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ESR spectroscopy has been used to characterise the reaction 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*2IpcBH2 (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 alpha-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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Reference:
Tetrahydrofuran – Wikipedia,
Tetrahydrofuran | (CH2)3CH2O – PubChem