Reaction Rates and Practical Hints

The first propagation step concerns the reaction of the substrate R X with the tributylstannyl radical. R-X can be a halide (but not a fluoride), a chalco-genide, an isocyanide, a nitro compound or some more complicated radical precursor group such as xanthates, Barton and/or Kim esters.14 The rate of this step depends on X, the weakness of the R-X bond and the stability of the ensuing radical R (roughly speaking, the more stable the radical the weaker is the R-X bond) some typical rate constants are collected in Table 4.1.15 

As one would expect from the respective bond strengths, the order for the halides is R-I R-Br R-Cl for the same R group and selenides react faster than sulfides for the chalcogenides. In general, when the bond strengths are comparable, monovalent halides tend to be more reactive than divalent 

Tributyl- and triphenyl tin hydrides are now commercially available, the former being much cheaper as their preparation involves merely distilling a mixture of hexabutylditin oxide and polymethylhydrosiloxane.18 

The main practical difficulty when working with stannanes, apart from the somewhat nauseating stench, which seems to cling forever to glassware, is the purification of the product and the complete removal of organotin residues. Various tricks have been invented over the years, as follows  

In any case, its advisable to use freshly distilled material, as better yields are invariably obtained and the purification is often easier. In some instances, the 

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