EPR and computational studies of the formation and beta-scission of cyclic and acyclic dialkoxyalkyl radicals

Abstract

EPR spectroscopy and density functional theory have been applied to study the formation and subsequent β-scission of a series of dialkoxyalkyl radicals. Abstraction of hydrogen by photochemically-generated tert-butoxyl radicals from acyclic acetals R1O(R2O)CHR3, and from cyclic analogues derived from diols, takes place mainly from the acetal carbon atom to give radicals of the type R1O(R2O)ĊR3 and relative rates of abstraction have been determined in competition experiments. When R3 = phenyl or vinyl, the activating influence of these substituents on hydrogen-atom abstraction is smaller than might be expected, probably because delocalisation of the unpaired electron on to the unsaturated group comes at the expense of planarisation at Cα, in opposition to the natural pyramidalising tendency of the two α-alkoxy groups. Absolute rate constants and Arrhenius activation parameters for β-scission of R1O(R2O)ĊR3 have been determined by a steady-state EPR method and the results can be understood in terms of angle-strain and stereoelectronic effects. β-Scission of selected cyclic dialkoxyalkyl radicals that carry a phenyl or vinyl substituent at the radical centre has been investigated using density functional theory at the UB3LYP/6-31G(d,p) level. Computed activation parameters are in good agreement with the experimental results, where comparison is possible. Both experiment and theory indicate that benzylic 2-phenyl-1,3-dioxan-2-yl radicals undergo β-scission more readily than the corresponding allylic 2-vinyl-1,3-dioxan-2-yl radicals.