Stability, zwitterion-cyclopropanone

This relative stability of cyclopropanone-zwitterion in five-membered rings can be generalized, as a similar behavior is found, for instance, in the Ram-berg-Backlund reaction of a-halogenated sulfones. (The mechanism of this reaction is known, and involves the formation of an episulfone heterocyclic analog of the cyclopropanone.) When this intermediate is formed in a strained system such as propellane 41, Paquette and coworkers suggest that it opens to a zwitterion in order to explain the formation of the substitution product 42. 

Perhaps more informative are the data in Table 22 for the relative rates of reaction of various cyclopropanones with furan. The results are consistent with the mechanism given in Scheme 32 where k >kz [furan], although the kinetic data do not distinguish between the closed cyclopropanone form and the zwitterionic intermediate. Increasing substitution would, of course, increase the stability of the allyl cation. 

Numerous more recent experiments (reviewed in ref. 12) have confirmed the general accuracy of the mechanism shown in Scheme 4, and the actual intermediate involved depends upon the relative stabilities of the zwitterions/oxyallyl cations and the corresponding cyclopropanones. These will be affected by the choice of solvent and structural features of the starting ketone, such as the degree of substitution and ring strain (in cyclic halo ketones). A recent example in which an oxyallyl intermediate and a cyclopropanone intermediate were both intercepted is shown in Scheme 8. ... 

On the other hand, a substrate with an axial halogen has a semi-U geometry. In the presence of a base, the developing carbanion (in the transition state), which can initially be described by an sp orbital, does not have the appropriate stereochemistry to react with the axial C-Cl bond (sec Scheme 7). In an aprotic medium the carbanion carbon has rehybridized into a p orbital stabilized by overlap with the carbonyl p orbitals, and the formation of a zwitterion is observed. Cyclization of the latter will lead to the two isomeric cyclopropanones 14 and 15, which will yield the two rearranged derivatives 14a and 15a by ring opening. 

Another example shows that under typical Favorskii rearrangement conditions, substituents stabilize the zwitterion and inhibit cyclopropanone formation 1-bromo- and 3-bromo-l,l,3-triphenylpropanones do not yield esters. In these cases the equilibrium between the cyclopropanone and the zwitterion is totally displaced in favor of the latter. (See Scheme 9.)... 

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. 

Accordingly, the evolution of the Favorskii rearrangement can explained as being a function of the relative stability of the zwitterion relative to the cyclopropanone. This stability is itself dependent on the nature of solvent (protic or aprotic) and on structural features (substitution and/or ring strain). 

We will now discuss the stability of the zwitterion relative to the cyclopropanone. 

Relative Stability of Zwitterion and Cyclopropanone A. Acyclic Substrates... 

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