Baeyer villiger rearrangement, Lecture notes of Organic Chemistry

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MOLECULAR

REARRANGEMENTS

UNIT-III

LECTURE SCHEDULE:

Lecture- 13 :

THE BAEYER–VILLIGER REARRANGEMENT

Migratory Aptitude. Mechanism. Stereochemistry. Applications.

THE BAEYER–VILLIGER REARRANGEMENT

The treatment of ketones with a peroxy-acid (RCO 3 H) leads to the production an ester. An oxygen atom is ‘inserted’ next to the carbonyl group.

Enolizable ß-diketones do not react. -Diketones can be converted to anhydrides.

With aldehydes, migration of hydrogen gives the carboxylic acid.

Baeyer–Villiger reactions are among the most useful of all rearrangement reactions, and the most common reagent is m -CPBA ( meta -chloroperbenzoic acid) because it is commercially available.

 The reaction is often applied to cyclic ketones to give lactones.

Migratory Aptitude:

 The most electron-rich alkyl group (more substituted carbon) migrates first.  The general migration order:

 The results of the oxidation of^18 O-labeled benzophenone have strongly endorsed the fact that the second step is rate-determining.  In this study, the result revealed that phenylbenzoate retains the labeled oxygen in the carbonyl group

Baeyer–Villiger reaction of Unsaturated Ketones:

Peracids may epoxidize alkenes faster than they take part in Baeyer–Villiger reactions, so unsaturated ketones are not often good substrates for Baeyer–Villiger reactions.

 There are three possibilities

1. Peracids can selectively epoxidize.
2. Peracids can selectively carry out Baeyer–Villiger Oxidation.
3. Can carry out both reactions

 It is difficult to predict the outcome & it depends on-

1. Electrophilic nature of the ketone
2. 2. Nucleophilic nature of the alkene

Q; why does this reaction work, and why the C=C double bond here is particularly unreactive? Ans: Secondary group migrates in preference of the primary group. The alkene is not as reactive as expected because of steric crowding.

STEREOCHEMISTRY:

In Baeyer–Villiger reaction insertion of oxygen goes with retention of stereochemistry i.e., Starting material configuration is retained in the product. This is a general feature of Baeyer– Villiger reactions, even when inversion would give a more stable product.

Example: In this benzylic ketone, racemization would be expected but, as retention is the rule, it gives only one product.