Cyclopropanones mechanism

The net structural change is the same for both mechanisms. The energy requirements of the cyclopropanone and semibenzilic mechanism may be fairly closely balanced.87 Cases of operation of the semibenzilic mechanism have been reported even for compounds having a hydrogen available for enolization.88 Among the evidence that the cyclopropanone mechanism operates is the demonstration that a symmetrical intermediate is involved. The isomeric chloro ketones 12 and 13, for example, lead to the same ester. 

Quantum mechanical/molecular mechanical study on the Favorskii rearrangement in aqueous media has been carried out.39 The results obtained by QM/MM methods show that, of the two accepted mechanisms for Favorskii rearrangement, the semibenzilic acid mechanism (a) is favored over the cyclopropanone mechanism (b) for the a-chlorocyclobutanone system (Scheme 6.2). However, the study of the ring-size effects reveals that the cyclopropanone mechanism is the energetically preferred reactive channel for the a-chlorocyclohexanone ring, probably due to the straining effects on bicycle cyclopropanone, an intermediate that does not appear on the semibenzilic acid pathway. These results provide new information on the key factors responsible for the behavior of reactant systems embedded in aqueous media. 

The quasi-Favorskii rearrangement obviously cannot take place by the cyclopropanone mechanism. The mechanism that is generally accepted (called the semibenzilic mechanisml57)... 

Although several mechanisms have been proposed for this reaction, only cyclopropanone mechanism (Scheme 1) and benzilic acid mechanism (Scheme 2) are displayed here. 

The unsymmetrical mechanism is also called the semibenzilic mechanism. Generally, a weakly acidic hydrogen is present a to the carbonyl. When there is no such a -hydrogen, the reaction is called a quasi-Favorskii, and involves the same type of mechanism. The geometry which is necessary for this process to occur is the same in both the acyclic and cyclic series. Obviously in the latter case the reactivity will be directly related to the halogen stereochemistry. Ring strain will also play an important role and will often be the decisive factor in determining which of the semibenzilic and the cyclopropanone mechanisms obtain. 

Cyclopropanone Mechanism and Experimental Evidence for the Symmetrical Intermediate... 

Since the occurrence of a cyclopropanone mechanism was now considered to be beyond doubt, the studies carried out on the Favorskii rearrangement were directed toward a better understanding of the process leading to this cyclopropanone. The first step of the symmetrical mechanism leads to the formation of a carbanion enolate (Scheme 2) by an a hydrogen abstraction with... 

Table 3. The halogen is secondary in all cases except in 19e where it is tertiary all the substrates have protons in aThese are apt to be removed by the base to give the enolate in a according to the cyclopropanone mechanism. From an inspection of Table 3, it appears that the overall behavior of this kind of substrate is the same. They practically all give ring contraction products whatever the nature of the solvent, with the exception of compound 19e which has a tertiary halogen. In this case the reactivity is solvent dependant in protic media, substitution is observed, whereas ring contraction occurs under aprotic conditions.

The three types of compounds reported in Table 6 are more strained than those in Table 3. This factor, as well as others, such as the differences in reactivity and in experimental conditions, the steric and electronic factors, are all much more favorable for a semibenzilic rather than for a cyclopropanone mechanism. For derivative 67 of the cubane series, Eaton and Cole proposed such a mechanism. For the bicyclo[2.2.1]heptane derivatives, with the exception of 76, no a or a hydrogens are available, so that the semibenzilic mechanism is the only one possible (quasi-Favorskii). However, in the latter compounds, the aa -dihalogenated or the polyhalogenated derivatives can undergo ring opening. We will now discuss the influence of the steric and electronic factors on this secondary reaction. 

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