Organoboron antioxidants, 2 boric acid derivatives as secondary antioxidants

Abstract

AbstractBoric acid esters and arylboronic esters and amides are known to be good primary antioxidants, but they are also capable of destroying hydroperoxides effectively. We have found three principal ways of hydroperoxide decomposition. At first, simple alkyl, but also other esters catalytically destroy hydroperoxides, e. g. cumyl and tert‐butyl hydroperoxide, after an induction period. In a second route, reduction of the hydroperoxides takes place under transformation of oxidizable parts of the organoboron compound. Boric acid catechol esters, for example, reduce tert‐butyl hydroperoxide giving a black precipitate, a reaction product of the instable o‐benzoquinone. Catechol and hydroperoxide in the absence of boron compound do not react. Finally, p‐phenylboronic acid esters reduce hydroperoxides to give alcohol and the corresponding boric acid ester. Many boron derivatives can react in two or all three ways, e. g. 2‐phenyl‐1, 3, 2‐benzodioxaborole. 1 mol of this compound destroys 2 mol of cumyl hydroperoxide giving cumyl alcohol, and more than 10 mol to give acetone and phenol. In side reactions, cumyl hydroperoxide is converted into dicumyl peroxide and acetophenone. The last compound indicates a reaction passing via radicals; but in all cases, this reaction is not important.