Radical Additions to Other Unsaturated Groups

The cis-selectivity of these reactions can be rationalised by invoking a pseudo-chair transition structure. Transition structure A leading to the trans product suffers from a 1,3-diaxial interaction between the alkyl substituent and an axial hydrogen atom on the ring. This unfavourable interaction is 

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. 

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