NAMED ORGANIC REACTIONS
* The Baeyer villiger oxidation is used to oxidize ketones to esters by using peroxy acids.
Note: The Baeyer villiger rearrangement is a regioselective reaction. In above case, the R' group is assumed to possess greater migratory aptitude and hence only one product is formed preferentially.
* This reaction involves the oxidative cleavage of C-C bond.
* The reagents which can be employed in Baeyer villiger oxidation include:
* Cyclic ketones furnish lactones (cyclic esters).
* The aldehydes may give carboxylic acids or formates. In the latter case, alcohols are finally formed due to hydrolysis of unstable formates under the reaction conditions.
* Initially the peroxy group is added to the carbonyl carbon to give a Criegee like intermediate. Then one of the group attached to carbonyl carbon is migrated on to the electron deficient oxygen atom in a concerted step, which is the rate determining step.
* The substituents which can stabilize the positive charge can migrate readily. The migratory aptitude of various substituents is approximately:
3o-alkyl > cyclohexyl > 2o- alkyl > benzyl > aryl > 1o - alkyl > methyl.
* The electron withdrawing groups (-I groups) on peroxy acids enhance the rate of the reaction.
* Thus the Baeyer villiger oxidation of unsymmetrical ketones is regioselective.
* As the rearrangement is a concerted process, the configuration of the migrating chiral substituent is retained.
* In case of aldehydes, usually the hydrogen atom is migrated preferentially and thus by furnishing carboxylic acids. But formates are also produced when the migrating group is other than the hydrogen. This is possible when the other substituent is a tertiary alkyl group or electron rich vinyl or aryl group (e.g. Dakin reaction).
* One of the competing reaction is the formation of epoxide when a double bond is present in the molecule especially at low temperatures in neutral solvents.
1) In the following Baeyer villiger oxidation, the preferential migration of more substituted secondary alkyl group is observed (regioselective) without disturbing its chiral integrity i.e., the configuration at the chiral carbon is retained in the product (stereospecificity).
2) Cyclic ketones furnish lactones as illustrated below.
3) Cyclohexyl group migrates preferentially over methyl group as illustrated below:
4) In the following example, the subsequent hydrolysis of the ester gives the desired alcohol. Another example of regioselectivity and stereospecificity.
5) Baeyer villiger oxidation is preferred over the epoxidation of double bond by the peracid as illustrated in the following reaction.
6) The lactone formed can be reduced to a dihydric alcohol.
7) The bacteria strain variants which produce BVMO can be employed in Baeyer villiger oxidation.
8) In the following reaction, the aldehyde is oxidized to formate due to preferential migration of aryl group. But it undergoes hydrolysis under the reaction conditions to yield a phenol. This reaction is similar to that of Dakin reaction.
9) As illustrated below, the aldehyde group is oxidized to carboxylic acid due to preferential migration of the hydride ion. The aryl group with electronegative halogen groups has less migratory aptitude. Remember the groups which can stabilize positive charge possess greater migratory aptitude.
10) The greater migratory aptitude of aryl group over the -CH2 group can be observed in the following example.
11) The -CH2 group is migrated preferentially in the following reaction. The -CH-CF3 group has less migratory aptitude due to electron withdrawing nature.
Author: Aditya vardhan Vutturi