Thiyl radical with silane

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. 

In an analogous process, the reactions of unsubstituted and 2-substituted allyl phenyl sulfides with (TMSlsSiH give a facile entry to allyl fns(trimethylsilyl) silanes in high yields (Reaction 26). In this case, the addition of (TMSlsSi radical to the double bond is followed by the S-scission with ejection of a thiyl radical, thus affording the transposed double bond. Hydrogen abstraction from (TMSlsSiH by PhS radical completes the cycle of these chain reactions. ... 

The low reactivity of alkyl and/or phenyl substituted organosilanes in reduction processes can be ameliorated in the presence of a catalytic amount of alkanethiols. The reaction mechanism is reported in Scheme 5 and shows that alkyl radicals abstract hydrogen from thiols and the resulting thiyl radical abstracts hydrogen from the silane. This procedure, which was coined polarity-reversal catalysis, has been applied to dehalogenation, deoxygenation, and desulfurization reactions.For example, 1-bromoadamantane is quantitatively reduced with 2 equiv of triethylsilane in the presence of a catalytic amount of ferf-dodecanethiol. 

Reaction 7.74) [84], That is, (TMS)3Si radical added to the double bond of allyl sulfides, giving rise to a radical intermediate that undergoes (3-scission with the ejection of the thiyl radical. Hydrogen abstraction from the silane completes the cycle of these chain reactions. 2-Functionalized allyl tris(trimethylsilyl)si-lanes (71) have been employed in the radical-based allylation reactions. 

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]. 

Since the deoxygenation of compound 38 was well characterized using classical Barton-McCombie conditions, Roberts chose this same model substrate to illustrate the viability of silane/thiol reduction protocol. Reduction with this system gave a good yield of 39, however, it was observed that minor diasteromer 40, resulting from radical-induced epimerization at C-5, could also be isolated. The rationale for the formation of 38 was potentially due to the thiyl catalyst abstracting H from the substrate faster than from the silane. The authors propose that the use of a better donor, such as diphenylsilane, could prevent such side-products from occurring. 

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