Radical Clock Kinetic Studies - Practical Aspects

4 Radical Clock Kinetic Studies - Practical Aspects 

The most convenient radical clock studies are based on radical chain reactions because these processes usually have high conversions with small amounts of initiation. Two general types of radical chain processes have been extensively employed in clock studies, the tin hydride method and the PTOC-thiol method. 

Tin hydride reaction sequences are usually initiated by thermal decomposition of azo-Z /5-isobutyrylnitrile (AIBN), which has a decomposition half-life of about 1 h at 80 °C. Although other initiators can be used at different temperatures, AIBN is 

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]. 

The fundamental steps in the PTOC-thiol method are illustrated in Fig. 4 [10]. The radical source is one of Barton s PTOC esters or a related thiohydroxamic acid derivative [11, 12] these are made from the corresponding carboxylic acids. The acyloxyl radicals produced in the initiation step rapidly decarboxylate to give the radical of interest. This radical reacts with a hydrogen atom donor, a thiol in this 

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Radical Clock Kinetic Studies - Practical Aspects 323 == COOEt BuaSnH ... 

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