Favorskii rearrangement

The Favorskii rearrangement ( often spelled Favorskii rearrangement Favorskii rearrangement English ) is a name reaction in synthetic organic chemistry, the wurde.Die named after the Russian chemist Alexei Yevgrafovich Faworski ( 1860-1945 ) Faworski rearrangement describes the rearrangement of α - halo ketones or α - haloaldehydes in a basic medium and should not be confused with the Favorskii reaction.

History

The Favorskii rearrangement of α - halo ketones in basic medium leads to carboxylic acids or carboxylic acid esters. Starting substances usually chlorine or bromine ketones, rarely iodoketones. Suitable bases include hydroxides, alkoxides, and amines. With ammonia carboxamides may arise.

α - chlorocyclohexanone supplies with potassium hydroxide ring contraction, the potassium salt of cyclopentane carboxylic acid, which can be converted by neutralization to cyclopentane carboxylic acid:

Faworski realized that α, β -unsaturated carboxylic acids occur in troubleshooting some aliphatic dihalo ketones of the type RCH2CX2COCH2R ' with dilute potassium hydroxide solution.

Reaction mechanisms

Reaction mechanism 1

The exact sequence named after Faworski rearrangements has long been controversial. Finally, it has been recognized that at least two mechanisms are possible. The first possible mechanism begins with the deprotonation of the α - Halogenketonens 1 by the base used. After cleavage of the halide ion a cyclopropanone structure created 3 The positively polarized carbon atom of the carbonyl group is then nucleophilic attack by the base used. The result is the compound 4, which is protonated by a rearrangement by a water molecule. The result is the desired compound 6 is reacted with a one alcoholate, a carboxylic acid ester is produced 6 (R = organyl radical, such as alkyl group). If, however, one with potassium or sodium hydroxide to the reaction product 6 is a carboxylic acid (R = H).

An example of the mechanism of the reaction of the α - Chlorcyclohexanons is 7 Here arises an intermediate of cyclopropanone structure 8 as Loftfield proved by labeling with 14C.

Reaction mechanism 2

Another mechanism begins with the nucleophilic attack of the base used for the positively polarized carbon atom of the α - Halogenketonens 9 then the halide is eliminated and the desired compound 11 is formed. Depending on the base used, a carboxylic acid or a carboxylic acid ester can be produced.

Favorskii rearrangement while the chlorocyclopentanone is not known, provides 2- Halogencyclobutanon 12 ring contraction cyclopropane carboxylic acid 14, in addition to 2- Hydroxycyclobutanon and illustrates an example of the mechanism described dar.

The French chemist provided evidence that in this case hydroxide attack on the carbonyl carbon atom and the adjacent C-C bond is cleaved. This reaction mechanism is similar to the benzylic acid rearrangement and was therefore referred to as " Semibenzilsäure rearrangement ".

Use

In organic synthesis Favorskii rearrangement are always used when alternative syntheses of carboxylic acids (esters ) are more complicated than that of n α - haloketones.

A major use is the Favorskii rearrangement in cyclic compounds, since an n- membered ring to the next lower ( n-1) -membered ring can be converted into a rule.

Particular attention aroused a synthesis of the strained hydrocarbon cubane by Eaton, in a Favorskii rearrangement ( Semibenzilsäure mechanism) was a key step.