Electron spin resonance studies of the structures and reactions of trialkoxysilyl radicals

Abstract

Photochemically generated t-butoxyl radicals abstract hydrogen from the silanes (RO)₃SiH (R = Me, Bu^t, or Me₃Si) to give the corresponding silyl radicals (RO)₃Si˙, which have been studied in liquid solution using e.s.r. spectroscopy. All three silyl radicals are strongly pyramidal at silicon, as judged from the magnitudes of the ²⁹Si hyperfine splittings, and the values of a(²⁹Si) accord reasonably well with those predicted from a previously established linear relationship between a(²⁹Si) for X₃Si˙ and ¹J(²⁹Si–H) for X₃SiH. Trialkoxysilyl radicals abstract halogen rapidly from alkyl bromides and less readily from alkyl chlorides; the relative reactivities of Bu^tBr and PrBr towards (Bu^tO)₃Si˙ and Me₃Si˙ are similar. Addition of (Bu^tO)₃Si˙ to carbonyl compounds is somewhat slower than addition of Me₃Si˙. Addition of (RO)₃Si˙ to ethylene and to 1,1-di-t-butylethylene takes place readily to give β-silylalkyl radicals. Non-equivalence of the β-protons in (Bu^tO)₃SiCH₂ĊH₂ is attributed to chirality of the (RO)₃Si group and lineshape analysis provides Arrhenius parameters for the process exchanging the environments of the two protons; this process is identified as hindered rotation about the Si–O bonds.