The effects of β-alkoxy substituents on radical reactions: halogen-atom abstraction from alkyl chlorides

Abstract

EPR spectroscopy has been used to measure the relative rates of halogen-atom abstraction from RCH₂Cl [R = Pr, MeOCH₂, (MeO)₂CH, (MeO)₃C, Me₃Si] by the metalloid-centred radicals Me₃N→ḂHBu, Et₃N→ḂH₂ and Et₃Si˙(M˙) in cyclopropane and in oxirane solvents at 160–260 K. β-Methoxy substituents increase the rate of reaction, especially for chlorine abstraction by the highly nucleophilic amine–boryl radicals. The accelerating influence of the β-oxygen substituents is attributed to a polar effect which operates in the transition state and involves charge transfer from M˙ to the CH₂–Cl group. The relative reactivities of the cis- and trans-isomers of 5-chloro-2-tert-butyl-1,3-dioxane indicate that the substituent effect of a β-alkoxy group does not depend markedly on the orientation of the β–C–O bond with respect to the rupturing C–Cl bond. It is concluded that the β–C–O bond dipole interacts electrostatically through space with the dipolar C ⋯ Cl ⋯ M grouping in the transition state and thus stabilises the latter relative to the reactants. This interpretation is supported by the results of ab initio molecular calculations for model systems. The EPR spectra of the β-alkoxyalkyl radicals are discussed in relation to their preferred conformations as deduced on the basis of the Heller–McConnell equation. The applicability of the latter equation is confirmed by ab initio calculations for the ethyl radical.