Homolytic reactions of ligated boranes. Part 12. Amine–alkylboranes as polarity reversal catalysts for hydrogen-atom abstraction by t-butoxyl radicals

Abstract

E.s.r. spectroscopy has been used to show that abstraction of electron-deficient hydrogen atoms from α-C-H groups in esters, ketones, and acetic anhydride by electrophilic t-butoxyl radicals is catalysed by amine–alkylboranes, in particular by trimethylamine–thexylborane (Me₃N→BH₂Thx; Thx =–CMe₂CMe₂H)(2). The single-step abstraction by Bu^tO˙ is replaced by a two-step catalytic cycle in which Bu^tO˙ first abstracts electron-rich hydrogen from the amine–alkylborane to give a nucleophilic amine-alkylboryl radical, which subsequently abstracts with high regioselectivity the electron-deficient α–hydrogen from the carbonyl compound. Both steps of this catalytic cycle are promoted by favourable polar effects, while such effects militate against direct α-hydrogen abstraction by Bu^tO˙. The rate coefficient for hydrogen abstraction from (2) by Bu^tO˙ to give the aminel alkylboryl radical Me₃N→BHThx, has been estimated to be 4.7 × 10⁷ dm³ mol^–1 s^–1 at 189 K in cyclopropane. Relative reactivities of carbonyl compounds towards α-hydrogen-atom abstraction by amine-alkylboryl radicals have been determined in competition experiments at 189 K and shown to depend on enthalpic, polar, and steric factors. Towards Me₃N→BHThx, the relative molar reactivities of MeCO₂Et, MeCH₂CO₂Et, and Me₂CHCO₂Et are 1 : 4.5 : 0.4, while towards the less bulky Me₃N→BHMe they are 1 : 6.7 : 7.3, showing the importance of steric effects in determining the rate of hydrogen abstraction. Ab initio molecular-orbital calculations have been carried out at the MP3/6-31 G**//HF/6-31 G** level for H₃N→BH₂Me and H₃N→BHMe and the strengths of the B–H and B–C bonds in ammonia–methylborane are estimated to be 432 and 404 kJ mol^–1, respectively.