Hydrogen peroxide oxidation of organoboranes

Hydrogen peroxide oxidation of organoboranes is exothermic. Careful, dropwise addition of 30% hydrogen peroxide to the organoborane will provide sufficient heating to maintain a reaction temperature in the 40-50°C range. 

The electrophilic character of boron is again evident when we consider the oxidation of organoboranes. In the oxidation phase of the hydroboration-oxidation sequence, as presented in Figure 6.11, the conjugate base of hydrogen peroxide attacks boron. Hydroperoxide ion is formed in an acid-base reaction in step 1 and attacks boron in step 2. The empty 2p orbital of boron makes it electrophilic and permits nucleophilic reagents such as HOO to add to it. 

The oxidation of a trialkylborane may be effected by per-benzoic acid or by aqueous hydrogen peroxide in the presence of alkali.20 A detailed systematic study of the reaction parameters (oxidation temperature, base concentration, hydrogen peroxide concentration) of the latter method has led to the development3 of a standard and common procedure for oxidizing organoboranes, and is illustrated in the present procedure. 

Stereochemistry. Brown4 has shown that the carbonylation reaction and the carboethoxymethylation reaction are highly stereospecific in that the original boron-carbon bond is converted into a carbon-carbon bond with retention of configuration. These reactions thus are similar to oxidation of organoboranes with alkaline hydrogen peroxide and animation with hydroxylamine-O-sulfonic acid. 

Oxidation of organoboranes to alcohols is usually effected with alkaline hydrogen peroxide. The reaction is of wide applicability and many functional groups are unaffected by the reaction conditions, so that a variety of substituted alkenes can be converted into alcohols by this procedure. Several examples have been given above. A valuable feature of the reaction is that it results in the overall addition of water to the double (or triple) bond, with a regioselectivity opposite to that from acid-catalysed hydration. This follows from the fact that, in the hydroboration step, the boron atom adds to the less-substituted carbon atom of the multiple bond. Terminal alkynes, for example, give aldehydes in contrast to the methyl ketones obtained by mercury-assisted hydration. 

The electrophilic character of boron is again evident when we consider the oxidation of organoboranes (Mechanism 6.5). The conjugate base of hydrogen peroxide is... 

Brown, H. C., Snyder, C., Rao, B. C. S., Zweifel, G. 1986. Organoboranes for synthesis. 2. Oxidation of organoboranes with alkaline hydrogen peroxide as a convenient route for the cis hydration of alkenes via... 

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