Structural Features on the Reactivity of Acyclic and Cyclic Substrates

Influence of Structural Features on the Reactivity of Acyclic and Cyclic Substrates 

In open-chain compounds, where rearrangement products are more easily obtained, 2-bromo-2,4-dimethyl-3-pentanone gives the a-alkoxyketone as a major product even when the reaction is carried out under aprotic conditions.Consequently two factors seem to be decisive in inhibiting cyclopropanone formation and yielding the a-alkoxyketone even in an aprotic solvent. The first is the degree of substitution, which can play a role in open-chain or cyclic compounds. The second operates only in cyclic systems and is ring strain which can be calculated by force field techniques. This kind of strain can be due to valence angle deformations or to transannular nonbonded interactions or bonded interactions.  

According to what has been discussed until now, structural features should be capable of stabilizing a zwitterionic intermediate. Evidence for such an intermediate has been obtained by Smith and Gonzalez in the case of substitution 

Accordingly it appears that the results concerning the relative stabilities of the zwitterion and the cyclopropanone as a function of substitution can be extended to the Favorskii rearrangement in open-chain or cyclic compounds. In the latter case, both substitution and ring strain have to be considered. 

It is reasonable to predict that in compounds not bearing many substituents or in unstrained cyclic compounds, the contribution of the cyclopropanone will be more important than that of the zwitterion and 

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