Stereoselectivity in Radical Reactions

Most radicals located on saturated bonds are jt-radicals with a planar configuration and may be depicted with the free spin located in a p-orbital (1). Because such radical centers are achiral, stereochemical integrity is lost during radical formation, A new configuration will be assumed (or a previous configuration resumed) only upon reaction. Stereoselectivity in radical reactions is therefore dependent on the environment and on remote substituents. 

The general concepts of stereoselectivity in radical reactions have been illustrated in a number of recent books. Readers are referred to those books for a thorough treatment [1,2]. The following sections deal with a collection of applications where silanes act as mediators for smooth and selective radical strategies, based on consecutive reactions. 

Stereochemistry in radical reactions for organic synthesis has not been studied very extensively, because mild or low temperature-promoted radical reaction methods are extremely limited and the stereoselectivity in radical reactions is generally rather poor. Recently, however, stereoselective organic synthesis with radical reactions has become popular, since mild radical reaction methods such as the Barton decarboxylation, Et3B-initiated Bu3SnH reaction, etc. have been developed. Normally, low temperature-initiated radical reactions induce high stereoselectivity. 

A. L. J. Beckwith, Regioselectivity and stereoselectivity in radical reactions. Tetrahedron 37 3013 (1981). 

Regio- and stereoselectivity in radical reactions leading to heterocycle... 

To understand stereoselectivity in radical reactions it is first necessary to have a general understanding of the principles of radical reactions such as understanding of the importance of bond dissociation energies, steric effects, stereoelectronic effects, and radical polarity in radical reactions. Radicals are now valued synthetic intermediates because they can be used for transformations that are often difficult to accomplish by other methods. The kinds of protection schemes that are often essential for synthetic sequences of ionic reactions are rarely required for radical reactions. 

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