Reductions radical chain, triethylsilane

The reaction of thiyl radicals with silicon hydrides (Reaction 8) is the key step of the so-called polariiy-reversal catalysis in the radical chain reduction. The reaction is strongly endothermic and reversible with alkyl-substituted silanes (Reaction 8). For example, the rate constants fcsH arid fcgiH for the couple triethylsilane/ 1-adamantanethiol are 3.2 x 10 and 5.2xlO M s respectively. 

This reaction looks attractive because triethylsilane is non toxic and its by-products are low boiling point compounds easily removed by distillation. However, the alkanethiols have a strong bad smell and the remaining traces could be difficult to eliminate from the solution in certain cases, which make them undesirable in the synthesis of fine chemicals. Therefore, it appeared of interest to test the use of thiols supported on polyHIPE in these radical chain reductions. 

Reduction of RX to RH.1 In the presence of di-r-butylhyponitrite (initiator) and a thiol,2 triethylsilane reduces alkyl chlorides, bromides, or iodides to alkanes in >91% yield by a chain reaction in which the thiol effects transfer of H from the silane to an alkyl radical. This reduction is generally effected with a R3SnH, which is toxic and more difficult to remove from the products. 

Again, no reaction was observed in the absence of mercaptoethanol. The mechanistic steps for the transformation were elucidated by the Chatgilialoglu et ah and are represented in Scheme 4.13, in analogy with the pathway reported for the radical reduction of aromatic azides with triethylsilane in toluene, with the addition of silyl radicals to the azide function, liberation of nitrogen and formation of silyl-substituted aminyl radical. The thiol is the hydrogen atom donor to this intermediate and it can be regenerated by its interaction with the silane, thus propagating the chain. The hydrolysis of the silylamine occurred... 

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