Migration peroxide groups

The most widely used reaction of organoboranes is the oxidation to alcohols, and alkaline hydrogen peroxide is the reagent usually employed to effect the oxidation. The mechanism, which is outlined below, involves a series of B to O migrations of the alkyl groups. The R—O—B bonds are hydrolyzed in the alkaline aqueous solution, generating the alcohol. 

Steric and conformational factors are also important, especially in cyclic systems.233 There is a preference for the migration of the group that is antiperiplanar with respect to the peroxide bond. In relatively rigid systems, this effect can outweigh the normal preference for the migration of the more branched group.234... 

The acid catalyzed rearrangements of peroxides, in which the migrating groups show the same migration aptitudes as in carbonium ion rearrangements, may be represented as rearrangements of oxygen cations.89-91... 

The decomposition of the peroxide IX, which gives acetophenone when the conditions are such as to favor the radical mechanism, gives methyl ethyl ketone if the reaction is run in acid. This is because of the superior migration aptitude of phenyl groups in real or incipient oxygen cations.112... 

It is interesting to discern the different migratory aptitudes displayed by spirocyclic ketones on exposure to hydrogen peroxide [252] (Eq. 226,227). Although participation of the selenium atom to direct the attack of the peroxide has been formulated, the results are also consistent with an electronic explanation. Thus, the rapid formation of selenoxide renders the spirocyclic center acceptor-like through polarity alternation, and the migration of the methylene group becomes more favorable. 

If carbonylation of a trialkylborane is conducted in the presence of water, the water promotes migration of a second alkyl group from the boron centre to the carbon atom derived from carbon monoxide. Subsequent oxidation by alkaline hydrogen peroxide gives a ketone which bears two substituents derived from the trialkylborane. A pathway which accounts for this is shown in Figure B3.3. 

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