Chapter 3: Nonchain Reactions

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Chapter 3 is in a portable document file (pdf) and can be viewed by clicking the blue Chapter 3 button. The drawing below the button pictures a mechanism for a nonchain reaction involving a carbohydrate radical. Beside and below the drawing is a summary of Chapter 3.

This drawing contains an example of a nonchain radical reaction of a carbohydrate.
Summary of Chapter 3

Transition-metal-generated radicals are involved in most nonchain, radical reactions of carbohydrates. In some of these reactions the transition metal accepts an electron, and in others it is an electron donor. The carbohydrate radicals thus produced undergo typical radical reactions, such as addition to a compound with a multiple bond and hydrogen-atom abstraction. Mangan-ese(III) acetate and ammonium cerium(IV) nitrate both react with CH-acidic compounds (for example, compounds containing β-dicarbonyl groups) to produce electrophilic radicals that add readily to compounds with electron-rich double bonds (e.g., glycals).

Bis(cy­clo­pentadienyl)titanium chloride (Cp2TiCl) reacts with glycosyl halides to produce pyranos-1-yl radicals. In the absence of a radical trap these radicals generate anomeric mixtures of glycosyl titanium compounds that undergo β‑elimination to form glycals. Radical intermediates also are pro­duced when Cp2TiCl causes reductive opening of epoxide rings.

The samarium(II) iodide–hexamethylphosphoramide (SmI2–HMPA) complex often serves as an electron donor in radical-forming reactions where a carbohydrate sulfone or halide is the electron acceptor. Organocobalt and organomercury compounds generate radicals by carbon–cobalt and car­bon–mercury bond homolysis, respectively. These compounds form carbon-centered radicals by both thermal and photochemical reaction. Carbon–cobalt bonds also undergo enzymatic cleavage, but in nonbiological settings photochemical bond homolysis is most common.

Photolysis of a variety of carbohydrates produces radicals that participate in nonchain reactions. Excited carbonyl compounds generate radicals by hydrogen-atom abstraction and by C–C bond fragmentation. Oxygen–iodine bonds cleave homolytically upon photolysis to produce highly reactive, alkoxy radicals.