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

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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, Bus), 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 ButOOBut, 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.

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