Organoboranes for synthesis. 5: Stoichiometrically controlled reaction of organoboranes with oxygen under mild conditions to achieve quantitative conversion to alcohols

Abstract

The reaction of organoboranes with oxygen under mild conditions can be controlled to give an essentially quantitative conversion of all three alkyl groups on boron to the corresponding alcohol. The controlled oxidation is a very clean reaction, with only minor amounts of carbonyl and hydrocarbon products formed. All organoboranes react quite rapidly in the initial stages, but vary considerably in the time required to achieve the desired uptake of oxygen. In contrast to oxidation by alkaline hydrogen peroxide, a portion of this reaction proceeds through alkyl radicals, thus resulting in some loss of stereospecificity. Oxidation of mixed organoboranes reveals that the relative rates of oxidation of alkyl groups on boron are consistent with a radical mechanism, with tertiary ⪢ secondary ⪢ primary in the rate of oxidation. The selective oxidation of one alkyl group in the presence of the other is not possible, due to small differences in relative rates of oxidation. However, thexyl and cyclohexyl groups can be selectively removed from boron in the presence of alkenyl groups. Thus, controlled oxidation of thexyldialkenylborane affords pure dialkenylborinic acid.