Illogical disconnection

Although all these strategies imply, in some respect, a reactivity inversion of some of the starting reagents or synthons, the term "reactivity inversion" (or Umpolung as proposed by Seebach in 1979) [1] is commonly used in the context of the strategy known as "illogical disconnections" [2]. 

In contrast with all the heterolytic disconnections considered so far, which have been called "logical" in the sense that they meet all the requirements already mentioned (see Heading 3.5), we will cal "illogical disconnections" those disconnections which, although they do not fulfil some of these requirements, are still useful since they introduce an element of flexibility which is highly desirable in designing complex organic syntheses. 

In contrast to "illogical disconnections", in "plausible disconnections" a reasonable or, at least, plausible mechanism exists which involves the motion of electron pairs and leads to stable fragments, either ions or molecules. In any case, however, the "principle of maximum simplicity" does not hold here, since one of the resulting fragments is always a dissonant three-membered ring. 

Let us now consider a dissonant 1,6-dicarbonyl system, which provides a good example of a [3,3]-sigmatropic rearrangement. The "illogical disconnection" would lead to an a,p-unsaturated ketone and a "homoenolate" anion ... 

Since disconnection of any of the C-C bonds of a dissonant system leads to two fragments of identical reactivity (either nucleophilic or electrophilic), in the synthetic direction the reactivity of one of the two fragments must be inverted (see 5.1, "illogical disconnections"). Another possibility, however, is to oxidise or to reduce the resulting fragments to radical species. 

Clearly, these too will be illogical disconnections but ve can get round the problem in a different way by using a disconnection which actually links up the two... 

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