IV. Radical Addition
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A. Intermolecular Reaction
1. Addition to a Multiple Bond
Addition of a carbon-centered radical to a multiple bond in an unsaturated compound is an elementary reaction that forms a new carbon–carbon bond (eq 14 and eq 15). One way for this to happen is for a radical centered on one of the carbon atoms in a pyranoid or furanoid ring to add to an unsaturated noncarbohydrate. Examples of this type of addition are found in the reactions shown in eq 16,12 where a radical is centered on C-2 in a pyranoid ring, and eq 17,13 where the radical center is on C-3' in a furanoid ring. It is also possible to have the radical center located on a carbon atom that is in an open-chain structure (eq 18).14 Addition can involve a noncarbohydrate radical or a carbohydrate radical adding to an unsaturated carbohydrate (eq 1915 and eq 2016, respectively). When the radical and the compound to which it is adding are both carbohydrates, reaction creates complex structures quickly (eq 20).
Heteroatoms play a role in radical addition when a radical centered on a nitrogen, phosphorous, silicon, sulfur, or tin atom adds to an unsaturated carbohydrate; for example, a nitrogen-centered radical is involved in the reaction shown in eq 19,15 and the adding radical in the reaction pictured in eq 21 is sulfur-centered.17 Radicals also add to unsaturated carbohydrates in which the multiple bond contains one or two heteroatoms. An example of this type of reaction is given in eq 22, where addition is to a carbon–sulfur double bond.18 Similar radical addition reactions occur with carbohydrates containing carbon–oxygen, carbon–nitrogen, and nitrogen–oxygen multiple bonds.
2. Addition That Forms a Radical with a Hypervalent Atom
Although most addition reactions consist of a radical adding to a multiple bond, reaction that does not involve a double or triple bond also can take place. This happens when addition produces a radical in which an atom has an expanded octet (eq 23). Such a reaction is thought to take place when a telluride, such as 3, reacts with a methyl radical (eq 24).19
B. Intramolecular Reaction (Radical Cyclization)
Radical cyclization (eq 25) is an intramolecular version of radical addition that merits special mention due to the synthetic importance of new ring formation. Five- and six-membered rings are created most often, but larger rings also can be produced. A typical radical cyclization reaction is shown in eq 26.20