Homolytic reactions of ligated boranes. Part 15. Comparative studies of amine–boranes as donor polarity reversal catalysts for hydrogen-atom abstraction

Abstract

ESR spectroscopy has been used to characterise the reactions of the amine–boryl radicals produced by hydrogen-atom abstraction from a variety of amine–borane complexes by photochemically generated t-butoxyl radicals. The complexes Me₃N→BH₂R (R = Me₂CHCMe₂, Buⁿ, Buⁱ, Bu^s), 1,1-dimethyl-1,2-azaborolidine, 1-methyl-cis-1-azonia-5-boratabicyclo[3.3.0]octane, Me₂NCH₂CH₂NMe₂·2lpcBH₂(Ipc = isopinocampheyl), Me₃SiCH₂NMe₂→BH₃, and Me₃N→BH₃ were investigated. All the amine–boryl radicals rapidly abstract halogen from alkyl bromides and chlorides at 170 K. Specific alkyl radicals can be generated for ESR studies at low temperature by UV irradiation of a solution containing Bu^tOOBu^t, Me₃N→BH₂Buⁿ, and the corresponding alkyl chloride. The amine–borane complexes act as donor polarity reversal catalysts for the overall abstraction of acidic hydrogen from HCC(O) groups in esters, lactones, ketones, imides, and related compounds. Relative rates of catalysed hydrogen-atom abstraction from MeCO₂Et, MeCH₂CO₂Et, and Me₂CHCO₂Et have been determined and competitive abstraction from the two different types of α-CH groups in MeC(O)CHMe₂ has been similarly quantified. The relative reactivities of the amine–boryl radicals can be understood in terms of a balance between enthalpic, polar, and steric factors and the merits of the different amine–boranes as polarity reversal catalysts for the overall abstraction of hydrogen from acidic C–H groups by alkoxyl radicals are assessed. The origin of the polar effects observed in hydrogen-atom abstraction reactions is discussed in terms of the electronegativity difference between the attacking and departing radicals and a simple approach for the quantitative description of polar effects is outlined.