Thiyl radical with silicon hydride

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. 

Table 3.5 Rate constants for the reactions of thiyl radicals with silicon hydrides in cyclohexane at 60 °C [34]...

If one of the reactions in a radical chain sequence is too slow to compete effectively with radical-radical reactions, the chain will collapse. Slow reactions of simple silanes such as Et3SiH with alkyl radicals precludes their use in the tin hydride method. Although quite reactive with alkyl radicals, thiols and selenols fail in the tin hydride method because the thiyl and selenyl radicals do not react rapidly with organic halide precursors. Nonetheless, it is possible to use thiols and selenols in tin hydride sequences when a Group 14 hydride is used as a sacrificial reducing agent. The thiyl or selenyl radical reacts with the silane or stannane rapidly, and the silicon- or tin-centered radical thus formed reacts rapidly with the organic halide [8], In practice, benzeneselenol in catalytic amounts has been used in radical clock studies where BusSnH served as the sacrificial reductant [9]. 

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