The mechanism of polarity-reversal catalysis as involved in the radical-chain reduction of alkyl halides using the silane–thiol couple

Abstract

The mechanism by which thiols promote the radical-chain reduction of alkyl halides by a variety of simple silanes, such as Et₃SiH and Ph₃SiH, has been investigated in detail. Kinetic studies of the thiol-catalysed reduction of 1-bromooctane and of 1-chlorooctane by Et₃SiH in cyclohexane at 60 °C are consistent with a mechanism that involves reversible abstraction of hydrogen by the thiyl radical from the silane, followed by abstraction of halogen from the octyl halide by the resulting triethylsilyl radical and quenching of the derived octyl radical by the thiol to give octane. On the basis of this mechanism, rate constants for abstraction of hydrogen from Et₃SiH by the adamantane-1-thiyl radical (k_XSH) and for transfer of hydrogen in the reverse direction (k_SiH) were determined as 3.2 × 10⁴ M⁻¹ s⁻¹ and 5.2 × 10⁷ M⁻¹ s⁻¹, respectively, at 60 °C. The equilibrium constant k_XSH/k_SiH is thus 6.2 × 10⁻⁴ at 60 °C and corresponds to Δ_rH ≈ Δ_rG = +20.4 kJ mol⁻¹ for abstraction of hydrogen from Et₃SiH by 1-AdS˙, implying that the Si–H bond in the silane is stronger by ca. 20 kJ mol⁻¹ than the S-H bond in the alkanethiol. The silanethiol (Bu^tO)₃SiSH was found to be a more effective catalyst than 1-AdSH, because k_XSH is greater (1.3 × 10⁵ M⁻¹ s⁻¹) while k_SiH is very similar (5.1 × 10⁷ M⁻¹ s⁻¹). The value of k_XSH/k_SiH is now 2.6 × 10⁻³ at 60 °C and thus the S–H bond in this silanethiol is stronger by ca. 4 kJ mol⁻¹ than that in 1-AdSH. The proposed mechanism for alkyl halide reduction is strongly supported by kinetic studies of the thiol-catalysed H/D-exchange between R₃SiH/D and XSH/D and the thiol-catalysed racemisation of (S)-Bu^tMePhSiH, radical-chain processes that provide independent confirmation of the values of k_XSH derived from octyl bromide reduction. The value of Δ_rH determined in this work indicates that abstraction of hydrogen from Et₃SiH by an alkanethiyl radical in cyclohexane solvent is ca. 11 kJ mol⁻¹ less endothermic than implied by the difference in the currently-favoured experimental gas-phase dissociation enthalpies for the Et₃Si–H and MeS–H bonds.