Homolytic organometallic reactions. Part V. Homolytic bimolecular substitution at boron by the triplet state of ketones

Abstract

If a mixture of an aliphatic ketone and a trialkylborane is irradiated with u.v. light, the ketone triplet brings about a bimolecular homolytic substitution at the boron centre, and the superimposed e.s.r. spectra of the two radicals which are formed can be observed; the ultimate products of the reaction result from the coupling of these radicals. R¹R²CO*+ BR³₃→ R¹R²ĊOBR³₂+ R³·→ products

By monitoring the intensities of the spectra when two different boranes, or a borane and an olefinic quencher, compete for reaction with acetone triplets, or when substitution and fragmentation reactions of di-isopropyl ketone triplet compete, the above mechanism was established, and kinetic parameters for the substitution reaction were derived. Typically, the reaction between triplet acetone and tri-n-butylborane has a rate constant of 7 × 10⁶M^–1 s^–1 at 20°.