412013-04-8Relevant academic research and scientific papers
Ligated Boryl Radicals. Part 2. Electron Spin Resonance Studies of Trialkylamine-Boryl Radicals
Baban, Jehan A.,Marti, Vernon P. J.,Roberts, Brian P.
, p. 1723 - 1734 (2007/10/02)
Transient amine-boryl radicals R3N->BH2 (1) have been generated in solution by hydrogen-atom abstraction from a variety of amine-borane complexes R3N->BH3, and studied by e.s.r. spectroscopy.The e.s.r. spectrum of (1; R = Me), and not of BH3>-. as previously proposed, is also detected after γ-irradiation of polycrystalline Me3N->BH3 at 77 K.The hyperfine splitting constants for (1) indicate a pyramidal arrangement of ligands about the boron centre, in contrast to the effectively planar geometry at the radical centre in isoelectronic primary alkyl radicals.In solution, (1; R = Et) rapidly abstracts halogen from alkyl bromides and less readily from alkyl chlorides; at 246 K butBr is only 1.2 times more reactive than PrnBr.The radical (1; R = Et) adds to MeCN to give an iminyl radical and displaces a methyl radical from MeNC; it fails to add to ethylene, but adds readily to the more electron-deficient double bond in trimethyl(vinyl)silane.Amine-boryls undergo β-scission much more readily than their isoelectronic alkyl radical counterparts; this difference is attributable to the greater exothermicity of the former cleavage.The aziridine-and azetidine-boryl radicals undergo rapid ring opening; C-N cleavage in the cis-isomer of the 2-methylaziridine-boryl radical gives mainly the secondary alkyl radical, while the trans-isomer gives mainly the less stable primary alkyl radical.Substituted but-3-enylamine-boryl radicals, analogues of the hex-5-enyl radical, undergo regioselective 1,5-exo-cyclisation to give analogues of cyclopentylmethyl.Relative rates of β-scission and cyclisation of amine-boryl radicals have been determined in competition with their abstraction of bromine from ethyl bromide.
