Silylaminyl radicals. Part 2. Free radical chain halogenation of hydrocarbons using N-halogenobis(trialkylsilyl)amines

Abstract

The liquid-phase halogenation of a number of hydrocarbons and of 1-chlorobutane by N-halogenobis-(trialkylsiyl)amines has been studied using product analysis techniques. The reactions take place by free radical chain mechanisms which involve the propagation steps generalised in equation (A) and (B)(X = Br or Cl). At 353 K, the molar reactivities of toluene (benzylic C–H) and cyclohexane towards (R₃Si)₂N·+ RH →(R₃Si)₂NH + R·(A), R·+(R₃Si)₂NX → RX +(R₃Si)₂N·(B)(Me₃Si)₂N· are approximately equal and toluene is 5.2 times more reactive than perdeuteriotoluene. The relative rates of hydrogen abstraction by (Me₃Si)₂N· and (Bu^tMe₂Si)₂N· from the primary, secondary, and tertiary C–H groups in 2-methylbutane show that the silylaminyl radicals are not only highly reactive but also sterically demanding. Thus, at 333 K the average primary C–H reactivity is 0.6 times that of the tertiary C–H towards attack by (Me₃Si)₂N·, but 4.2 times that of the tertiary C–H towards attack by the more bulky (Bu^tMe₂Si)₂N·. Silylaminyl radicals are much more reactive in hydrogen abstraction than are analogous dialkylaminyl radicals and this difference is interpreted in terms of thermodynamic and polar effects which arise because of the σ-donor–π-acceptor nature of the trialkylsilyl substituent.