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

摘要

Laser flash photolysis (LFP) of di-tert-butyl peroxide or dicumyl peroxide at ambient temperatures in the presence of Et_3N~BH_3 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~BH_3 by cumyloxyl radicals show a smaU solvent dependence, e.g. 12 x 107 and 2.2 x 107 dm_3 mol-l S-I in isooctane and acetonitrile, respectively: Rate constants for halogen atom abstraction by E_4N~BH; and BH_3'- from a number of chlorides and bromides were determined by LFP and by. competitive kinetics, e.g., for E4N~BH; + CCI_4/PhCH_2CIICH_3(CHJ_2CI, k = 4.4 x 109/1.1 x 107/5.1 x 105 dm_3 mol-l S-I and for BH_3' -+ CCIJPhCH_2CI, k = 2.0 x 109/3.0 x 107 dm_3 mol-l S-I. Rates of addition of E4N~BH; to 1- and I, I-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 Et_3N~BH; is by far the most nucleophilic radical for which kinetic data are available. A few rate constants for 3bstraction of hydrogen from electron-deficient carbon by E_4N~BH_2' are also reported.