A Guidebook to Mechanism in Organic Chemistry

Migration to Electron-Deficient Oxygen Atoms

MIGRATION TO ELECTRON-DEFICIENT OXYGEN ATOMS

It might reasonably be expected that similar reactions could occur in
which the migration terminus is an electron-deficient oxygen atom:
such rearrangements are indeed known.

(i) The Baeyer-Villiger oxidation of ketones

Treatment of ketones with hydrogen peroxide or organic peracids,

O

• RC—O—OH,

results in their conversion to esters:

O O

II H.O, ^ I!

R—C—R —^-V ft—C—OR

The rate-determining step has been shown to be the acid-catalysed

addition of the peracid to the ketone and the reaction is believed to

follow the course:

O <

II H®

R—C—R =s=i R—i

O

II

R—C—OR <

(XXXIV)

The initial adduct (XXXI) undergoes ready loss of an anion and

migration of one of the R groups with its electron pair to yield a

protonated form (XXXIII) of the final ester (XXXIV). In support of

the above mechanism it has been shown by using wO labelled oxygen

that the carboqyl oxygen in the original ketone becomes the carbonyl

oxygen in the final ester. The R group has been shown to migrate with

retention of configuration and bearing in mind that a cation of the

RO® type, such as.(XXXIIX is likely to be extremely unstable, it

»H

:—R

Oil

(^1) I
R' • C—O—OH „ 1. „ _
—> R—t—R O
If*
oJ-o—
2)—H®
C R'
II
(XXXI)
R—C®
I
OR
(XXXIII)
,-ft' c-o°
OH
R—C^$)
®o-«^
(XXXII)