Absolute rate constants for the autoxidation of organometallic compounds. Part II. Benzylboranes and 1-phenylethylboranes

Abstract

The kinetics for the free-radical chain autoxidation of several benzylboranes and 1-phenylethylboranes at 30° have been examined. The autoxidations of the dialkoxyboranes are highly autocatalytic. This is due both to an increasing rate of chain initiation and to an increasing oxidizability of the partially oxidized material as the reaction progresses. In the initial stages, chain propagation involves the abstraction of hydrogen from the carbon adjacent to the boron atom. It is postulated that as the oxidation proceeds hydrolysis and condensation reactions produce oxybisboranes and that these compounds are autoxidized via rapid S_H2 reactions at boron in which an alkylperoxyl radical replaces an alkyl radical. The final products are those that would be expected of an S_H2 reaction at boron in the starting compounds (alkylperoxydialkoxyboranes and mixed trialkoxyboranes).

The rate-controlling step for chain propagation in the autoxidations of the boroxines and of tribenzylborane involves peroxyl radical attack at boron. The following propagation rate constants were obtained: tribenzylboroxine. 1 × 10⁶ l mol^–1 s^–1; tri-(1-phenylethyl)boroxine, 4 × 10⁶ l mol^–1 s^–1; and tribenzylborane, 5 × 10⁶ l mol^–1 s^–1.