IV. Summary

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Chemoselectivity refers to ability of a reagent or intermediate (e.g., a free radical) to react with one group in a molecule in preference to a different, but potentially reactive, group present in the same molecule. Since most carbohydrate radicals trace their beginnings to reactions involving either the tri-n-butyltin [Bu₃Sn·] or tris(trimethylsilyl)silyl [(Me₃Si)₃Si·] radical, chemoselectivity in the reactions of these radicals plays a central role in carbohydrate radical formation. In many reactions a second opportunity for chemoselectivity arises when an initially formed, carbon-centered radical reacts selectively with another molecule present in solution.

The tri-n-butyltin radical adds to carbon–carbon, carbon–oxygen, and carbon–sulfur multiple bonds in a reversible fashion; consequently, for chemoselective reaction to occur, the reverse reaction must be blocked in some manner. Preventing reversal of radical addition usually is achieved by hydrogen-atom abstraction, addition to a multiple bond, or fragmentation of the adduct radical. Silicon–carbon bonds tend to be stronger than tin–carbon bonds so addition of some silyl radicals to unsaturated compounds is not reversible at normal reaction temperatures. The tris(trimethylsilyl)silyl radical, however, does add reversibly to unsaturated compounds.

Carbon-centered radicals tend to be quite chemoselective intermediates. They add readily to electron-deficient, carbon–carbon, multiple bonds but are less reactive in group and atom replacement reactions than (Me₃Si)₃Si· and Bu₃Sn·