Baeyer–Villiger oxidation

The Baeyer -Villiger oxidation is a reaction in organic chemistry. Here, a ketone is converted to esters by reaction with peracid:

Cyclic ketones are converted to lactones with ring expansion. The reaction is catalytically accelerated by the presence of Lewis acids such as BF3.

The Baeyer -Villiger oxidation is named after the German chemist Adolf von Baeyer (1835-1917) and the Swiss chemist Victor Villiger ( 1868-1934 ).

Reaction mechanism

A particularly suitable percarboxylic acid for the Baeyer -Villiger reaction is peroxytrifluoroacetic acid (2).

The ketone 1, and the percarboxylic acid 2 react to form the unstable tetrahedral intermediate 3 with a weak O-O bond. After heterolytic cleavage of the OO bond migrates one of the alkyl groups (top: R ) to an O atom. The result is an ester 4 Besides the catalytic acceleration by Lewis acids, the nature of the leaving group [ above: trifluoroacetate (5 )] and the "wandering " group a significant influence on the reaction rate.

In unsymmetrically substituted ketones generally infiltrated the group which can stabilize the positive charge of the carbonium ion better if their migration is not sterically hindered.

The relative migration trend from the largest to the lowest tendency to migrate:

H> tert- alkyl > sec - alkyl = phenyl > primary alkyl > methyl

Experimental investigations of the Bayer - Villiger oxidation of 18O -labeled benzophenone showed that the carbonyl group of the ketone unchanged in the final product ( benzoate ) appears:

Aldehydes

Aldehydes are oxidized to carboxylic acids in general, as an H atom in most cases, the largest migration tendency ( for exceptions, see Dakin reaction). This reaction has synthetically, however, of minor importance, as there are many other methods for the oxidation of aldehydes to carboxylic acids, which are simpler and atom-economical feasible.

Technical application

Caprolactone, a raw material for the production of polycaprolactone a plastic selected from the group of thermoplastics can be produced industrially by a Baeyer -Villiger oxidation of cyclohexanone with peracids, such as peracetic acid, per-benzoic acid or m-chloroperbenzoic acid. The oxidation can however also be accomplished by means of catalysts by oxygen. In the laboratory, we used per-benzoic acid as an oxidizing agent:

Enzymatic variant

The use of Bayer - Villiger monooxygenases ( BVMO ) allows the enantioselective synthesis of chiral lactones, with the subclass of cyclohexanone monooxygenases ( CHMO ) has proven to be particularly effective. May be oxidized to (S)- 4-methyl- ε -caprolactone, for example, 4- methylcyclohexanone enantioselectivity (> 96 % ee). The mechanism of enzymatic Bayer - Villiger reaction was investigated.