Absolute rate constants for some reactions of the triethylamine-boryl radical and the boraneradical anion

Abstract

Laser flash photolysis (LFP) of di-tert-butyl peroxide or dicumyl peroxide at ambient temperatures in the presence of Et₃N→BH₃ or BH₄⁻ generated the title radicals which were found to have broad, featureless absorptions in the visible region. Rate constants for H-atom abstraction from Et₃N→BH₃ by cumyloxyl radicals show a small solvent dependence, e.g. 12 × 10⁷ and 2.2 × 10⁷ dm³ mol⁻¹ s⁻¹ in isooctane and acetonitrile, respectively. Rate constants for halogen atom abstraction by Et₃N→BH₂˙ and BH₃˙⁻ from a number of chlorides and bromides were determined by LFP and by competitive kinetics, e.g., for Et₃N→BH₂˙ + CCl₄/PhCH₂Cl/CH₃(CH₂)₂Cl, k = 4.4 × 10⁹/1.1 × 10⁷/5.1 × 10⁵ dm³ mol⁻¹ s⁻¹ and for BH₃˙⁻ + CCl₄/PhCH₂Cl, k = 2.0 × 10⁹/3.0 × 10⁷ dm³ mol⁻¹ s⁻¹. Rates of addition of Et₃N→BH₂˙ to 1- and 1,1-substituted olefins increase dramatically as the electron affinity of the olefin increases, confirming the nucleophilic character of amine-boryl radicals. A comparison of the present results with literature data for the addition of olefins of four nucleophilic carbon-centered radicals proves that Et₃N→BH₂˙ is by far the most nucleophilic radical for which kinetic data are available. A few rate constants for abstraction of hydrogen from electron-deficient carbon by Et₃N→BH₂˙ are also reported.