Trialkyltin reagents, toxicity

Trialkyltin hydrides represent an important class of reagents in organic chemistry because of their utility in radical reactions. However, problems of toxicity and the difficulty of product purification made trialkyltin hydrides less than ideal reagents. Several workup procedures and structurally modified trialkyltin hydrides have been developed to facilitate the separation of tin residues from the reaction mixture. Tris(trimethylsilyl)silicon hydride has also been synthesized and is often used successfully in radical reactions. However, its reactivity is different from that of trialkyltin hydrides in a number of important respects. Other tin hydride surrogates are also available. ... 

Trialkyltin hydrides are common reagents in organic radical chemistry, but the toxic by-products are extremely difficult to remove from reaction products. To facilitate their removal, a pyrene-functionalized tin hydride has been prepared (entry 20) 25 After standard solution-phase radical reactions with the stannane, filtration through activated carbon traps the tin species to afford pure products. 

Sml2-mediated radical cyclisations involving alkyl, alkenyl and aryl radical intermediates can be used to construct efficiently five-membered and, in certain cases, six-membered ring systems. This approach provides a useful alternative to trialkyltin hydride-mediated methods as toxic reagents and problematic tin byproducts are avoided. In addition, the use of Sml2 to induce radical cyclisations has led to the development of a number of powerful, radical/anionic sequential processes for the construction of complex systems. Sequential reactions involving radical-alkene/alkyne cyclisations are discussed in Chapter 6. 

Special care is required to handle trialkyltin hydrides and the waste they generate, and standard laboratory purification techniques often leave toxic levels of tin compounds in the product [88]. The industrial application of these methods has been hindered by the need to remove these tin-containing contaminants. Methods catalytic in tin have been developed [89-91], and tin hydride reagents modified to make their removal easier [92-98], but the need for alternatives to tin... 

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