groups, possesses a cis-trans-conrigumtion and corresponds to the case of a 1,7-sigmatropic migration via an intermediate eight-membered ring compound. 

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