V. Summary

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 double bond and hydrogen-atom abstrac­tion. Manganese(III) acetate and ammon­ium cerium(IV) nitrate both react with CH-acidic compounds, such as those with β-dicarbonyl substit­uents, to produce electrophilic radi­cals that add readily to electron-rich double bonds (e.g., those present in 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.