Equilibrium zwitterion-cyclopropanone

A conceivable mechanism for this reaction considers that malonate anion initially reacts as a base to give an equilibrium mixture of zwitterion 2 and cyclopropanone 3 intermediates, which have been suggested to be involved in the Favorskii rearrangement. ... 

On the other hand, it is clear that the axial stereochemistry favors zwit-terion formation (Schemes 7 and 8). This zwitterion is the precursor of the cyclopropanone, or is in equilibrium with it. The in or out cycliza-tion of the zwitterion would then be controlled by steric factors in such a way that only the less hindered cyclopropane would be formed." This is the case of the esters formed with retention of configuration from 3a-bromo-8,13-epoxy-2-labdanone (18a) and from trans-9-chlorodecalone 19a. ... 

It is quite a critical question to know whether the cyclopropanone and the zwitterion are mesomeric forms, or two real chemical species in equilibrium with each other. According to Burr and Dewar the unsubstituted zwitterion is not a mesomeric representation of the unsubstituted cyclopropanone in fact, the zwitterion is an individual chemical species which is the precursor of the cyclopropanone, Unfortunately, there is no experimental proof of this hypothesis as cycloaddition reactions are not observed when nonsubstituted cyclopropanones are interacted with dienophiles. It should be noted that this kind of cycloaddition reaction probably proceeds via a dipolar mechanism. ... 

However, evidence for the existence of a zwitterionic intermediate has been obtained by Bordwell when the zwitterion is substituted. In this case, Bordwell assumed an equilibrium between the zwitterion and the corresponding cyclopropanone. This was confirmed by Fort, who was able to trap the zwitterion by a cycloaddition reaction. 

Consequently, in absence of substitution, the zwitterion is either a mesomeric form of the cyclopropanone or a species in equilibrium with the cyclopropanone but in the latter case, this equilibrium would be totally displaced toward the cyclopropanone, a view in agreement with the theoretical results of Greenberg and Liebman. ... 

When the zwitterion or the cyclopropanone is included in a ring system, we think that there is indeed an equilibrium between the two individual species. The latter will depend on the ring strain, which is directly related to the ring size. For example, in the case of five-membered rings, the equilibrium is totally displaced toward the zwitterion. " ... 

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.)... 

Dimethylcyclopropanone remains in equilibrium with zwitterionic oxyallyl cation 92 in situ and undergoes D-A reaction with furan. Similarly, allene oxide 92a generated from silyl epoxide gives cyclopropanone and oxyallyl cation, which is trapped as furan adduct [79]. 

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