The mechanism of polarity-reversal catalysis as involved in the radical-chain reduction of alkyl halides using the silane–thiol couple
作者:Yudong Cai、Brian P. Roberts
DOI:10.1039/b206552p
日期:——
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 (kXSH) and for transfer of hydrogen in the reverse direction (kSiH) were determined as 3.2 à 104 Mâ1 sâ1 and 5.2 à 107 Mâ1 sâ1, respectively, at 60 °C. The equilibrium constant kXSH/kSiH is thus 6.2 à 10â4 at 60 °C and corresponds to ÎrH
â
ÎrG
=
+20.4 kJ molâ1 for abstraction of hydrogen from Et3SiH by 1-AdSË, implying that the SiâH bond in the silane is stronger by ca. 20 kJ molâ1 than the S-H bond in the alkanethiol. The silanethiol (ButO)3SiSH was found to be a more effective catalyst than 1-AdSH, because kXSH is greater (1.3 à 105 Mâ1 sâ1) while kSiH is very similar (5.1 à 107 Mâ1 sâ1). The value of kXSH/kSiH is now 2.6 à 10â3 at 60 °C and thus the SâH bond in this silanethiol is stronger by ca. 4 kJ molâ1 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)-ButMePhSiH, radical-chain processes that provide independent confirmation of the values of kXSH derived from octyl bromide reduction. The value of ÎrH determined in this work indicates that abstraction of hydrogen from Et3SiH by an alkanethiyl radical in cyclohexane solvent is ca. 11 kJ molâ1 less endothermic than implied by the difference in the currently-favoured experimental gas-phase dissociation enthalpies for the Et3SiâH and MeSâH bonds.
通过对各种简单的硅烷(如Et3SiH和Ph3SiH)促进硅烷自由基链还原烷基卤化物的机制进行了详细研究。在60 °C的环己烷中,使用Et3SiH对1-溴辛烷和1-氯辛烷进行的硫醇催化还原的动力学研究与以下机制一致:硫基自由基从硅烷中可逆地抽取氢,随后由产生的三乙基硅基自由基从辛烷卤化物中抽取卤素,最后由硫醇熄灭生成的辛烷自由基,从而生成辛烷。根据该机制,从Et3SiH中被合适的Adamantane-1-硫基自由基抽取氢的速率常数(kXSH)和反向氢转移的速率常数(kSiH)分别在60 °C下确定为3.2 × 10^4 M⁻¹ s⁻¹和5.2 × 10^7 M⁻¹ s⁻¹。因此,平衡常数kXSH/kSiH在60 °C下为6.2 × 10⁻⁴,并对应于ΔrH ≈ ΔrG = +20.4 kJ mol⁻¹,意味着硅烷中的Si–H键比烷基硫醇中的S–H键强约20 kJ mol⁻¹。发现硅硫醇(ButO)3SiSH是一种比1-AdSH更有效的催化剂,因为kXSH更大(1.3 × 10⁵ M⁻¹ s⁻¹),而kSiH非常相似(5.1 × 10⁷ M⁻¹ s⁻¹)。因此在60 °C下,kXSH/kSiH的值为2.6 × 10⁻³,这意味着该硅硫醇中的S–H键比1-AdSH中的强约4 kJ mol⁻¹。提议的烷基卤化物还原机制得到了硫醇催化的H/D交换(在R3SiH/D与XSH/D之间)以及硫醇催化的(S)-ButMePhSiH的消旋化的动力学研究的强有力支持,这些自由基链过程进一步确认了从辛烷溴化物还原得出的kXSH的数值。本研究中确定的ΔrH值表明,在环己烷溶剂中,烷基硫基自由基从Et3SiH中抽取氢的反应比目前流行实验气相解离焓(针对Et3Si–H和MeS–H键)暗示的要低约11 kJ mol⁻¹。