VI. Electron Transfer

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A. Reactions of Carbohydrate Radicals

Transition-metal complexes can act as electron transfer agents when reacting with carbohydrate radicals (eq 42 and 43). In the reaction shown in eq 44, titanium donates an electron during formation of the carbon-titanium bond between titanocene(III) chloride (Cp₂TiCl) and the pyranos-l‑yl radical 8.^31,32 In the reaction shown in eq 45, ammonium cerium nitrate [(NH₄)₂Ce(NO₃)₆] accepts an electron from the pyranos-1-yl radical 9 to convert it into the corresponding cation.^33,34 Other compounds that serve as electron donors in reactions with carbohydrate radicals are SmI₂, Cr(EDTA)^2-, [Ru(bpy)₃]²⁺, and Co(dmgH)₂py. (The structures of the ligands in these compounds are pictured in Figure 1). In addition to (NH₄)₂Ce(NO₃)₆, Mn(OAc)₃ also acts as an electron acceptor in reactions with carbohydrate radicals.

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B. Formation of Carbohydrate Radicals

When an electron is transferred to a carbohydrate chloride, bromide, iodide, or sulfone, the resulting radical anion reacts to form a carbohydrate radical (Scheme 4). The two electron donors participating in most of these reactions are SmI₂ (eq 46)³⁵ and Cp₂TiCl (eq 47),³⁶ but other transition-metal complexes [e.g., Cr(EDTA)^2-] are able to function in this capacity. Although the radical anion 10 is pictured in Scheme 4 as a discrete intermediate, in some instances cleavage of the RX bond may be simultaneous with electron transfer.

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Solvated electrons, which are more reactive as electron donors than transition-metal complexes, combine with esterified carbohydrates to produce radical anions. These radical anions then expel carboxylate anions to form carbohydrate radicals (eq 48).³⁷

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Reaction of an aldehyde or ketone with samarium(II) iodide produces a samarium ketyl (eq 49),³⁸ an intermediate considered to be a hybrid of structures 11-13 (Figure 2). These ketyls exhibit reactivity characteristic of carbon-centered radicals.

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