Homolytic organometallic reactions. Part II. The kinetics and rate constants for the autoxidation of organoboron compounds in solution

Abstract

The kinetics of the autoxidation of a series of n-, s-, and t-butylboron compounds in iso-octane at 30° have been investigated. The reactions of tri-n- and tri-s-butylboranes (and of triphenylborane) were normally controlled by the rate of diffusion of oxygen into the solution but proceeded at measurable rates in the presence of pyridine which reduced the concentration of the boranes by complexation.

In every case, the kinetics were compatible with a homolytic chain reaction involving the following steps.[graphic omitted]

In the early stages of the reaction, initiation is by the direct reaction between borane and oxygen, but, as the reaction proceeds, the unimolecular decomposition of the alkylperoxyborane, or its bimolecular reaction with alkylborane become more important.

Values for the ‘oxidizability’ of the different boranes at the same rates of initiation [K_p/(2K_t)^1/2], and of the rate constants for the bimolecular homolytic substitution by the alkylperoxy radicals at the boron centre (K_p) are listed. These latter processes are influenced by counterbalancing electronic and steric effects in the alkyl groups, and by the presence of oxygen ligands on the boron, but in suitable cases they are considerably faster than the corresponding reaction at hydrogen, which is the corresponding propagation step in the autoxidation of a hydrocarbon.