Semibenzilic 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. 

It has been found that the bromo ketones 10-7a-c can rearrange by either the cyclopropanone or the semibenzilic mechanism, depending on the size of the ring and the reaction conditions. Suggest two experiments that would permit you to distinguish between the two mechanisms under a given set of circumstances. 

Most other systems studied have bridgehead halogens, and special attention has been paid to the reactions of the kind shown in Scheme 35. The yields of rearrangement products are uniformly good, and deuterium incorporation results imply that the semibenzilic mechanism operates for the smaller ring... 

The related dihalo ketone (28) rearranges smoothly at 0 °C, whilst bromo ketone (29) requires prolonged heating at 155 C (Scheme 37). This is anticipated, since formation of the intermediate (30) required for the semibenzilic mechanism is sterically disfavored. 

The mechanism shown, the semibenzilic mechanism, operates only when the ketone cannot enolize on the side opposite the leaving group, as in aryl ketones. When the ketone can enolize, a different mechanism is operative. This mechanism is discussed in Chapter 4. 

Mechanism 2 (the semibenzilic acid mechanism) looks better, but labeling studies show that the two C atoms a to the ketone become equivalent in the course of the reaction, which is consistent only with mechanism 1 (the electrocyclic mechanism). The rearrangement of a-chloro-o-phenylacctonc to methyl hydrocinnamate is also consistent only with the electrocyclic mechanism if the semibenzilic mechanism were operative, then methyl 2-phenylpropionate would be the product. 

To summarize, when H atoms are present on the a-carbon opposite the leaving group, the electrocyclic mechanism usually operates when they are not, the semibenzilic mechanism operates. Why does the electrocyclic mechanism proceed more quickly than the more reasonable semibenzilic mechanism for eno-lizable a-haloketones Deprotonation and electrocyclic ring closing are both very rapid reactions—the latter even when a strained ring is formed—and they must simply be faster than HO- addition and migration, despite what our chemical intuition tells us. 

The net structural change is the same for both mechanisms. The energy requirements of the cyclopropanone and semibenzilic mechanisms may be fairly closely balanced, as instances of the operation of the semibenzilic mechanism have been reported even... 

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. 

One could actually wonder why the a-halogenated cyclopentanone does not undergo ring contraction by a semibenzilic mechanism. As discussed below, the conditions required for such a mechanism to be operative are very narrowly related to the carbon halogen stereochemistry which must be equatorial. We have just seen that in a-halocyclopentanones, the carbon halogen bond is pseudoaxial, a stereochemistry which is more suitable to zwitterion formation. 

The semibenzilic mechanism requires (Scheme 12) a nucleophilic addition at the carbonyl carbon. Then, in a concerted process, the halogen is ejected by the 1,2-anionotropic migration of the CJ-C bond. 

Several structural requirements appear to be necessary in order to be able to observe the semibenzilic mechanism. Thus the acidity of the a hydrogen plays an important role. Indeed, it should be weakly acid in order to favor nucleophilic attack at the carbonyl carbon. Of course, this kind of mechanism will be facilitated by the absence of an a hydrogen the rearrangement is then known under the name of quasi-Favorskii. Since both substrates just discussed react via the semibenzilic mechanism, we shall discuss them simultaneously. 

Competition of the Cyclopropanone and the Semibenzilic Mechanisms as a Function of the Ring Strain and of the Base Strength... 

In the presence of sodium hydroxide and deuterated solvents (experiments 1 and 3), derivatives 59 and 60 undergo the Favorskii rearrangement via the semibenzilic mechanism, whereas it is the symmetrical mechanism which is involved with 61 in experiment 7. At first glance, it is only the difference in ring strain (the only variable factor) which is at the origin of this different... 

The analysis of the Favorskii rearrangement semibenzilic mechanism sheds light on some experimental and structural factors which allow a differentiation from the symmetrical mechanism. 

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. 

Finally, in polycyclic strained derivatives of the cage type, the semibenzilic mechanism will be preferred because of the equatorial halogen and the fairly high ring strain, which actually prohibits cyclopropanone formation. In most of... 

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