Stereodynamical processes in β-substituted symmetric isopropyl radicals

Abstract

Addition of trimethylsilyl (Me₃Si·) and methylthiyl (MeS·) radical to Me₃SiCH₂–CHCH₂ and MeSCH₂–CHCH₂ yields, respectively, the radicals Me₃SiCH₂ĊHCH₂SiMe₃(I) and MeSCH₂ĊHCH₂SMe (II) which were observed by e.s.r. spectroscopy. Restricted rotation about the ·CH–CH₂ bonds was detected below –100 °C and the free energy of activation determined [3.4 for (I) and 2.8₅ kcal mol^–1 for (II), respectively]. The features of the line shapes also showed that the conformation of (I) is symmetric whereas that of (II) is asymmetric. The geometry of these conformations was also assessed and the angle ϕ between the 2p_z orbital of ·CH and the C–X bond was found to be 4–5°§ when X = SiMe₃(I) and 10–11° when X = MeS (II). The dependence of the β-splitting on the conformation and on the electronic properties of silicon and sulphur was studied by INDO calculations and the hypothesis that sulphur-containing radicals must be distorted to account for their low a_Hβ splitting is challenged.